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القسم الاول من معجم الاجهزة للتي تبدأ بالحرف A هام جدا جدا للمهتمين بالعمل الفني
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I a (alpha) Average percent
A age change in resistance per degree of pure metal resistance of device between O°C and 100°C. Usually designated by the Greek letter
I alpha, with units of W /W / 0C.
! aberration Deviation from ideal behavior by ( a lens, optical system, or optical component.
: i. Aberration exists in all optical systems, and ~. i designers must make trade-offs among the \] different types depending on how much .;t aberration they will tolerate.
ABI Application Binary Interface; to run without porting.
'... abnonnally distributed uncertainty In the ISA recommended practice ISA-RP67.04.022000 this term denotes uncertainties that do not have a normal distribution.
abort In data processing, to terminate a computer operation before its normal conclusion.
abrasion 1. The act of removing surface material from a substance through sliding or roll
.. ing contact with the hard particles made of i the same or another substance. The particles
themselves may be loose or part of another .surface brought into contact with the first. 2. A surface blemish caused by roughening or scratching.
abrasive 1. Particulate matter, usually having sharp edges or points, that can be used to shape and finish workpieces in grinding, honing, lapping, polishing, blasting, or tumbling processes. Depending on the process, abrasives may be loose, formed into solid shapes, glued to paper or cloth, or suspended in a paste, slurry, or air stream. 2. Any substance capable of removing material from a surface through the act of abrasion. 3. A material formed into a solid mass, usually fired or sintered, and used to grind or polish workpieces; common forms are grinding wheels, abrasive disks, honing sticks, cones, and burrs.
., ABS Acrylonitrile butadiene styrene; frequently used for enclosures; good high-temperature resistance with high impact strength and cold-temperature impact, good overall chemical resistance; can be adversely affected by ultraviolet.
;absolute accuracy error The deviation of the analog value at any code from its theoretical value after the full-scale range has been calibrated. Expressed in percent, ppm, or fractions of 1 LSB.
. absolute address An address that indicates the exact storage location at which the refer
enced operand is to be found or stored in the actual machine code address numbering system. It is synonymous with specific address and actual address and related to absolute code.
absolute alann An alarm caused when a system has detected a variable that has exceeded a set of prescribed high- or low-limit conditions.
absolute altimeter See terrain clearance indicator.
absolute altitude Distance between an aircraft or spacecraft and the actual surface of a planet or natural ~atellite.
absolute code Coding that uses machine instructions with absolute addresses. Synonymous with specific code.
absolute encoder An electronic or electromechanical device that produces a unique digital output (in coded form) for each value of an analog or digital input. In an absolute rotary encoder, for instance, you can determine directly the position following any incremental movement, without having to refer to the starting position.
absolute feedback In numerical control, the action of assigning a unique value to each possible position of a machine slide or actuating member.
absolute humidity The weight of water vapor in a gas-water vapor mixture per unit volume of space occupied, as, for example, grains or pounds per cubic foot.
absolute instrument An instrument that determines the value of a measured quantity in absolute units by making a simple physical measurement.
absolute measurement A measured value that is expressed in terms of fundamental standards of distance, mass, and time.
absolute pressure 1. The pressure measured relative to zero pressure (vacuum). 2. The combined local pressure that is induced by some source and the atmospheric pressure at the location of the measurement. 3. Gauge pressure plus barometric pressure expressed in the same units of measurement.
absolute programming In numerical control, the process of using a single point of reference to determine all positions and dimensions.
absolute stability A linear system is absolutely stable if there exists a limiting value of the open-loop gain such that the system is stable for all lower values of that gain and unstable for all higher values.
absolute value error The magnitude of the error disregarding the algebraic sign or, if a vectorial error, disregarding its direction.
3
I absolute viscosity / accelerometer
absolute viscosity A measure of the internal shear properties of fluids. It is expressed as the tangential force per unit area at either of two horizontal planes separated by one unit thickness of a given fluid, with one of the planes fixed and the other moving with unit velocity.
absorbance An optical property expressed as log (l/T), where T is the transmittance.
absorptance The fraction of the incident light absorbed by something.
absorption [Sci) 1. The reduction in intensity of a beam of electromagnetic or particulate rJdiation as it passes through matter. This reduction is chiefly due to interactions with atoms or electrons or with their electric and magnetic fields. [Comm] 2. In fiber-optic cable, the loss of power resulting from the conversion of optical energy into heat. This loss is usually caused by impurities such as transition metals and hydroxyl ions. .
absorption band A region of the electromagnetic spectrum in which a given substance exhibits a high absorption coefficient compared to adjacent regions of the spectrum.
absorption coefficient An inherent material property that is expressed as the fractional loss in radiation intensity per unit mass or per unit thickness. It is determined over an infinitesimal thickness of the given material at a fixed wavelength and bandwidth.
absorption curve A graph showing the variation of transmitted radiation through a fixed sample while the wavelength material of a given thickness is changed at a uniform rate.
absorption dynamometer A device for measuring mechanical force or power by converting the mechanical energy into heat in a friction mechanism or bank of electrical resistors.
absorption-emission pyrometer An instrument for determining gas temperature by measuring the radiation emitted by a calibrated reference source both before and after the radiation passes through the gas, where it is partly absorbed.
absorption hygrometer An instrument for determining the water vapor content of the atmosphere by measuring the amount of water absorbed by a hygroscopic chemical.
absorption meter An instrument for measuring the quantity of light transmitted through a transparent medium by means of a photocell or other light-detecting device.
absorption spectroscopy The study of the wavelengths of light that are absorbed by materials and the relative intensities at which different wavelengths are absorbed. This
technique can be used to identify materials and measure their optical densities.
AC Altemating current; electric current that reverses its direction at regularly recurring intervals, s~ch as 60 times/second (60 Hz).
accelerated hfe test A method for estimating the .reli~bil~ty or dura~ility of a product by subjectIng It to operatmg conditions above its maximum ratings.
accelerating agent 1. A ?ubstance that increaS~5.a chemical te,a.ction rate. 2. A chemical that h~stt~ns the curing of rubber, plastic, cement or adhesives, and may also improve their properties. Also known as accelerator.
accelerating electrode An auxiliary electrode in an electron tube that is maintained at an applied potential so as to accelerate electrons in a beam.
acceleration The time rate of change of velocity; the second derivative of a distance function with respect to time.
acceleration error The maximum difference at any measured value within the specified' range, between output readings taken with and without the application of specified constant acceleration along specified axes. Note: See transverse sensitivity when applied in the context of acceleration transducer.
acceleration limit The maximum vibration and shock acceleration that the transducer can accept in either direction along its sensitive axis without permanent damage. This limit is usually stated as ± -8's. The acceleration limits are usually much wider than the acceleration range and thereby represent a measure of the overload capability of the transducer. [ISA/RP-37.2 (R1995)]
acceleration range The range of accelerations over which the transducer has the specified linearity. [ISA/RP-37.2 (R1995)]
acceleration time [Camp] 1. The amount of time it takes a mechanical component of a computer to go from rest to running speed. [Sci] 2. The measurement of the time required for any object to reach a predetermined speed.
accelerators A feature of some computer applications to create keyboard commands that provide altematives to screen menus. for selecting choices. The keystroke has speCIal meaning within that particular application,
. " which means more involved, but usually more "user- friendly," procedures can be saved for making requests or entries.
accelerometer 1. An instrument for measuring acceleration or an accelerating force su.ch as gravity. If the instrument includes'p!oVlsions for making a recorded output, It 15
4
I acceptance angle / accumulator
called an "accelerograph," 2, A transducer used to measure linear or angular acceleration.
acceptance angle In fiber-optic cable, the half angle of cone within which incident light is totally reflected internally by the fiber core,
acceptance test A series of tests used to demonstrate the capabilities and workability of a new system and usually conducted by
: j the manufacturer to show the customer that , ~ the system is in working order.
~. access The state in which it is permissible to .;~ place information into, or retrieve informa\, tion, from a storage device. ,r access, direct storage (DSA) The procedure
j,
whereby data are transferred to or from storage that is essentially coincident with the normal operation of a computer, without disturbing the central processing unit registers. [ISA/RP-55.1-1975 (R1983)] , access, random 1. The process of obtaining
, data from, or placing data into, storage such that the time required for such access is independent of the location of the data most recently obtained or stored, 2, A type of storage device in which the access time is effec
;~ tively independent of the location of the ! c,
data. [ISA/RP-55.1-1975 (R1983)] access, serial The process of obtaining data
~
from, or placing data into, storage when ,! there is a sequential relation governing the t C time it takes to access successive storage I locations. [ISA/RP-55.1-1975 (R1983)] I, access code A group of alphanumeric characj, ters that identifies the user to the system so
information can be placed or retrieved byi' other devices in the system. ; J access line The portion of a leased communiJ cation line, such as in a telephone system,
that permanently connects the user with the
'I" serving central office or wire center.
~ access method Any of the data-management techniques available to the user for transfer
t ring data between main storage and an input/output device.", access privilege The right or permission to i.
~ access (read or write) a file that is granted by1 I the processor after such permission has been 1 requested.
access procedures The procedure by which~ the devices attached to the network gain~ access to the medium. The access procedure" typically includes provisions for guarantee
ing fairness in the sharing of the network bandwidth between attached devices. The most common access procedures for LANs are CSMA/CD, token bus, token ring, and slotted ring. See MAC.
access time 1. The interval between a request for stored information and the delivery of that information; often used to refer to the speed of memory. 2. The time interval that is characteristic of a storage unit. A measure of the time required to locate information in a storage position and make it available for processing or to return information from the processing unit to a storage location.
access unit interface (AUI) The optional interface between a data station that uses an IEEE 802.3 LAN and a transceiver or modem. The AUI makes it possible to transparently connect a data station to either baseband or broadband media.
accessible 1. A system feature that is viewable by and interactive with the operator, and allows the operator to perform user-permitted control actions, for example, make set point changes and auto-manual transfers or perform on-off action. 2. The capacity to be touched with a standard test finger or test pin, when used"as 'specified in Section 6.2 of ANSI!ISA-82.02.01-1999 (IEC 1010-1 Mod). [ANSI!ISA-82.02.01-1999 (IEC 1010-1 Mod)]
accessible area An area that is routinely or periodically entered by plant personnel in their performance of routine functions during normal plant operation and in accordance with applicable health physics procedures. [ISA-67.03-1982]
accessible isolation valve The isolation valve that is nearest the measured process on an instrument-sensing line, which is available to personnel during normal plant operation. The root valve mayor may not perform the function of the accessible isolation valve, depending on where it is located. [ANSI! ISA-67.02.01-1999]
accessible part A part that can be touched during normal use or operator servicing. [ANS/lSA-82.01.01-1999 (IEC 1010-1 Mod)
accessible surface A surface to which a flammable or combustible mixture has access. [ANSI/lSA-12.01.01-1999]
accessible terminal A node in an electronic network that is configured to allow it to be connected to an external circuit.
accessory A peripheral device that supports a main system function, such as a floppy disk drive or printer.
Accredited Standard Committee A standard committee accredited to ANSI.
accumulator 1. The register and associated equipment in the arithmetic unit of the computer in which arithmetical and logical operations are performed. 2. A unit in the digital computer where numbers are totaled, that is,
5
I accuracy / accuracy rating
accumulated. Often the accumulator stores one operand and when it receives any second operand it performs the indicated operations on them and then forms and stores the result. It is related to an adder. [Proc] 3. A pressure vessel containing water and steam that is used to store the heat of the steam for use at a later time and at some lower pressure: 4. A relatively large-volume chamber or other hydraulic device that receives fluid lmder low hydraulic power, stores it, and then discharges it at high hydraulic power,
, I
six-place numerals; nevertheless, a properly computed four-place numeral might be more accurate than an improperly computed six-place numeral. [ANSI/ISA-51.1-1979 (R1993)]
accuracy, mean (data processing) Mean accuracy is precisely defined as (100 - E) % FR., where the mean error E is expressed as a percentage of full range (FR.). It is common practice, however, to equate mean accuracy with the value of tl1e m.~an error. That is, mean accuracy is cornmonly stated as OJ %
~ after which it is ready to repeat the cycle. 5. A chamber or vessel for storing low-side liquid refrigerant in a refrigeration system. 6. An accumulator is also referred to as a receiver, a "reflux receiver", or a "reflux drum."
accuracy 1. The ratio of the error to the fullscale output or the ratio of the error to the
FR. whereas a more precise and acceptable statement is that mean accuracy is 99.9% FR. [ANSI/ISA-51.1-1979 (R1993)]
accuracy, measured The maximum positive and negative deviation observed when testing a device under specified conditions and by a specified procedure. Note 1: Measured accu
l
j!j
Jj
jIoutput, as specified, expressed as a percent racy is usually measured as an inaccuracy and ~
age. Note 1: Accuracy may be expressed in terms of units of measurand or as being within ±[number] percent of full-scale output. Note 2: The term accuracy should be used only for generalized descriptions of characteristics. It should not be used in specifications. The term error is preferred in specifications and other specific descriptions of transducer performance. [ISA-37.1-1975 (R1982)] 2. In process instrumentation, the degree of conformity of an indicated value to a recognized, accepted standard value or ideal value. [ANSI/ISA-51.1-1979 (R1983); AI\;SI/ISA-75.05.01-2000] 3. The degree to which an indicated value matches the actual value of a measured variable. 4. Quantitatively, the difference between the measured value and the most probable value for the
expressed a~ accu.racy. Note 2: Measured accuracy is fYPk4l11y expressed in terms of the measured vanable, percentage of span, percentage of upper-range value, percentage of scale length, or percentage of actual output reading. [ANSI/ISA-51.1-1979 (R1993)]
accuracy, reference See accuracy, rating. accuracy, total (data processing) Total accu
racy is precisely defined as (lao - Emax) % P.5. where the maximum error Emax is expressed as a percentage of full-scale value. Total accuracy is a measurement of the worst-case effect of all the errors present in the analog subsystem. [ISA/RP-55.1-1975 (R1983)]
accuracy measured test The procedure for this test is described in ISA Standard ANSI/ ISA-51.1-1979 (R1993).
II
same quantity, when the latter is determined from all available data and critically adjusted for sources of error. 5. The deviation or error by which an actual output varies from an expected ideal or absolute output. Each element in a measurement system contributes to errors, which should be separately specified if they contribute significantly to the degradation of total system accuracy. 6. In an analog-to-digital converter, accuracy is tied to resolution. A 13-bit A/D, as used in the controller, for example, can resolve to one part in 213 or 8,192, so the best accuracy as a percentage of full-scale range is theoretically 1/8192, or about 0.0125 percent.
accuracy (data processing) The degree of freedom from error, that is, the degree of con
accuracy rating In process instrumentation, a number or quantity that defines a limit that errors will not exceed when a device is used under specified operating conditions. Note 1: When operating conditions are not specified, reference operating conditions shall be assumed. Note 2: As a performance specification, accuracy (or reference accuracy) shall be assumed to mean the accuracy rating of the device when used at reference operating conditions. Note 3: Accuracy rating includes the combined effects of conformity, hysteresis, dead band, and repeatability errors. The units being used should be stated explicitly. It is preferable to use a ± sign before the number or quantity. Accuracy rating can be expressed in a number of forms. The follow
formity to truth or to a rule. Accuracy is contrasted with precision. For example, four-place numerals are less precise than
ing five examples are typical: (a) Accuracy rating can be expressed in terms of the measured variable. Typical expression: The accu
a
6
I
1
I AC/DC coupling I acoustic inertance
j
! . !
~ .
,
racy rating is ± (C, or ± iF. (b) Accuracy rating can be expressed in percentage of span. Typical expression: The accuracy rating is ±-0.5% of span. (This percentage is calculated using scale units such as degrees F, psig, etc.). (c) Accuracy rating can be expressed in percentage of the upper-range value. Typical expression: The accuracy rating is ± 0.5% of upper-range value. (This percentage is calculated using scale units such as kJ'a,.degrees F, etc.). (d) Accuracy rati,og is expressed in percentage of scale length. Typical expression: The accuracy rating is ± 0.5% of scale length. (e) Accuracy rating is expressed in percentage of actual output reading. Typical expression: The accuracy rating is ± 1% of actual output reading. [ANSI/ISA-51.1-1979 (R1993)]
AC/DC coupling A selection on the front panel of some readout instruments. The ACcoupling position switches a capacitor into the input conductor. This is used for piezoelectric accelerometers to remove the power supply bias voltage. The DC-coupling position removes this capacitor from the input line. This is used for transducers that have an output at zero frequency (such as piezoresistive accelerometers and proximity probes), and it is desired to make measurements below 1 Hz.
ACE Advanced Computing Environment initiative; alliance of more than twenty firms who support software standards for MIP architecture based on common ABI and API. Also known as asynchronous communications elements.
achromatic Optical elements that are designed to refract light of different wavelengths at the same angle. Typically, achromatic lenses are made of two or more components of different refractive index and are designed to be uses at visible wavelengths only.
acid cleaning The process of cleaning the interior surfaces of steam-generating units by using an inhibitor so as to prevent corrosion and then subsequently draining, washing, and neutralizing the acid with a further wash of alkaline water.
acid wash A chemical solution that contains phosphoric acid, which is used to neutralize residues from alkaline cleaners and to simultaneously produce a phosphate coating that protects a surface of metal from rusting and prepares it for painting.
acidity Represents the amount of free carbondioxide mineral acids and salts (espeCially sulphates of iron and aluminum) that
7
hydrolize to yield hydrogen ions in water. Acidity is reported as milliequivalents per liter of acid, ppm acidity is reported as calcium carbonate, and pH is reported as the measure of hydrogen ion concentration.
acid-resistant Able to withstand chemical attack by strongly acidic solutions.
AC input module An I/O module that converts process-switched AC into logic levels for use in the Pc.
ACK A. transmission.control character that is transmitted by a receiving device as an affirmative response to a sending device. Also see acknowledge.
acknowledge 1. The sequence action that indicates that a new alarm has been recognized. [ANSI/ISA-18.1-1079 (R1992)] 2. A message sent between peer entities to indicate that data was properly received.
Acme screw thread A type of power-transmission thread tha t is made in four series29° general purpose, 29° stub, 60° stub, and 10° modified square. The number of threads per inch is not standardized according to shank diameter.
acoustic Related to sound. acoustic compliance The reciprocal of acous
tic stiffness. acoustic coupler A type of communications
device that converts digital signals into audio tones that can be transmitted over the public telephone network through a conventional handset.
acoustic dispersion The separation of a complex sound wave into its various frequency components. Acoustic dispersion is usually caused by variation of the wave velocity in the medium that has the sound frequency. It is usually expressed in terms of the rate of change of velocity with frequency.
acoustic generator A transducer for converting electrical, mechanical, or some other form of energy into sound waves.
acoustic holography A technique for detecting flaws or regions of inhomogeneity in a part by subjecting it to ultrasonic energy. This produces an interference pattern on the free surface of water in an immersion tank. Laser holography can then read the interference pattern to produce an image of the test object.
acoustic impedance The complex quotient that is obtained by dividing the sound pressure on a surface by the flux through the surface.
acoustic inertance A property that is related to the kinetic energy of a sound medium, which equals Za/26j, where Za is the acoustic
_
acoustic interferometer I activatIon analY~l~
I
IfI'
j
I i'
reactance andf is sound frequency. The usual units of measure are g/cm4
. Also known as acoL/stic mass.
acoustic interferometer An instrument for measuring either the velocity or frequency of sound pressure in a standing wave that has been established in a liquid or gas medium between a sound source and a reflector as the reflector is moved or the frequency is varied.
acoustic radiometer An instrument that measures sound intensity by determining unidirectional steady-state pressure when thesound wave is reflected or absorbed at a boundary.
acoustic reactance The imaginary component of acoustic impedance.
acoustic resistance The real component of acoustic impedance.
acoustic sensitivity The output of a transducer (not caused by rigid body motion) in response to a specified acoustical environment. This sensitivity is sometimes expressed as the acceleration in g rms that is sufficient to produce the same output that is induced by a specified sound pressure-level spectrum with an overall value of 140 dB referred to as 0.0002 dyne per sq em rms. [ISA-RP37.2-1982 (R1995)]
acoustic signature In sonar applications, the profile that is characteristic of a particular undersea object (or class of objects), for example, the profile of a school of fish or a sea-bottom formation.
acoustic spectrometer An instrument for analyzing a complex sound wave by determining the volume (intensity) of sound-wave components that have different frequencies.
acoustic stiffness A property related to the potential energy of a medium or its boundaries that equals 2 <3 fla' where Z" is the acoustic reactance and f is sound frequency. The usual units of measure for acoustic stiffness are dyne/ems.
acoustical ohm The unit of measure for acoustic resistance, reactance, or impedance. It equals unity when a sound pressure of one microbar produces a volume velocity of one cubic centimeter per second.
acoustics 1. The technology associated with the production, transmission, and utilization of sound, and the science associated with sound and its effects. 2. The architectural quality of a room-especially a concert hall, theater, or auditorium-that influences the ability of a listener to hear sound clearly at any location.
acousto-optic An interaction between an acoustic wave and a lightwave passing through the same material. Acousto-optic devices can be used for the deflection or modulation of beams, for signal processing, and for Q switching.
acousto-optic glass Glass whose composition is designed to maximize the acousto-optic effect.
AC output module An I/O module that converts PC logic levels into output switch action for AC.load control.. ;" .
SIGNALFROM CPU ISOLATOR
AC OUTPU
acquisition time The interval in which a saml2kHmd;.hold circuit acquires an input signal to within specification.
acronym A word formed from the initial letters of a long or complex technical term. For example, ~M is the acronym for "random access memory."
acrylonitrile butadiene styrene (ABS) A plastic frequently used for enclosures because it provides good high-temperature resistance with high-impact strength and cold-temperature impact as well as good overall chemical resistance. ABS can be adversely affected by ultraviolet. See ABS.
ACSE Association control service element; for ISO layer 7/ for MAP 3.0.
ACT Acoustic charge transport; technology that processes signals in their natural analog domain, taking advantage of the 10,000-to-1 simplification that results when using analog rather than digital circuits. Converts analog input signal into discrete- time signal, sampled in timet not quantitized in amplitude.
actinicity The ability of radiation to induce chemical change.
actinometer 1. An instrument for measuring the actinic quality of radiation-that is, its relative ability to induce chemical change. 2. An instrument for measuring the flux density of solar radiation.
action, air-to-close See fail-open. action, air-to-open See fail-close. action object An object that supports the
instantiation and deletion of blocks and objects within a resource. [ISA-TR50.02, Part 9-2000]
activation analysis A method for determining composition, especially the concentration
8
----------_._--- ~- -- ....
I active alarm point / actuator, diaphragm type
.. i'
, I'
' .. j
FI> t. .~
of trace elements. The composite substance is bombarded with neutrons, and the wavelengths and intensities of the characteristic gamma rays emitted from the activated nuclides are measured.
active alarm point See alarm point. active application The software application
on a computer that the keyboard is now focused in a Windows environment. Usually, it is the "top" window that is "open."
active device Any component, device, or circuit that introduces gain or has functional direction. The active device is usually considered to be any device except pure capacitance, inductance, resistance, or combinations of these. In current loop applications, the active device is a device capable of supplying current for the loop.
active medium The material in a laser that produces the amplified stimulated emission. The name of the laser identifies the active medium.
active transducer A transducer whose output waves are produced by power derived from a source other than any of the actuating waves, but whose output power is controlled by the actuating waves.
active window The window that appears "on top" of other windows in a Windows computer environment, thereby overlapping and obscuring from view other windows (applications) that are also open. The active window is the window that is currently in use. See active application.
active-matrix liquid crystal displays (AMLCDs) A liquid crystal display (LCD) technique in which the pixels on the screen are controlled by voltage signals applied in rows and columns. An array of thin film transistors (TFIs), with one transistor per pixel, keeps the pixels energized at all times so there is no need to reenergize on each scan. Thus, the pixels respond faster and are brighter than passive-matrix liquid crystal displays (PMLCDs). Compare with passivematrix liquid crystal displays.
ActiveX Short for "active component extension." A binary reusable software object (COM component) that plugs into object linking and embedding (OLE) software, thereby allowing different software packages to communicate and interact with one another in a networked environment. This makes it possible to integrate plant floors using the Internet and intranets. Since the adv2nt of Internet, Microsoft has preferred the term ActiveX over OLE because of the
9
former's expanded scope (and marketing appeal).
activity [Comm]1. The presence of a signal or noise at the input terminals of a fieldbus device in which the signal or noise is higher than the receiver signal level threshold of that device. [ANSI/ISA-50.02, Part 2-1992] [Sci] 2. The ratio of the escaping tendency of the component in a solution to the escaping tendency of a standard state. The ion concentration multiplied by an activity coefficient is equal to the activity. .
activity-based costing An information system that maintains and reports data on the activities, products, and processes of a plant or company.
actual address See absolute address. actual flow The actual volume of liquid pass
ing through the flowmeter in a unit time. It is computed by applying all necessary corrections for the effects of temperature, pressure, air buoyancy, and the like to the corresponding readings indicated by the calibrator. [ISA-RP31.1-1977]
actuate To put into action or motion. actuating error signal See signal, actuating
error. actuation signal The set point minus the con
trolled variable at a given instant. Same as error.
actuator 1. A pneumatic, hydraulic, or electrically powered device that supplies force and motion so as to position a valve's closure member at or between the open or closed position. [ANSI/ISA-75.05.01-2000]2. A part of the final control element that translates the control signal into action by the final control device in the process. Typical examples are motors, solenoids, cylinders, and so on. 3. A device responsible for actuating a mechanical device such as a control valve. 4. A device that actuates.
actuator, bellows type A fluid-powered device in which the fluid acts upon a flexible convoluted member, known as the bellows, so as to provide rotary motion to the actuator stem. [ISA-75.01-1985 (R1995)]
actuator, diaphragm type A fluid-powered device in which the fluid acts upon a flexible member, termed the diaphragm, so as to provide linear motion to the actuator stem. [ANSI/ISA-75.05.01-2000]
actuator, double-acting I adaptive control, adaptive tuning
PRESSURE CONNECTION ,>-<-_~I::...----.... (VENT FOR REVERSE ACTION)
DIAPHRAGM PLATE
DIAPHRAGM
SEAL VENT (PRESSURE CONNECTION FOR REVERSE ACTION)
ACTUATOR SPRING
I. --' - ACTUATOR STEM
Actuator, Diaphragm Type
actuator, double-acting An actuator in which the power supply acts both to extend and retract the actuator stem. [ISA-75.01-1985 (R1995)]
actuator, electric type A device that converts electrical energy into motion. [ANSI/ISA75.05.01-2000]
actuator, electrohydraulic type A self-contained device that responds to an electrical signal by positioning an electrically operated hydraulic pilot valve so that it allows pre~surized hydraulic fluid to move an actuahng piston, bellows, diaphragm or fluid motor in order to position a valve stem. [ANSI/ISA75.05.01-2000]
actuator, electromechanical type A device that uses an electrically operated, motordriven gear train or screw to position the actuator stem. Such actuators may operate in response to either analog or digital electrical signals. The electromechanical actuator is also referred to as a "motor gear train actuator." [ANSI/ISA-75.05.01-2000]
actuator, fluid motor type A fluid-powered device that uses a rotary motor to move the actuator stem. [ANSI/ISA-75.05.01-2000]
actuator, hydraulic type A fluid-powered device that converts the energy of an incompressible fluid into motio~. .
actuator, piston type A flUid-powered deVice in which the fluid acts upon a movable cylindrical m:ember, termed a piston, so as to provide linear motion to the actuator stem. [ANSI/lSA-75.05.01-2000]
actuator, pneumatic A device that converts the energy of a compressible fluid, usually air, into motion.
actuator, single-acting An actuator in which the power supply acts in only one direction. In a spring and diaphragm actuator, for example, the spring acts in a direction opposite to the diaphragm thrust. Single-acting spring and diaphragm actuators may be fur
Il ther classified in terms of the direction of the
stem movement on the increasing fluid pressure: (a) air to extend the actuator stem and (b) air to retract the actuator stem.[ANSI/ ISA-75.05.01-2000]
actuator, vane type A fluid-powered device in which the fluid acts upon a movable pivoted member, known as the vane, so as to provide rotary motion to the actuator stem. [ANSIjISA-75.05.01-2000]
..actuator effective area The net area of a piston, bellpws, ,\~ne, or diaphragm that is acted on by fluid .pressure in order to generate actuator output force. [ANSI/ISA75.05.01-2000]
actuator environment 1. The temperature, pressure, humidity, radioactivity, and corrosiveness of the atmosphere surrOl.mding the actuator. 2. The mechanical and seismic vibration transmitted to the actuator through the piping or heat that is radiated toward the actuator from the valve body. [ANSI/ISA75.05.01-2000]
~ctuator stem or actuator shaft The part that connects the actuator to the valve stem or shaft and transmits motion (force) from the actuator to the valve. Rotary-motion valves have shafts,·while linear-motion valves generally have stems. [ANSI/ISA-75.05.01-2000]
actuator travel time See stroke time. ACU Automatic calling unit; Dialing device
that permits a business machine to automatically dial calls over a communications network (auto answer j dial modem).
Ada A Pascal-based, real-time systems programming language developed by CII-Honeywell Bull under a contract to develop weapons system tracking for the United States Department of Defense. It has since been used throughout the federal government for applications far beyond this initial purpose.
adapter Device that makes possible compatibility between different equipment.
adapting See self-adapting. adaptive control 1. A control system that
uses its previous experience to adjusts its response to its inputs. 2. Automatic means used to change the type or influence (or both) of control parameters in such a way as to improve the performance of the control system. [ANSI/lSA-51.1-1979 (R1993)]. See control, adaptive.
adaptive control, adaptive tuning 1. Continuously adjusting the gain (proportioning action) of the control loop from a signal external to that loop. Sometimes other parameters are (also) modified, particularly integral (reset action). 2. When referring to
10
adaptive dithering / adhesive bonding
advanced control techniques, the term has come to have the broader connotation of a system of advanced process control that is capable of automatically adjusting (adapting) itself to meet a desired output despite shifting control objectives and process conditions or unmodeled uncertainties in the process dynamics. Such control is often performed through neural networks and/or fuzzy logic coupled with traditional PIDtype algorithms.
adaptive dithering A form of dithering for displaying graphics on a system that has a different- sized palette. In this form of dithering, the program looks to the overall image on the system to determine the best set of colors or shapes.
adaptive optics Optical components that can be made to change the way in which they reflect or refract light. In practice, the term usually means mirrors with adjustable surface shapes.
adaptive system A system that displays the ability to learn, change state, or otherwise react to stimuli. It is capable of adapting itself to changes in its own environment.
adaptor bushing The part that attaches a close-coupled diaphragm actuator to the bonnet of the diaphragm valve body. [ANSI/ ISA-75.05.01-2000)
ADCCP Advanced Data Communications Control Procedure; bit-oriented communica
'/j tions protocol standard defined by ANSI. AID converter (ADO A hardware device
that converts analog data into digital form; also called an "encoder."
add time The time required for one addition, not including the time required to get and return the quantities from storage.
ADD 1. See false add and OR. 2. See sum. adder A device that forms, as output, the
sum of two or more numbers that are presented to it as inputs. Often such devices include no data-retention feature, that is, the output signal remains only as long as the input signals are present. Related to accumulator.
This capability is one of the powerful benefits of a good distributed control system.
address 1. An identification, represented by a name, label, or number, of a register or location in storage. Addresses also are part of an instruction word along with commands, tags, and other symbols. 2. The part of an instruction that specifies an operand for the instruction.
address bus The highway that links the subcomponents of the ~crocomputersystem along which addniss data are transferred.
address field That part of an instruction or word that contains an address or operand.
address format The arrangement of the address parts of an instruction.
addressing The means whereby the originator or control station selects the unit to which it is going to send a message.
addTessing mode Method for addressing a location that is used for data storage.
address modification The hardware action in which an instruction's effective operand address is computed by some sequence of the following two operations (as prescribed within the instruction): (a) indexing--adding an index to the address; (b) indirect addressing--using the intermediate computed address to obtain another address from memory.
address register A register in which an address is stored.
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address resolution The process of relating a logical address to a physical address.
add time The time required for one addition, not including the time required to get and return the quantities from storage.
adequately ventilated area An area that has a ventilation system (natural or artificial) that, at a minimum, prevents the accumulation of gases or vapors to an explosive level. Most standards and recommended practices recommend having systems that prevent levels in excess of 25 percent of the lower flammable (explosive) limit (LFL or LEL). Note: Adequate ventilation of an area is by itself not an effective means for preventing dust
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'; adder-subtrador A device whose output is a representation of either the arithmetic sum or difference, or both, of the quantities represented by its operand inputs.
additive primaries In color reproduction, red, green and blue. When lights of these colors are added together, they produce the sensation of white light.
add-Oil A component or device that is added to a system to increase storage capacity, modify architecture, or upgrade performance.
explosions. [ISA-12.01.01-1999) adhesion 1. A bonding between two surfaces
that is usually applied to localized welding at high points under substantial contact pressures. 2. Bonding between two surfaces, assisted by an adhesive substance.
adhesive Any substance capable of bonding two surfaces together.
adhesive bonding A commercial process for fastening parts together in an assembly by
11
I adhesive strength / aerosol
using only glue, cement, resin, or other adhesive.
adhesive strength The strength of an adhesively bonded joint, usually measured in tension (perpendicular to the plane of the bonded joint) or in shear (parallel to the plane of the joint).
adiabatic Taking place without any exchange of heat between the process system and another system or its surroundings.
adiabatic temperature The theoretical temperature that would be attained by the products of combustion provided that the entire chemical energy of the fuel, the sensible heat content of the fuel, and the combustion air above the datum temperature were transferred to the products of combustion. This assumes that (a) combustion is complete, (b) there is no heat loss, (c) there is no dissociation of the gaseous compounds formed, and (d) inert gases play no part in the reaction.
adjacent channel In FM/FM telemetry, the modulated signal bandwidth immediately below or above the channel of interest. See FM/FM.
adjacent equipment The auxiliary equipment that may be located adjacent to the valve or actuator. [ANSI/ISA-75.05.01-2000]
can function independently and simultaneously. The asymmetrical component means that the upload speeds are slower than the download speeds, but most delays occur during downloading.
adsorption The concentration of molecules of one or more specific elements or compounds at a phase boundary. The concentration usually occurs at a solid surface that bounds a liquid or gaseous"medium containing the
... specific element or compound. advanced process control (APC) Process con
trol strategies beyond straightforward PID loop control, which is usually defined as "classical" advanced control. These advanced strategies involve a combination of PID loops, dead-time compensators, lead/ lag feedforward function blocks, and singlevariable constraint controllers.
advisory alert An alert object that has an assigned priority value of less than 8. [ISATR50.02, Part 9-2000]
AE Application entity; active element within an ISO layer. Also known as: architectural engineer; application enabler.
A&E Architecture and Engineering; company that designs and builds the process plant. l
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adjustment The process of altering the value AEB Australian Electrotechnical Board; stanof some circuit element or some component dards association of Australia, a member of of the mechanism of an instrument, control lEe.
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ler, or auxiliary device so as to bring the indication to a desired value within a specified tolerance. This desired value usually corresponds to an independently determined value of the measured variable within a specified tolerance.
adjustment, span A means provided in an instrument to change the slope of the inputoutput curve. See span shift. [ANSI/ISA-51.11979 (R1993)]
adjustment, zero A means provided in an instrument to produce a parallel shift of the input-output curve. See zero shift. [ANSI/ ISA-51.1-1979 (R1993)]
ADO Ampex Digital Optical; in video development, a traditional video editing system component that allows video frames to appear to be in motion.
ADPCM Adaptive differential pulse code modulation; encoding format for compressing and storing audio information in digital format.
ADSL Asymmetric digital subscriber line; a technique that compresses the amount of data needed to send computer, voice, and video data over normal phone lines (a compared to coaxial cable) so those three services
iAECMA Association Europeenne des Constructeurs de Materiel Aerospatial (also known as the European Association of Aerospace Industries). European standards group for aerospace industry.
AENOR Asociaci6n Espanola de Normalizaci6n y Certificaci6n; standards group in Spain.
aeolight A type of glow lamp whose inten
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1sity of light output varies with a signal voltage applied to it. Its construction employs a cold cathode and an envelope filled with a mixture of gases.
aerator Any device for injecting air into a material or process stream.
aerodynamics A branch of mechanics that deals with the motion of gases, such as air, and with the forces acting on solids in relative motion with respect to a gas.
aerograph Any self-recording instrument carried aloft to take meteorological data.
aerometer An instrument for determining the density of air or other gases.
aerosol A dispersion of fine liquid or solid particles in a gas. For instance, both smoke and fog are aerosols.
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I Aex / air bind
Aex A required prefix for apparatus that meet one or more types of protection as
i defined in the National Electric Code: Sec. i
\. tion 505-2(c) for installation in accordance '. with Article 505 of the National Electrical
Code, NFPA 70: 1999. [ANSI/ISA-12.01.011999]
AFNOR Association Fran<;aise de Normalisation; standards group itt france.
afterglow Luminosity that persists in a gas after an electrical discharge passes through it. The'phenomenon is sometimes utilized to measure flow.
AGA Advanced Graphics Architecture; chip set for driving high-resolution multimedia tools. Also known as: American Gas Association, a national trade association founded in 1918 in the United States, composed of about three hundred gas distribution and transmission companies to act as clearinghouse on gas energy information, and act as a catalyst in technical and energy policy matters, and as a voice for gas industry.
age hardening Raising the strength and hardness of an alloy by heating a supersaturated solid solution at a relatively low temperature so as to induce precipitation of a finely dispersed second phase. Also known as aging and precipitation hardening.
agent 1. A system that acts on behalf of another system. 2. A computing program
. ~- that performs some information-gathering or processing task in the background, usually some very small, well-defined task. Some believe that the human mind consists of thousands or millions of agents working in parallel, so "true" artificial intelligence (AI) machines should also contain many agents along with some system for arbitrating among the competing results of these agents' activity.
I agglomeration Any process for converting a
mass of relatively fine solid material into a mass of larger lumps.
aggregate 1. Natural sand, gravel, and crushed stone that is mixed with cement toJ make mortar or concrete. 2. A structured collection of data elements. [ISA-TR50.02, Part l. . 9-2000]
! ~ aging 1. The alteration of the characteristics
of a device because of use. 2. Operating a product before shipping it so as to stabilize component functions or detect early failures. 3. Any time-dependent change in the properties of a material, but especially age hardening at room or slightly elevated temperatures. 4. Curing or stabilizing parts or materials
through long-term storage outdoors or under closely controlled storage conditions.
agitator A device for mixing, stirring, or shaking liquids or liquid-solid mixtures so as to keep them in motion.
AGP Accelerated graphics port; provides the capability for three-dimensional and fullmotion video graphics in workstations.
AGVS Automated Guided Vehicle System; controls vehicles that proceed along predeterminedro~tesar guide-paths, performing scheduled material-handling tasks without operators.
AHP Analytical Hierarchy Process; approach to using multiple criteria in decisions that aids in the meaningful organization of information, provides the ability to easily change weights and ratings for "what-if" analysis, and verifies that the approach to evaluation is logical and consistent.
AlA Application integration architecture; method of Digital Equipment Corporation for allowing programs to be portable between VMS and ULTRIX.
AIFF Audio Interchange File Format; method for saving digital audio in electronic memory, used for exchanging data between computers. Developed by Apple and used in Macintosh computers with a compression standard called Macintosh Audio Compression/Expansion (MACE).
air 1. The mixture of oxygen, nitrogen and other gases that, with varying amounts of water vapor, forms the atmosphere of the earth. [ISA-77.41-1992] 2. Air implies the use of any suitable and normally clean, dry, safe gas. [ISA-RP60.9-1981]
air, dry 1. Air with which no water vapor is mixed. This term is used comparatively since in nature air always has some water vapor, which, being a gas, is dry. 2. A papermaking term used to describe dry pulp that contains about 10 percent moisture.
air, saturated Air that contains the maximum amount of the vapor of water or other compound that it can hold at its temperature and pressure.
air-atomizing oil burner A burner for firing oil in which the oil is atomized bv compressed air, which is forced into ~nd through one or more streams of oil. This action breaks the oil into a fine spray.
air bearing A device that lubricates motion with flowing air. A linear air bearing floats a table on air as it travels a straight line.
air bind An air pocket in a pump, conduit, or piping system that prevents liquid from flowing past it. Also known as liquid trap.
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air binding / air cushion
l air binding The inclusion of air in a space, placement of air from the measuring vessel
hindering the flow of some other gas or liq during the calibration run. The standard air uid. (50% R.H.) for correcting to weights in vac
air blast The flow of air at a high velocity, usually for a short period.
airborne Carried in the atmosphere either by being transported in an aircraft or by being dispersed in the atmosphere.
air-bubbler liquid-Ievel.:detector A device for indirectly measuring the level of liquid in a vessel-especially a corrosive liquid, viscous liquid, or liquid containing suspended solids. This detector consists of a standpipe, open at the bottom and closed at the top, that is connected to an air supply whose pressure is maintained slightly above the maximum head of the liquid in the vessel. Air bubbles out of the bottom of the pipe, ensuring that the internal pressure is equal to the head of the liquid in the vessel (this pressure is measured by a simple gauge or transducer). Also known as purge-type liquid-level detector.
air-bubbler specific-gravity meter Any of several devices that measure specific gravity by determining the differential pressure between two air-purged bubbler columns. The devices ordinarily use either of two principles for determining specific gravity-comparing sample density with the density of a known liquid, or comparing the pressure between two bubbler columns immersed at different depths in the process liquid.
i.eN Rf/';"Ul()i
(T'P)
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uum has a density of 1.217 kg/m3 at 288.7 K and 1.013 250E + 05 Pa. When weighings are made against weights, the buoyancy force on these weights must also be considered. For brass weights, the net effect of air buoyancy in air at standard conditions is about 0.015 percent. [ISA-RP31.1-1977]
air compressor A machine that raises the pressur~ of air a~ve atmospheric pressure and normally delivers it to an accumulator or distribution system.
air condenser 1. A heat exchanger for converting steam into water in which the heat-transfer fluid is air. Also known as air-cooled condenser. 2. A device for removing oil or water vapors from a compressed-air line.
air-conditioned area See area, air conditioned. air conditioning Controlling the atmospheric
environment in a confined space by measuring and continually adjusting factors such as temperature, humidity, air motion, and concentrations of dust, gases, odors, pollen, or microorganisms.
air consumption The maximum rate at which air is consumed by a device within its operating range during steady-state signal conditions. Note: Air consumption is usually expressed in cubic feet per minute (f~ /min) or cubic meters per hour (m3 /h) at a standard (or normal) specified temperature and pressure. [ANSl/lSA-51.1-1979 (R1993)]
air-cooled engine An engine, such as an internal combustion engine, whose waste heat is removed directly by a flowing stream of air-either a stream blown across the engine's external surfaces or one blown through internal cooling passages.
air-cooled heat exchanger A device for removing heat from a process fluid by passing it through a bank of finned tubes that are cooled by blowing or drawing a stream of air across the tube exteriors. [ANSl/lSA-51.11979 (R1993)]
air curtain A stream of high-velocity, conditioned air that is directed downward across an opening such as a door or window so as to exclude insects and exterior drafts, pre
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vent heat transfer through the opening, andI....J_
space. air buoyancy The lifting effect or buoyancy air cushion 1. A mechanical device that uses
"- make it possible to air condition the interior
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of the ambient air that acts during a weigh trapped air to absorb shocks or arrest motioning procedure involving open gravimetric without shock. 2. The partly confined stream calibrations. This effect is caused by the dis- of low-pressure, low-velocity air that sup
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I air cylinder I air resistance
ports a vehicle known as an air-cushion vehicle, ground-effect machine, or hovercraft. This air allows the vehicle to travel equally well over water, ice, marshland, or relatively level ground.
" air cylinder A cylindrical body for storing compressed air, for compressing air with a piston, or for driving a piston with compressed air,
air deficiency Insufficient air, in an air-fuel mixture, to supply the oxygen that is theoretically required to completely oxidize the fuel.
air ejector A device for removing air or noncondensable gases from a confined space, such as the shell of a steam condenser, through the eduction created by a fluid jet.
air entrainment The artificial infusion of a semisolid mass such as concrete or a dense
r-'··
slurry with minute bubbles of air, especially by mechanical agitation.
air filter A device for removing solid particles such as dust or pollen from a stream of air, especially by causing the airstream to pass through layered, porous material such as cloth, paper, or screening.
airfoil-vane fan A device for creating a stream of moving air by drawing it into a fan casing near the hub and propelling it centrifugally with a rotor whose vanes are curved backward from the direction of rotation.
air furnace Any furnace whose combustion air is supplied by natural draft or whose internal atmosphere is predominantly heated air.
air gap The space between two ferromagnetic elements of a magnetic circuit.
air gauge 1. A device for measuring air pressure. 2, A device for precisely measuring physical dimensions by measuring the pressure or flow of air from a nozzle against a workpiece surface and then relating the measurement to the distance between the nozzle and the workpiece,
air hoist A lifting or hauling tackle whose power is provided by air-driven pistons (for reciprocating motion) or by air motors (for rotary motion).
air infiltration The leakage of air into a setting or duct.
,., air knife A device that uses a thin, flat J'et off air to remove excess coating material from sheet stock such as paper.
air lance A device for directing a high-velocity stream of pressurized air into a process vessel or against a surface such as a boiler wall to remove unwanted deposits.
air lock 1. An intermediate chamber between an environmentally controlled confined
space and the outside atmosphere that allows personnel and materials to enter by sealing a door between the chamber and the confined space. After the door is opened to the outside to admit personnel or materials, it is closed and sealed. The environmental conditions in the chamber are changed to match those in the confined space, then an interior door is opened to permit entry into the confined space. The process is reversed when exiting the confined space. 2. See air bind. ',' .'
air meter A device for measuring the flow of air or other gas and expressing it as weight or volume per unit time.
air moisture Water vapor suspended in air. air monitor A warning device that detects
airborne radioactivity or chemical contamination and sounds an alarm when the radiation, gas, or vapor level exceeds a preset value.
air motor An engine that produces rotary motion by using compressed air or other gas as the working fluid.
air nozzle An air port that has direction and appreciable length so as to direct an air stream.
air permeability A method of measuring the fineness of powdered materials, such as portland cement, by determining the ease with which air passes through a defined mass or volume.
air port An opening through which air passes.
air preheater A heat exchanger for transferring some of the waste heat in flue gases from a boiler or furnace to incoming air, thus increasing the efficiency of combustion.
air purge 1. A flow of air through a furnace, boiler gas passages, and associated flues and ducts that effectively removes any gaseous combustibles and replaces them with air. Purging may also be accomplished by an inert medium. [ISA-77.41-1992] 2. The removal of undesired matter by replacing it with air.
air purging Removing airborne contaminants, gases, or odors from a confined space by introducing fresh, clean air.
air regulator A device for controlling airflow-for example, a damper to control the flow of air through a furnace or a register to control the flow of heated air into a room.
air reheater A device in a forced-air heating system that adds heat to air circulating in the system.
air resistance The opposition offered to the passage of air through any flow path.
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air ride / alarm-only instrument
air ride An air-suspension system installed on some moving-van trailers to provide shock and vibration control for moving delicate equipment. [ISA-RP60.11-1991]
air separator A device for separating materi als of different density, or particles of different sizes, by means of a flowing current of air.
air set A device that is used to control the supply air pressure to the valve actuator and its auxiliaries. [ANSI!ISA-75.05.01-2000]
airspeed The speed of an airborne object with respect to the surrounding air mass. In calm air, airspeed is equal to ground speed. True airspeed is a calibrated airspeed that has been corrected for pressure and temperature effects caused by altitude and for compressibility effects at high airspeeds.
air spring A device commonly used in place of a mechanical spring in heavy vehicles to support the vehicle's body on its running gear. The energy-storage element is an air-filled container with an internal elastomeric bellows or diaphragm.
air supply (AS) 1. The supply of air used in pneumatic instrumentation as a power supply. 2. Plant air supply (PA). 3. Instrument air (IA). 4. The energy supply for pneumatic instrumentation.
air thermometer A device for measuring temperature in a confined space by detecting variations in the pressure or volume of air in a bulb placed inside the space.
airtight Sealed to prevent the passage of air or other gas; impervious to leakage of gases across a boundary.
air vent A valve opening in the top of the highest drum of a boiler or pressure vessel
18.1-1979 (R1992)] See annunciator. 5. An audible, visual, or physical presentation that is designed to alert the instrument user that a specific level of gas concentration has been reached or exceeded. [ISA-92.02, 03, 04 and 06.01-1998] 6. An instrument, such as a bell, light, printer, or buzzer, that indicates when the value of a variable is out of limits. 7. A piece of equipment that generates a visual or audible signal that attracts attention. [ISARP12.4-1.99..6] 8. A warning signal presented whenever a criticar deviation state frofu. normal conditions occurs in a process. Technically, alarm is a condition (not an event). An event is when the alarm condition begins; another event is when that co;n.dition ends. 9. The detection of a block leaving a particular state as well as its returns back to that state. [ISA-TR50.02, Part 9- 2000]
alarm, maintained An alarm that returns to normal after p.eing acknowledged. [ANSI! ISA-18.1-1979 (R1992)]
alarm, momentary An alarm that returns to normal before being acknowledged. [ANSI! ISA-18.1-1979 (R1992)] An indication that is .used to alert an operator about an abnormal operating condition. [ANSI!ISA-77.13.011999]
alarm extensions, electrically operated Usually a highly sensitive induction-type device for signaling high or low flows or deviations from any set flow. The device consists of a sensing coil positioned around the extension tube of the rotameter. When the metering float moves into the field of the coil it causes a low-level signal change that is usually amplified to a level suitable for performing annunciator or control func
for the purpose of venting air. tions. [ISA-RP16.4-1960] air vessel An enclosed chamber, partly filled alarm extensions, magnetically actuated A
1 with pressurized air, that is connected to a piping system to counteract water hammer or promote the uniform flow of liquid. [ISARP60.9-1981]
airy disk The central bright spot produced by a theoretically perfect circular lens or mirror. The spot is surrounded by a series of dark and light rings, which are produced by diffraction effects.
alarm 1. A device or function tha t signals the existence of an abnormal condition by making an audible or visible discrete change, or both, so as to attract attention to that condition. [ANSI!ISA-5.1-1979] 2. An abnormal process condition. 3. The sequence state when an abnormal process condition occurs. 4. A device that calls attention to the existence of an abnormal process condition. [ANSI!ISA
device attached to the meter body that contains an electrical switch and is magnetically actuated by the metering float extension so as to signal a high or low flow. The switch is adjustable with respect to the float position over a range that is equal to the travel of the metering float. Standard switch ratings are usually 0.3 amperes for a nO-volt, 60-cycle AC supply (five amperes or more if relays are used). [ISA-RP16.4-1960]
alarm module (point or sequence module) A plug-in assembly that contains the sequence logic circuit. Some alarm modules also contain visual display lamps or lamps and windows. [ANSI/ISA-18.1-1979]
alarm-only instrument An instrument that provides alarm(s) but does not have an inte
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I alarm point I alignment
gral display device to indicate gas concentration levels. [ISA-92.02, 03, 04 and 06.01-1998]
1. alarm point The sequence logic circuit, visual
display, auxiliary devices, and internal wir
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ing that are related to one visual display. [ANSI/ISA-18.1-1979]
alarm point, active An alarm point that is wired internally and completely equipped. The window is labeled to identify a specific monitored variable. [ANSI/ISA-18.l-1979) .
alarm point, future (blank) An alarm point that is wired internally and equipped except for the plug-in alarm module. The window is not labeled to identify a monitored variable. [ANSI/ISA-18.1-1979]
alarm point, spare An alarm point that is wired internally and completely equipped. The window is not labeled to identify a monitored variable. [ANSI/ISA-18.1-1979]
alarm set point 1. The selected input value at which an indication, alarm, or other output function is initiated. The value of the process variable and the corresponding value of the transmitted signal at which an alarm is initiated. Example: An electronic pressure transmitter is calibrated 0-200 psi. A highpressure alarm needs to be sounded if the process reaches 175 psi. The transmitter output is a 4-2 mA signal. The alarm set point would be 175 psi with an increasing pressure. To calibrate a switch activated by the mA signal, the following formula is used:
(PValarmsetpointJ.(TransmitterlRVl) 16 Jl 40 I ."----'-'-_-'-.CC.-"'-'-'----'--_C--.:. X mA + . = a arm scrpoJOl[( Transm itta span
The alarm set point would be 175 psi, which is represented by an 18 mA increasing signal. 2. The selected gas concentration level(s) at which an indication, alarm, or other output function is initiated. [ANSI/ISA-12.13.012000] or activated [ISA-92.06.01-1998]
alarm severity A selection of levels of priority for the alarming of each input, output, or rate of change.
alarm system An integrated combination of detecting instruments and visible or audible warning devices that actuates when an environmental condition or process variable exceeds some predetermined value.
, alarm valve A device that detects water flow and sounds an alarm when an automatic sprinkler system is activated.
ALARP As low as reasonably practical; acceptable control system failure designation based upon IEC 1508 specification.
ALE Application link enabling; allows a message exchange between different applications within or between computer systems.
alert Alarms and events. [ISA-TR50.02, Part 9-2000] See process condition; see also sequence state.
alert box In data processing, a window that appears on a computer screen to alert the user of an error condition.
alg~braic addero .. An ~ctronic or mechanical device that can ~titomaticallyfind the algebraic sum of two quantities. .
algorithm 1. A finite set of well-defined rules for solving a problem in a finite number of steps. [ISA-TR50.02, Part 9-2000] 2. Detailed procedures for giving instructions to a computer. 3. Contrast with heuristic and stochastic. 4. A recursive computational procedure. 5. A step-by-step procedu,re for solving a problem or accomplishing an end. 6. Sometimes used to refer to a firmware or a software program.
algorithmic language A language designed for expressing algorithms.
algorithmic-oriented language (ALGOL) Computer language designed by a committee of the Association for Computing Machinery and European computer industry representatives. Useful for mathematical problem-solving and the first block-oriented computer language.
alias 1. When varying signals are sampled at equally spaced intervals, two frequencies are considered to be aliases of one another if they canr..ot be distinguished from each other by an analysiS of their equally spaced values. 2. False signals in the frequency domain caused by an excessive sampling interval for digi tizing.
aliasing 1. A peculiar problem in data sampling, where data are not sampled enough times per cycle, and the sampled data cannot be reconstructed. 2. In digital bitmapped graphics, a jagged boundary along the edges of shapes and different-colored shapes within an image.
aliasing error An inherent error in time-shared telemetry systems where improper filtering is employed prior to sampling.
alidade 1. An instrument used in the plane-table method of topographic surveying and mapping. 2. Any sighting device for making angular measurements.
alignment The condition of proper orientation of machine-drive components to minimize vibratory forces unnecessary to power transmission.
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alkaline cleaner / alternate code complement
alkaline cleaner An alkali-based aqueous solution for removing soil from metal surfaces.
alkalinity The amount of carbonates, bicarbonates, hydroxides, and silicates or phosphates in the water. Reported as grains per gallon, or ppm, as calcium carbonate.
Allen screw A screw or bolt that has a hexagonal socket in its head and is turned by inserting a straight or bent hexagonal rod into the socket.
alligatoring 1. Cracking in a film of paint or varnish that is characterized by broad, deep cracks extending through one or more coats. Also known as "crocodiling." 2. The roughening of the surface of very coarse-grained sheet metal during forming. 3. The longitudinal splitting of flat slabs in a plane parallel to the rolled surface that occurs during hot rolling. Also called "fishmouthing."
allobar A form of an element in which the distribution of isotopes is different from the distribution in the naturally occurring form. Thus, an allobar has a different apparent atomic weight than the naturally occurring form of the element.
allocation A form of coordination control that assigns a resource to a batch or unit. Note: An allocation can be for the entire resource or for portions of a resource. [ANSI/ISA-88.01-1995]
allowable response time The limiting response time established in the safety analysis and documented in the plant's technical specifications. [ISA-67.06-1984]
allowable value A limiting value that the trip set point may have when it is periodically tested, beyond which appropriate action shall be taken. [ANSI/ISA-67.04.01-2000]
allowable working pressure See design pressure.
allowance The specified difference in limiting sizes-either minimum clearance or maximum interference between mating parts-that is computed mathematically from the specified dimensions and tolerances of both parts.
alloy A solid material that has metallic properties and is composed of two or more chemical elements.
alloy steel An alloy of iron and carbon that also contains one or more additional elements intentionally added to increase hardenability or to enhance other properties.
all-pass network A network that is designed to introduce phase shift or delay into an electronic signal without appreciably reducing amplitude at any frequency.
Alnico Any of a series of commercial iron-base permanent magnet alloys that contain varying amounts of aluminum, nickel, and cobalt as the chief alloying elements. The Alnicos are characterized by their ability to produce a strong magnetic field for a relatively small magnet mass and to retain their magnetism, with relatively insignificant loss in field strength when the magnetizing field is removed:
alphabet . The specific character set used by a computer. .
alphabetic word 1. A word consisting solely of letters. 2. A word consisting of characters from the same alphabet.
alpha counter 1. A system for detecting and counting energetic alpha particles. It consists of an alpha counter tube, amplifier, pulseheight discriminator, scaler, and recording or indicating mechanism. 2. An alpha counter tube and necessary auxiliary circuits alone. 3. A term sometimes loosely used to describe just the alpha counter tube or chamber itself.
alpha emitter A radionuclide that disintegrates by emitting an alpha particle from its nucleus.
alphanumeric 1. Pertaining to a character set that contains both letters and numerals, and usually by other characters. Synonymous with "alphameric." [ISA-RP55.1-1975 (R1983)] 2. The sequencing or ordering of a list using both initial letters and numbers.
alpha particle A positively charged, energetic atomic particle consisting of two protons and two neutrons that is identical in all measured properties with the nucleus of a helium atom. It may be produced by radioactive decay of certain nuclides or by stripping a helium atom of its electrons.
alpha-ray spectrometer An instrument used to determine the energy distribution in a beam of alpha particles.
alpha test Trying out a new product at a vendor's own company before subjecting it to a beta test. Software undergoes alpha testing as a first step in gathering user feedback.
altazimuth A sighting instrument that has both horizontal and vertical graduated circles so that both azimuth and declination can be determined from a single reading. Also known as astronomical theodolite and universal instrument.
alteration switch A manual switch on the computer console or a program-simulated switch that can be set on or off to control coded machine instructions.
alternate code complement In a frame synchronization scheme, a frame synchroniza-
I :. I, 18
i
I alternate immersion test I ambient temperature
.' i
tion pattern is complemented on alternate frames to give better synchronization.
alternate immersion test A type of accelerated corrosion test in which a test specimen is repeatedly immersed in a corrosive medium, then withdrawn and allowed to drain and dry.
alternating-current bridge A bridge circuit that utilizes an AC signal source and AC null detector. Generally, both in-phase (resistive) and quadrature (reactive) balance conditions must be established to balance the bridge. Some bridges require only one balance (resistive or reactive) and use a phase-sensitive detector.
alternating current plasma display panel (ACPDP) A type of liquid crystal display (LCD) that relies upon the emission of photons from gas that has been ionized by electric charge. Electrodes that are covered by insulation layers to protect them from working gas and therefore assuring longer life. Compare with direct current plasma display panel (DCPDP).
alternating-current thin-film electroluminescent (ACTFEL) A type of liquid crystal dis
; play (LCD) flat-panel display device. See electroluminescent displays.
,altigraph A recording pressure altimeter. altimeter An instrument for determining the
height of an object above a fixed level or reference plane sea level, for example; the aneroid altimeter and the radio altimeter are the most common types.
altitude 1. The vertical distance above a stated reference level. Unless otherwise specified, this reference is mean sea level. [ISA37.1-1975 (R1982)] 2. Height above a specified reference plane, such as average sea level. Usually given as a distance measurement in feet or meters regardless of the method of measurement.
altitude signals Reflected radio signals returned to an airborne electronic device from the land or sea surface directly underneath the vehicle.
ALU Arithmetic logic unit, which is the portion of the central processing unit (CPU) that performs arithmetic and logic functions (rather than memory organization and data transfer functions).
alum A general name for a class of double sulfates that contain aluminum and another cation such as potassium, ammonium, or iron.
Alumel Aluminum nickel alloy lIsed in the negative leg of a type K thermocouple. Trade name of Hoskins Manufacturing Company.
19
alumina The oxide of aluminum-AI20 3•
aluminizing 1. Applying a thin film of aluminum to a material such as glass. 2. Forming a protective coating on metal by depositing aluminum on the surface or a reacting surface material with an aluminum compound. The aluminum is then diffused into the surface layer at elevated temperature.
aluminum A soft, white metal that in pure form exhibits excellent. electrical conductivity and oxidation resistan,:=e. It is the base metal for'an extensive series of lightweight structural alloys used in such diverse applications as aircraft frames.and skin panels, automotive body panels and trim, lawn furniture, ladders, and domestic cookware.
ambient A surrounding or prevailing condition, especially one that is not affected by a body or process contained in it.
ambient air 1. Air to which the sensing element is normally exposed. [ISA-92.02, 03, 04 and 06.01-1998] 2. The air that surrounds the equipment. The standard ambient air for performance calculations is air at 80
o
p, 60 percent relative humidity, and a barometric pressure of 29.921 in. Hg. This gives a specific humidity of 0.013 lb of water vapor per lb of air.
ambient compensation The design of equipment or a measuring instrument such that changes in ambient conditions do not affect the performance of that equipment or the readings of that instrument.
ambient conditions 1. The conditions (pressure, temperature, etc.,) of the medium that surrounds the case of the transducer. [ISA37.1-1975 (R1982)] 2. The environment of an enclosure (room, cabinet, etc.) surrounding a given device or equipment.
ambient pressure See pressure, ambient. ambient pressure effects The change in sen
sitivity and the change in zero-measurand output caused by subjecting the transducer to a specified ambient pressure change. [ISA37.8-1982 (R1995)]
ambient pressure error The maximum change in output at any measured value within the specified range when the ambient pressure is changed between speCified values. [ISA-37.1-1075 (1982)]
ambient temperature 1. The temperature of the atmosphere encompassing the entire area of the instrument air system installation, including the compressor, piping, dryer, and the instruments themselves. 2. The temperature of the medium surrounding a device. [ANSI/ISA-7.0.01-1996]
II American standard pipe thread I amplitude linearity, shock
American standard pipe thread A series of specified sizes for tapered, straight, and dryseal pipe threads established as a standard in the United States. Also known as Briggs pipe thread.
American standard screw thread A series of specified sizes for threaded fasteners, such as bolts, nuts, and machine screws, established as a standard in the United States.
AM/FM Automated mapping/facility management; electronic mapping, a branch of the Geographic Information System (GIS). See GiS.
Amici prism Also known as a "roof prism." A right-angle prism in which the hypotenuse has been replaced by a roof, where two flat faces meet at a 90° angle. The prism performs image erection while deflecting the light by 90°. This is the same as rotating the image by 180°'Vreversing it left to right and at the same time inverting it top to bottom.
ammeter An instrument for determining the magnitude of an electric current.
sometimes referred to as Q. [ISA-RP37.2-1982 (R1995)]
amplifier 1. In process instrumentation, a device that enables an input signal to control power from a source independent of the signal. This makes the device capable of delivering an output that bears some relationship to, and is generally greater than, the input signal. [AN5.I/ISA51.1-1979 (R1993)] 2. Any device that can increase the magnitude of a
j
IIIjiiI, l
chemical deposition or by condensation. The method employed is dictated by the material's composition and ultimate use.
amp or ampere 1. A unit of measure used to define the rate of flow of electricity (current) in a circuit; the electric current produced by one volt acting through a resistance of one ohm. 2. The current that will deposit silver at the rate of 0.001118 grams per second, with the current flowing at 1 coulomb (6.25 x 108
electrons) per second. ampere-hour A quantity of electricity equal
to the amount of electrical energy passing a given point when a current of one ampere flows for one hour.
ampere-hour meter An integrating meter
amplitude of a second signal supplied from another source. 2. The ratio of the output-signal amplitude from an amplifier circuit to the input-signal amplitude from the control network, both expressed in the same units.
amplification factor The j.1 factor for the plate and control electrodes of an electron tube when the plate current is held constant.
amplification factor at resonant frequency The ratio of the maximum sensitivity'Of a transducer (at its resonant frequency) to' its ref~.rence sensitivity. The 1amplification factor at resonant frequency is 1
amorphous film A film of material deposited physical quantity, such as mechanical force on a substrate for corrosion protection, or or electric current, without significantly dis 1 insulation, for its conductive properties, or torting the wave shape of any variation with for a variety of other purposes. It is noncrys time associated with the quantity. 3. A com Italline and can be depOSited by evaporation ponent used in electronic equipment to raise
the level of an input signal so that the corresponding output signal has sufficient power to drive an output device such as a recorder orloudspeake~ .
amplifier (laser) A laser amplifier is a device that amplifies the light produced by an externallaser but lacks the mirrors needed to sustain oscillation and independently produce a laser beam.
amplifying Having an integral output amplifier. [ISA-S37.1-1975 (R1982)]
amplitude 1. A measure of the departure of a phenomenon from any given reference. 2. The maximum departure measured from the mean position of a wave form. 3. The difference between the average value of a sinu
j II 1
II!
IIi
i that measures the electric current flowing in soidal variation and the maximum (or a circuit and indicates the integral of current minimum) value. 4. The maximum deparwith respect to time. ture of motion measured from the mean
j ar I
ampere per meter The SI unit of magnetic position to an extreme. The units of amplifield strength; it equals the field strength tude are peak, peak-to-peak, and root mean
lao developed in the interior of an elongated, square. uniformly wound coil excited with a linear amplitude distortion A condition in an current density in the winding of one ampere amplifier or other device when the ampliper meter of axial distance. tude of the output signal is not an exact lin
(IHjA;\'
ampersand (&) In the typographical composi ear function of the input (control) signal. ation of screen displays and printing, the sym amplitude-frequency response See frequency bol for the Latin word et, meaning "and." response.
amplification 1. Increasing the amplitude of amplitude linearity, shock The closeness of
p ir
a signal by using a signal input to control the sensitivity to reference sensitivity over a AJ~
c(
20
amplitude linearity, vibration l analog electronic controller
stated range of acceleration amplitudes, process operator to manually position one or under shock conditions, usually specified as "within ±__percent for acceleration rise times longer than __ microseconds." [ISARP37.2-1982 (R1995)]
,lmplitude linearity, vibration Closeness of sensitivity to reference sensitivity over a stated range of acceleration amplitudes, at a stated fixed frequency. This linearity is usually specified as within "± [number] percent." [ISA-RP37.2·1982 (1995)] .
, .lmplitude modulation (AM) The process (or the results of the process) of varying the amplitude of the carrier at the same time as and in proportion to the variation in the modula ting signal.
MODULATING,' SIGNALI
~ \ CARRIER
WAVE
) J.
~ t v v v v v v v v V V 1
I , ,.,.
AMPLITUDE MODULATED
WAVES
MAX1UM M!NIMUM I INTERMEDIATE I AMPLITUDE AMPLITUDE AMPLITUDEI
amplitude noise Random fluctuations in the output of a light source or signal from other
, generating or detecting means. : . amplitude ratio A factor that expresses the ~' . ratio of the output amplitude to the input . ,amplitude when the input is sinusoidal. ; < amplitude response A measure of the time it ~. . takes for a defined change of amplitude to
occur. , AM rejection The removal of unwanted
amplitude modulation of a signal. Usually performed by using signal clipping or limit
" ing circuitry. : AIM station (automatic/manual station) In . control systems, a device that enables the
more valves. A single-loop station enables the operator to manually position a single valve, a shared station makes possible control of multiple valves, and a cascade station provides control of paired loops. •
analog 1. Having the form of continuously variable physical quantities, as in data. Contrast with digital. 2. The representation of numericql quantities by means of physical vaTia,bles, such as translation, rotation, voltage, or resIstance. '3. A wa¥eform is aI}alog if it is continuous and varies over an arbitrary range. Contrast with digital., .
analog backup Conventional analog instru.:~mentation that is provided to control a pro
cess if the computer system fails. analog channel A channel on which the
information transmitted over it can take any value between the limits defined by that channel. Voice channels are analog channels.
analog computer 1. A computer in which data are mainly represented in analog form. 2. A computer that operates on analog data by performing physical processes on them. Contrast with digital computer.
analog control The implementation of automatic control loops with analog (pneumatic or electronic) equipment. Contrast with direct digital control.
analog control system Classically, a system that consists of electronic or pneumatic single-loop analog controllers, in which each loop is controlled by a single, manually adjusted device.
analog data Data represented in a continuous form, as contrasted with digital data, which is represented in a discrete, discontinuous form. Analog data are usually represented by means of physical variables, such as voltage, resistance, rotation, and so on.
analog DC current signal A signal used for transmission that varies in a continuous manner according to one or several physical quantities. [ANSI/ISA-50.1-1982 (R1992)]
analog device A mechanism that represents numbers by physical quantities, for example, by lengths, as in a slide rule, or by voltage or currents, as in a differential analyzer or a computer of the analog type.
analog electronic controller Any of several adaptations of analog computers to perform control functions. They may produce an output signal tha tis directly related to the difference between a measured value and a predetermined set point or an output signal that is modified by rate-of-change or other feedback signals.
21
I
I analog hardware description language I Anderson bridge
.'
analog hardware description language (AHDL) A modeling language that is capable of representing both the structural and behavioral properties of analog circuits. Structural refers to the connectivity or net-list properties of a circuit; behavioral refers to the mathematical equations for individual components.
analog input 1. A continuously variable input. 2. A termination panel used to connect field "Yiring from the input device. See input, analog.
analog input module An I/O module that converts a process voltage or current signal into a multiple-bit form for use in the Pc. The signal is the analog of some process variable.
analog input point An alarm point for use with an analog-monitored variable signal, usually current or voltage. The logic circuit initiates an alarm when the analog Signal is above or below a set point. [ANSI/ISA-18.11979 (RI992)]
analog output 1. Transducer output that is a continuous function of the measurand, except as modified by the resolution of the transducer. [ISA-37.1-1975 (RI982)] 2. A continuously variable output (generally 4-20 rna or 3-15 psi). See also output, analog.
analog output module An I/O module that converts a multiple-bit number calculated in the PC into a voltage or current output signal for use in control.
analog sampling The process by which the computer selects individual analog input signals from the process, converts them into an equivalent binary form, and stores the data in memory.
analog signal An analog signal is a continuously variable representation of a physical quantity, property, or condition such as pressure, flow, temperature, and so on. The signal may be transmitted as pneumatic, mechanical, or electrical energy. See signal, analog.
analog simulation The solution of a set of simultaneous equations that are similar in form to the set of equations that describe the dynamic behavior of the subject system.
analog-to-digital (AID) 1. A device, or subsystem, such as a microprocessor that changes real-world analog data (as from transducers) in to a form that is compatible with binary (digital) processing. 2. The conversion of analog data into digital data. See analog-to-digital converter.
analog-to-digital converter (ADC) Any unit or device that is used to convert analog infor
ma~ion into ~pproximatecorresponding digi A~
tal mformatIon. See converter, analog to digital. ]
analysis The quantitative and/ or qualitative r. A~ determination of the constituent parts of some entity, object, or problem.
analysis, ultimate The chemical analysis of solid, liquid, or gaseous fuels. In the case of an coal or coke, the determination of the constituent carbon, hydrogen, sulfur, nitrogen, oxygen, and ash.
analysis of varianc.~ A method for partition ... ing the total vatiance.in experimental data ;
into components that can be assigned to specific sources.
analytical balance Any weighing device that an has a sensitivity of at least 0.1 mg.
analytical curve A graphical representation of some function of relative intensity in ~pectroscopic analysis plotted against some function of concentration. an
analytical gap The separation between the source electroo@6 in a spectrograph.
analytical limit The limit of a measured or an calculated variable established by the safety analysis to ensure that a safety limit is not exceeded. [ANSI/ISA-67.04.01-2000] an
analytical line The spectral line of an element that is used to determine its concentration in an spectroscopic analysis.
analytical scale In spectroscopic analysis, the an scale that results when an analytical curve is projected onto the intensity axis. It is often used in lieu of an analytical curve to permit ar. direct reading of spectral intensity as element concentration.
analyzer 1. Unattended instrumentation that continuously monitors a process stream. 2. Any of several types of test instruments, ordinarily one that can measure several dif ar ferent variables either simultaneously or sequentially. 3. In an absorption refrigeration system, the component that allows the mixture of water and ammonia vapors leaving ar the generator to come into contact with the relatively cool ammonia solution entering the generator. In this component, the mixture loses some of its vapor content.
AND 1. Logic output D exists if and only if ar all logic inputs A, B, and C exist. [ANSI/ISA5.2-1976 (RI992)]2. A logic operator that has the property that if P is an expression, Q is an expression, R is an expression ..., then the AND of P, Q, R ... is true if all expressions are true, false if any expression is false. ar
Anderson bridge A type of AC bridge that is especially suited to measuring the characteristics of extremely low Qcoils.
22
ANUF / angular momentum flowmeter
ANDF Architecture- Teutral Distributed For reflection and the angle of incidence are ma t; an OSF/1 term.
.AND gate A basic electronic circuit used in microprocessor systems. A logical 1 value on
. output is produced only if all of the inputs have logical 1 values.
anechoic chamber 1. A test room in which all the surfaces are lined with a sound-absorb-
I . ing material. Also known as a dead room. 2. A , room that is lined with a material that . absorbs radio waves of a particular fre~ quency or band of frequencies. It is used \ chiefly for tests at microwave frequencies, I such as a radar-beam cross section.
aremobiagraph A recording pressure-tube anemometer, such as a Dines anemometer, in which springs are used to make the output from the float manometer linear with wind speed.
anemoclinometer An instrument for determining tlTe inclination of the wind to a horizontal plane.
anemometer A device for measuring wind speed. If it produces a recorded output, it is known as an "anemograph."
anemoscope A device for indicating wind 'direction.
aneroid 1 ot containing or using liquid, as of a device or system.
angle beam In ultrasonic testing, a longitudinal wave from an ultrasonic search unit that
. enters the test surface at an acute angle. a:ngle modulation A type of modulation in ; which carrier-wave angle is varied in accor
dance with some characteristic of a modulat.ing wave. Angle modulation can take the form of either phase modulation or frequency modulation.
angle of elevation The angle between a horizontal plane and the observer's line of sight to an object that lies above the plane of the observer.
angle of extinction The phase angle of the stopping instant of anode current flow in a gas tube with respect to the starting instant of the corresponding half cycle of anode voltage.
angle of ignition The phase angle of the starting instant of anode current flow in a gas tube with respect to the starting instant of anode current flow in a gas tube with respect
'- to the starting instant of the corresponding positive half cycle of anode voltage.
. angle of incidence The angle between the direction of propagation of a ray of incident radiation and a normal to the surface it strikes. For a reflected wave, the angle of
23
equal. angle of repose A characteristic of bulk sol
ids that is equal to the maximum angle with the horizontal at which an object on an inclined plane will retain its position without •tending to slide. The tangent of the angle of repose equals the coefficient of static friction.
angle valve A valve design in which one port is collinear with the valve stem or actuator, and the other.porqs at a right angle to the valve stem. [AN'5I/I5A-75.05.01-2000j
angstrom (A) A unit of length defined as 1/6438.4696 of the wavelength of the red line in the Cd spectrum. It equals almost exactly 10-10 meters. Angstrom was once used almost exclusively to express the wavelengths of light and x-rays, but it has now been lar~ely
replaced by the 51 unit nanometer, or 10meters.
angular accelerometer' A device for measuring the rate of change of the angular velocity between two objects.
angular frequency A frequency expressed in radians per second. It equals two times the frequency in Hz.
angular misalignment In fiber-optic cables, the loss of optical power that is caused by a deviation from the optimum alignment of the fiber to the fiber at the coupling.
angular momentum The product of a body's moment of inertia and its angular velocity.
angular momentum flowmeter A device for determining mass flow rate. An impeller turning at constant speed imparts angular momentum to a stream of fluid passing
I angular velocity / antialias filter
through the meter. A restrained turbine located just downstream of the impeller removes the angular momentum, and the reaction torque is taken as the meter output. Under proper calibration conditions, the reaction torque is directly proportional to mass flow rate. Also called an "axial flowmeter."
angular velocity The rate of motion along a circular path, measured in terms of angle traversed per unit time.
animation The process or making an object move across a video screen by rapidly displaying a series of static pictures of it (icons), each in a slightly different position.
anisotropic Exhibiting different values of a property when characteristics of that property are measured along different directions or axes.
anisotropy Exhibiting different properties or other characteristics-strength or coefficient of thermal expansion, for instance--in different directions with respect to a given reference, such as a specific lattice direction in a crystalline substance.
annotate To add explanatory text to computer programming or any other instructions.
annotation A comment, note, or descriptive remark added to a printout, screen view, or even in memory itself.
annular nozzle A nozzle whose inlet opening is ring shaped rather than an open circle.
annulus 1. Any ring-shaped cavity or opening. 2. A plate that protects or covers a machine.
annunciator 1. A device or group of devices that call attention to changes in process conditions. An annunciator usually calls attention to abnormal process conditions, but may be used also to show normal process status. Usually included in this category are sequence logic circuits, labeled visual displays, audible devices, and manually operated push buttons. [ANSI/ISA-18.1-1979 (R1992)] 2. An electromagnetic, electronic, or pneumatic signaling device that either displays or removes a signal light, metal flag, or similar indicator or sounds an alarm, or both, when it detects the occurrence of a specific event. In most cases, the display or alarm is single-acting and after being tripped must be reset before it can indicate another occurrence of the event.
anode 1. The metal plate or surface that acts as an electron donor in an electrochemical circuit. Metal ions go into solution in an elec
electrochemical corrosion. 2. The negative electrode in a storage battery, or the positive electrode in an electrochemical cell. 3. The positive electrode in an X-ray tube or vacuum tube, where electrons leave the interelectrode space.
anode circuit A circuit that includes the anode-cathode path of an electron tube connected in series with other circuit elements.
anode supply voltage The voltage across the terminals of an electric power source connected in series in the anode circuit
anodic coating An oxide film that is produced on a metal by treating it in an electrolytic cell with the metal as the cell anode.
anodic protection Reducing the corrosion rate of a metal that exhibits active-passive behavior by imposing an external electrical potential on a part.
anodize To form a protective pilssive film (conversion coating) on a metal part, such as a film of AIZOJ on aluminum. This is done by making the part an anode in an electrolytic cell and by passing a controlled electric current through the cell.
anodizing A method of producing film on a metal surface that is particularly well suited for aluminum.
anomalous dispersion Inversion of the deri\'ative of the refractive index with respect to wavelength in the vicinity of an absorption band. .
ANSI American National Standards Institute; nonprofit, independent organization supported by trade organizations, industry, and professional societies for standards development and coordination in the United States. They represent the United States to the ISO; they defined ASCll.
ANSI screen control An ANSI standard that specifies a specific set of character sequences that instruct the computer to perform certain actions on the computer screen.
ANSI X3J3 Name given to the ANSr pL/r Language Standardization Committee.
antenna A device for sending or receiving radio waves. The term does not refer to the means for connecting the device to a transmitter or receiver. See also dipolt! antenlla and horn antellna.
antenna array A single mounting that contains two or more individual antennas coupled together to give specific directional characteristics.
antialias filter A low-pass filter that is designed to block frequencies greater than one-half the measuring rate.
trolyte at the anode during electroplating or
24 l
I antialiasing / aperiodic
antialiasing In digital graphics, a technique for reducing the jagged appearance of aliased bitmapped images, usually by insert
It ing pixels that blend boundaries, especially j .. color boundaries.
; I
I. anti-cavitation trim A trim style for control valves that by its geometry reduces or eliminates the tendency of the controlled liquid to cavitate. [ANSI/ISA-75.05.01-2000] It is a combination of plug and seat ring or plug and cage that by its geometry permits operation without cavitation or reduces the tendency to cavitate. This minimizes damage to the valve parts and to the downstream piping. [ANSI/ISA-75.05.01-2000]
anticipatory action See rate action. anticipatory control Changing a control sig
nal to one of the inputs to a process in response to a change in another input of the same process. Such control does not have self-correcting action as does closed loop (feedback) control. Anticipatory control is also called fecdforward control or open-loop control.
anticoincidence circuit A circuit with two inputs and one output that produces an output pulse only if either input terminal receives a pulse within a specified time interval. It does not produce a pulse if both input terminals receive a pulse within that interval.
o anticorrosive Containing, as of paint or grease, a chemical that counteracts corrosion
oo. or produces a corrosion-resistant film by reacting with the underlying surface.
. antifriction Used to describe a device, such as a bearing or other mechanism, that employs rolling contact with another part rather than sliding contact.
antihunt circuit A circuit designed to prevent oscillation in a feedback process control loop, thereby stabilizing it.
antimagnetic Made of nonmagnetic materials or employing magnetic shielding that blocks the influence of magnetic fields dur
._ ing operation, as of a device. antinodes The points, lines, or surfaces in a
medium that contains a standing wave " where some characteristic of the wave field is
at maximum amplitude. Also known as loops.
anti-noise trim A trim style for control valves that by its geometry reduces the noise generated by fluid flowing through the valve. [ANSI/ISA-75.05.01-2000]
, . .antireflective coating A coating designed to ~ ,. suppress reflections from an optical surface.
'; anti-reset windup A device, circuit, or software that prevents the saturation of the inte
gral mode of a controller, which occurs when control cannot be achieved. It helps to prevent the controlled variable from overshooting its set point when the obstacle to control is removed.
antiresonance A condition that exists between an externally excited system and the external sinusoidal excitation, such that any small increase or decreas.e in the frequency of the excitation signal causes the peak-to-peak amplitude of a speCified response to increase.
antiresonant Having 'very high (approaching infinity) impedance, as of an electric, acoustic, or other dynamic system.
antiresonant frequency A frequency at which antiresonance exists between a system and its external sinusoidal excitation.
antiskid Used to describe a material, surface, or coating that has been roughened or that contains abrasive particles in order to increase the coefficient of friction and prevent sliding-or slipping. Also known as "antislip." .
antisurge control Control that avoids the unstable operating mode of compressors known as "surge." See surge.
anvil 1. The part of a machine that absorbs the energy of a sharp blow. 2. A heavy block made of wrought iron, cast iron, or steel and used to support metal being forged in a smith. 3. The base of a forging press or drop hammer that supports the die bed and lower die. 4. The stationary contact of a micrometer caliper or similar gauging deo/ice.
AOE Application operating environment; design for UNIX by AT&T.
AOX Adsorbable organic halides, a consideration in EPA (United States) regulations.
AP Application process. [ISA-TR50.02, Part 92000] Also application platform; part of software systems management services that provide the environment for management application development, debugging, and execution.
APC Advanced process control; process control strategies beyond straightforward PIO loop control, which are usually defined as "classical" advanced control; APC involves a combination of PIO loops, dead time compensators, lead/lag feed forward function blocks, and single-\'ariable constraint controllers.
APD Avalanche photodiode; diode that exhibits internal amplification of photocurrent through avalanche multiplication of carriers in the junction region. See avalanche.
aperiodic Varying in a way that is not periodically repea ted.
25
aperiodically damped / application layer
aperiodically damped Reaching a constant value or stec.dy state of change without introducing oscillation.
aperture A hole in a surface through which light is transmitted. Apertures are sometimes called spatial filters, a more descriptive term when the aperture is placed in the Fourier (focal) plane.
aperture time The time required in a sample-and-hold circuit for the switch to open after the "hold" command has been given.
IIi(
! I ' II
API Application programming interface; a set of formalized software calls and routines that can be referenced by some application program so as to access underlying network services. Programs that use API-compliant calls can communicate with any others that use that same API; the interface between applications software and the application platform. Also known as American Petroleum Institute.
APL "A Programming Language;" a computer language developed by Kenneth Iverson and used mainly in scientific applications; known for its scope compactness, and facility with arrays, it has a highly specialized character set that can be mapped to keyboard.
APL "A Programming Language;" A powerful systems programming language developed by the International Business Machines Corporation.
APM Advanced Power Management; a standard for saving power by automatically turning off computer hardware when it is not in use.
APP Application Portability Profile; developed by NIST, includes X Windows, POSIX, SQL, Information Resource Dictionary System (see 1RDs) for database systems, Open Systems Interconnections (see 051), NFS (see NFs), COBOL, C, and Ada.
apparatus and systems of category "ia" Electrical apparatus and systems that contain intrinsically safe circuits that are incapable of causing ignition. This category has the following safety factor: the circuits remain safe when up to two countable faults are applied and, in addition, those non-countable faults give the most onerous condition. [ISA12.02.01-1999 (IEC 60079-11 Mod)]
apparatus and systems of category "ib" Electrical apparatus and systems that contain intrinsically safe circuits that are incapable of causing ignition. This category has the fol
give the most onerous condition. [ISA12.02.01-1999 (IEC 60079-11 Mod)]
apparent density The density of loose or compacted particulate matter as determined by dividing actual weight by volume occupied. The apparent density of the matter is always less than the true density of the material that comprises the particulate matter because the volume occupied includes the space devoted to pores or cavities between partil:les.
apparent flow' ·The uncorrected volume flow as indicated by the calibrator. [ISA-RP31.11977]
apparent viscosity The resistance to continuous deformation (viscosity) in a non-Newtonian fluid that is subjected to shear stress.
APPC Advanced peer-to-peer communications; network architecture definition by IBM that is specified as featuring high-level program interaction capabilities on a peer-topeer basis.
appearance pot~ntial The minimum electron-beam energy required to produce ions of a particular type in the ion source of a mass spectrometer.
applet A small application program frequently received by Internet users as part of a web page they are viewing.
applicable uncertainty (AU) That portion of the channel uncertainty that is applicable to a calculation of the minimum separation between set points. [ANSI/ISA-TR67.04.081996]
application 1. The system or problem to which a computer is applied. Computation, data processing, and control are often described as the three categories of application. 2. A program that provides functionality to end users. 3. A software functional unit that consists of an interconnected aggregation of function blocks, events, and objects, which may be distributed, may have interfaces with other applications, and may contain other applications. [ISA-TR50.02, Part 92000]
application enabler A software product that allows a software application to be developed rapidly by using productivity tools and standard components and by reusing previously developed software.
application layer A logical entity of the OSI digital communication model. It is the topmost of seven layers and the one that interfaces with the network user. The application
iIIIIIII
I !
lowing safety factor: the circuits remain safe when up to one countable fault is applied and, in addition, those non-countable faults
layer performs network services like file transfer and e-mail.
26
!
I
I application-oriented language / area
application-oriented language A problemoriented programming technique that employs statements that resemble the terminology of the user rather than of the programmer.
application program A program that performs a task specific to a particular end-user's needs. Generally, an application program is any program written on a program development operating system that is not part of a basic operating system. ...
application program interface (API) A set of formalized software calls and routines that can be referenced by an application program to access underlying network services. Programs that use API-compliant calls can communicate with any others that use that same API. APIs are the interface between applications software and the application platform.
application software Software that is specific to the user application in that it is the Safety Instrumented System functional description programmed into the PES (programmable electronic system) to meet the overall Safety Requirement Specifications. In general, application software contains logic sequences, permissives, limits, expressions, and so
- on, that control the appropriate input, output, calculations, and decisions necessary to meet the safety functional requirements. [ANSIIISA-84.01-1996]2. Programs that are unique to a specific process control system installation or other specific installations rather than of a general purpose and a broad applicability.
application-specific software A computer program that is adapted or tailored to specific user requirements for the purpose of collecting, manipulating, or archiving data or for process control.
. applied load 1. The weight carried or force sustained by a structural member in service. In most cases, the load includes the weight of the member itself. 2. Material carried by the load-receiving member of a weighing scale, not including any load necessary to bring the scale into initial balance.
applied shock Any rapidly applied load or , other form of excitation that produces shock
motion within a system. . approach idler The last idler passed before the
material on a belt reaches the weighbridge. [ISA-RP74.01-1984]
approved Acceptable to the authority that . has jurisdiction. The term is considered syn
onymous with listed and certified. [ANSII ISA-RP12.6-1995]
apron The part of the control center that encloses the area below the console mounting panel.
APT Automatically programmed tools; computer-aided part programming system for numerically controlled machine tools developed for multiaxis milling machines and for point-to-point and turning work.
aramid A liquid crystal polymer with exceptional tensile strength and a coefficient of expansion near tha.t 6f glass. Widely used in fiber-optic cables. .,
arbitration A form of coordination control that determines how a resource should be allocated when there are more requests for the resource than can be accommodated at one time. [ANSIIISA-88.01-1995]
arc 1. A segment of the circumference of a circle. 2. The graduated scale on an instrument for measuring angles. 3. A discharge of electricity across a gap between electrical conductors.
arc lamp A high-intensity lamp in which a direct-current electric discharge produces light that is continuous, as opposed to a flashlamp, which produces pulsed light.
arc line A spectral line in spectroscopy. architecture 1. The structure, functional, and
performance characteristics of a system, specified in a way that is independent of the system's implementation. 2. The arrangement and interconnection of the hardware components or modules that comprise the Safety Instrumented System. [ANSIIISA84.01-1996]
archival (archive) Long-term storage of data, usually onto some auxiliary storage medium, such as a separate disk or tape.
archival file In data processing, a store of seldom used data that must be retained for several years.
arcing device An electrical make/break component that is generally interpreted as being capable of producing an arc with enough energy to ignite a specific ignitable mixture. [ISA-12.01.01-1999]
ARCnet /I Attached Resource Computer network./I A token-passing network developed by Datapoint in 1977 that uses an active hub star at 2.5 Mbs, specifies only the bottom few layers of the ISO model, and combines a token-passing scheme with star, bus, or tree topologies rather than ring topology (such as token ring).
area 1. A component of a batch manufacturing site that is identified by physical, geographical, or logical segmentation within the site. Note: An area may contain process cells,
27
area, air-conditioned / argentometer
units, equipment modules, and control modules. [A SI/ISA-88.01-1995]2. A physical, geographical, or logical grouping determined by the site. An area may contain process cells, production units, and production lines. [ISA-95.00.01-2000]
area, air-conditioned A location that has a temperature at a nominal value that is maintained constant within a narrow tolerance at some point in a specified band of typical comfortable room temperatures. Humidity is maintained within a narrow specified band. Note: Air-conditioned areas also feature clean air circulation and are typically used for instrumentation, such as computers or other equipment, that requires a closely controlled environment. [ANSI/ISA-51.1-1979]
area, control room A location with heating and/or cooling facilities and whose conditions are maintained within specified limits. Such rooms mayor may not provide features for automatically maintaining constant temperature and humidity. Note: Control room areas are commonly provided for the operation of those parts of a control system that require ongoing operator surveillance. [ANSI/ISA-51.1-1979]
area, environmental A basic qualified location in a plant that has specified environmental conditions dependent on severity. Note: Environmental areas include air-conditioned areas; control room areas, heated and/or cooled; sheltered areas (process facilities); and outdoor areas (remote field sites). [ANSI/ISA-51.1-1979]
area, outdoor A location in which equipment is exposed to outdoor ambient conditions, including temperature, humidity, direct sunshine, wind, and precipitation. [ANSI/ISA51.1-1979]
area, sheltered An industrial process location, area, storage, or transportation facility that provides protection against direct exposure to the elements, such as direct sunlight, rain or other precipitation, or full wind pressure. The minimum and maximum temperatures and humidity in such areas may be the same as outdoors. Condensation can occur, and ventilation, if any, is provided by natural means. [ANSI/ISA-51.1-1979]
area classification The classification of hazardous (classified) locations by Class 1, II ,or III depending upon the presence of flammable gases or vapors, flammable liquids, combustible dust, or ignitable fibers or flyings. The classification can also be by Division 1 or 2 depending upon whether these materials
exist in an ignitable concentration under normal or abnormal conditions.
area classification (class) 1. Class I locations are those in which flammable gases or vapors are or may be present in the air in quantities sufficient to produce explosive or ignitable mixtures. 2. Class II locations are those that are hazardous because of the presence of combustible dust. 3. Class III locations are those that are hazardous because of the presence of easily ignitable fibers or fly
- -ings, but where such fibers or flyings are not likely to be suspended in the air in quantities sufficient to produce ignitable mixtures.
area classification (division) 1. Division 1 (hazardous) locations are those where concentrations of flammable gases or vapors exist (a) continuously or periodically during normal operations, (b) frequently during repair or maintenance or because of leakage, or (c) due to equipmef1t breakdown or faulty operation, which could cause the simultaneous failure of electrical equipment. 2. Division 2 (normally nonhazardous) locations are those in which the atmosphere is normally nonhazardous and may become hazardous only if the ventilating system fails, pipe lines are opened, or other unusual situations occur.
area classification (group) Identified groups of chemicals and compounds whose air mixtures have similar ignition and explosive characteristics for the purpose of testing, approval, and area classification. Group A: atmospheres containing acetylene. Group B: atmospheres containing butadiene, ethylene oxide, propylene oxide, acrolein, or hydrogen (or gases or vapors equivalent in hazard to hydrogen). Group C: atmospheres such as cyclopropene, ethyl ether, ethylene, or gases or vapors of equivalent hazard. Group D: atmospheres such as acetone, alcohol, ammonia, benzene, benzol, butane, gasoline, hexane, lacquer solvent vapors, naphtha, natural gas, propane, or gases or vapors of equivalent hazard. Group E: atmospheres that contain metal dusts. Group G: atmospheres that contain combustible dusts that have a resistivity of <105 ohm-cm2.
area meter A device for measuring the flow of fluid through a passage of fixed cross-sectional area, usually by using a weighted piston or float that is supported by the flowing fluid.
argentometer 1. A hydrometer is used to find the concentration of a silver salt in water solution.
28 ___ -4
I
4
argument I artificial intelligence (AI)
argument 1. In a mathematical function, the variable that, when a certain value is substituted for it, determines the value of the function. If y =j(x), then x, the independent variable, is the argument of the function. 2. An operand in an operation on one or more variables. See also parameter.
arithmetic ability The capability of perform! ing (at least) addition and subtraction on the
personal computer. arithmetic check See nzatlzematiq1l check. arithmetic element The portion of a mechan
1· ical calculator or electronic computer that performs arithmetic opera tions.
arithmetic expression An expression that contains any combination of data names,.. numeric literals, and named constants that is joined by one or more arithmetic operators in such a way that the expression as a whole can be reduced to a single numeric value.
arithmetic logic unit (ALD) A component of the central processing unit of a computer in which data items are compared, arithmetic operations performed, and logical operations executed.
arithmetic operation A computer operation , in which the ordinary elementary arithmetic
operations are performed on numerical quantities. Contrast with logical operation.
arithmetic operator Any of the operators + and - or the infix operators +, -, *, I, and **.
~ arithmetic unit The unit of a computing sys1. ' tern that contains the circuits that perform
arithmetic operations. . arm To make it possible for a hardware inter
rupt to be recognized and remembered. Contrast with disarm. See enable.
J armature 1. The core and windings of the rotor in an electric motor or generator. 2. The portion of the moving element of an instrument that is acted upon by magnetic flux to produce torque.
armored meter tube A variable-area meter tube (rotometer) of all-metal construction that utilizes a magnetic coupling between the
, float and an external follower. t ARPA Advanced Research Projects Agency;
operates within United States Department of Defense, which developed the first major packet- switched digital computer network.
~. ARP Address Resolution Protocol; TCP lIP process that maps Internet Protocol (IP) addresses to Ethernet addresses; required by TCP lIP for use with Ethernet.
ARPANet Advanced Research Projects Administration Network. A precursor to the Internet, developed in the late 1960s and early
1970s by the United States Department of Defense.
ARQ Automatic request for retransmission; in digital communications, where a receiver asks a transmitter to resend a block or frame, generally because of errors detected by the receiver.
array 1. An arrangement of elements in one or more dimensions. See also matrix and veclor. [Comp]2. In a computer program, a numbered, ordered c.ollection of elements, all of which have identica'l dataattributes. 3. An aggregate whose elements are of the same data type and may be uniquely referenced by subscripting. [ISA-TR50.02, Part 9-2000] [Eng] 4. A group of detecting elements that are usually arranged in a straight line (linear array) or in two-dimensional matrix (imaging array). 5. A series of data samples, all from the sam~ measurement point. Typically, an array is'assErihbled at the telemetry ground station for frequency analysis.
array dimension The number of subscripts needed to identify an element in the array.
array processing The capability of a computer to operate at a variety of data locations at the same time.
array processor A hardware device that processes data arrays. Fast Fourier transforms (FIT) and power-spectral density (PSD) are typical processes.
arrester A device that impedes the flow of large dust particles or sparks from a stack, usually through screening at the top.
arrow keys Keys on a computer keyboard tha t will move the cursor.
articulated arms (waveguides) A beamdirection arrangement in which light passes through a series of jointed pipes containing optics.
articulated structure A structure that is either stationary or movable, such as a motor vehicle or train, that is permanently or semipermanently connected so that its different sections can move relative to the others, usually by using pinned or sliding joints.
artifact 1. Any component of a signal that is extraneous to the variable represented by the signal. 2. In video development, the area within an image or characteristic of an image that is caused by system limitations, such as weird shimmering, jaggies, or other undesirable distortion. Also, in digital graphics, image imperfections caused by data compression.
artificial intelligence (AI) That branch of computing that studies capabilities that resemble human thought processes, such as reasoning,
29
artificial language / aspiration
learning, vision, aural recognition, and even self- improvement.
artificial language 1. A language specifically designed for ease of communication in a particular area of endeavor, but one that is not yet natural to that area. This is contrasted with a natural language which has evolved through long usage. 2. A programming language that is based on a prescribed set of rules established ahead of time, such as BASIC, COBOL, Pascal, and the like.
artificial radioactivity Radioactivity that.is induced by bombarding a material with a beam of energetic particles or with electromagnetic radiation.
artificial weathering Producing controlled changes in materials, such as surface appearance, under laboratory conditions that simulate outdoor exposure.
as built 1. A document revision that includes all the modifications that were performed as a result of actual fabrication or installation. As-built documents may have various issues that reflect various milestones such "as purchased," "as manufactured," "as insured," and "as commissioned.". [ISA-RP60.4-l990] 2. During construction, changes that are made in design and installation and marked up on drawings as built.
ascender In typography, that part of a lowercase letter that rises above the main body, as in the characters band d.
ASCII American Standard Code for Information Interchange; a widely used code in which alphanumerics, punctuation marks, and certain special machine characters are represented by lmique, seven-bit binary numbers. One hundred and twenty-eight different binary combinations are possible (27 = 128), thus 128 characters may be represented. ASCII was defined in ANSI X3.4-l986 and is normally used for asynchronous transmission.
ASCII file A text file that uses the ASCII character set.
as-fabricated The condition of a structure or material after assembly, without any conditioning treatment such as a stress-relieving heat treatment. Specific terms such as "as-welded," "as-brazed," or "as-polished" are used to designate the nature of the final step in fabrication.
as-fired fuel Fuel in the condition it was as it was fed to the fuel-burning equipment.
as found The condition in which a channel, or portion of a channel, is found after a period .. of operation and before recalibration (if
l"
needed). [ANSI/ISA-67.04.0l-2000; ISARP67.04.02-2000]
ash The noncombustible inorganic matter in the fuel.
ash content The incombustible residue that remains after burning a combustible material completely.
ash-free basis The method for reporting fuel analysis whereby ash is deducted, and other constituents are recalculated to total 100 percent...
ASI Actu,flt.oi: sensor interface; European "fieldbus" for binary sensors and actuators.
as left The condition in which a channel, or portion of a channel, is left after calibration or final setpoint device setpoint verification. [ANSl/ISA-67.04.01-2000; ISA-RP67.04.022000]
ASN.l Abstract Syntax Notation One. An ISO standard (DIS 8824) that specifies a c~m.onical method of data encoding. This standard is an extension of CCITT standard X.409.
ASN.l Abstract Syntax otation One; ISO IS 8824 and IS 8825 encoding and decoding structures.
ASPC Algorithmic statistical process control; closed loop version of normally open loop SPC.
aspect ratio [Comp] 1. The ratio of a symbol's height to its width. [ISA-S.5-l985] [Comm] 2. The ratio of frame width to height for a television picture. It is 4:3 in the United States, Canada and United Kingdom. [Eng] 3. In any rectangular structure, such as the cross section of a duct or tubular beam, the ratio of the longer dimension to the shorter. 4. A ratio of width to depth used to calculate resistance to flow in a rectangular elbow.
aspheric Not spherical or flat, as of the surface of optical elements. Lenses with aspheric surfaces are sometimes called "aspheres."
aspirating burner A burner in which the fuel is in a gaseous or finely divided form and is burned in suspension. The air for combustion is supplied by bringing into contact with the fuel air that has been drawn through one or more openings by the lower static pressure created by the velocity of the fuel stream.
aspiration Using a vacuum to draw up gas or granular material. This is often accomplished by passing a stream of water across the end of an open tube or through the run of a tee joint, where the open tube or branch pipe extends into a reservoir that contains the gas or granular material.
30
- ~ _.,.,........- ----------_._--._---~--'_._-----~-_ ... _---
I as-received fuel/astronomical theodolite
as-received fuel Fuel in the condition it is in as it received at the plant.
assemble To prepare a machine-language program from a symbolic language program
~ ~~ by substituting absolute code for symbolic operation codes and absolute or relocatable addresses for symbolic addresses.
assembler A program that translates symbolic source code into machine instructions by replacing symbolic operation codes with binary operation codes and symbolic addresses with absolute or relocatable addresses.
assembly [Eng] 1. A unit constructed of many parts or components and that functions in service as a single device, mechanism, or structure. [Comp] 2. A mid-level computer language.
assembly language A computer programming language, similar to a computer language, in which the instructions usually have a one-to-one correspondence with computer instructions in machine language and that utilizes mnemonics to represent instructions.
assembly list A printed list that is the by-product of an assembly procedure. It lists in logical instruction sequence all details of a routine, showing the coded and symbolic notation next to the actual notations established by the assembly procedure. This list
» ing is highly useful when debugging a .- . routine.-{-,\
assembly program See assembly system. assembly system A system comprised of two
elements, a symbolic language and an assembly program. The assembly program translates the source programs written in the symbolic language into machine language.
assign To designate a part of a system for a specific purpose.
assignable Permitting the channeling (or directing) of a signal from one device to another without the need for switching, patching, or changes in wiring. [ANSl/lSA5.1-1984 (R1992); lSA-5.3-1983]
,: assignment statement A program statement . that calculates the value of an expression and
assigns it a name (e.g., x = x + 5, Y=8). associated apparatus An electrical apparatus
in which there are both intrinsically safe circuits and non-intrinsically safe circuits. The latter can affect the safety of the former. Note: An associated apparatus may be either (a) an electrical apparatus that has an alternative type of protection for use in the appropriate explosive gas atmosphere or (b) an electrical apparatus not so protected and that should not therefore be used within an
31
explosive gas atmosphere. An example of the latter is a recorder that is not itself in an explosive gas atmosphere but is connected to a thermocouple that is situated within an explosive gas atmosphere where only the recorder input circuit is intrinsically safe. [ISA-12.02.01-1999 (IEC 60079-11 Mod)]
associated electrical apparatus An electrical apparatus in which the circuits are not all intrinsically safe but that contains circuits that can affect the safety of intrinsically safe circuits connected to it.
association [Sci] 1. The combining ot ions into larger ion clusters in concentrated solutions. [Comp] 2. A program-to-program logical relationship which may be dynamically established and torn down; may be assumed, may not be required. See connection.
associative memory A neural network architecture used in pattern recognition applications, in which the network is used to associate data patterns with specific classes or categories it has already learned.
associative storage A storage device in which storage locations are identified by their contents, not by names or positions. Synonymous with content-addressed storage. Contrast with parallel search storage.
astable circuit A circuit that alternates automatically and continuously between two unstable states at a frequency that is dependent on circuit constants, for example, in a blocking oscillator.
astable multivibrator A multivibrator in which each active device alternatelv conducts and is cut off for intervals of time as determined by circuit constants, without using external triggers.
astatic Without polarity; independent of the earth's magnetic field.
astigmatism A defect in an optical element that causes rays from a single point in the outer portion of a field of view to fall on different points in the focused image.
ASTM American Society for Testing and Materials; scientific and technical organization that develops material standards and testing methods.
astrodynamics A practical application of fundamental science to the problem of planning and controlling the trajectories of space vehicles.
astrolabe An instrument formerly used to find the altitudes of celestial bodies; a predecessor of the sextant.
astronomical theodolite See altazimuth.
I asymmetric rotor / attached equipment
asymmetric rotor A rotating machine element whose axis of rotation is not the same as its axis of symmetry.
asymmetrical compression A data compression system that requires more processing capability to compress an image than to decompress an image. Such systems are typically used for the mass distribution of programs on media such as CD-ROMs.
asymmetry potential The difference in potential between the inside and outside pHsensitive glass layers when they are both in contact with 7 pH solutions. This difference is caused by the deterioration of the pH-sensitive glass layers or by contamination of the internal fill of the measurement electrode. .
asynchronous [Comp]1. Circuitry or operation without common clock or timing signals. 2. A mode of operation in which an operation is started by a signal before the operation on which this operation depends is completed. When referring to hardware devices, it is the method in which each character is sent with its own synchronizing information. The hardware operations are scheduled by "ready" and "done" signals rather than by timer intervals. This implies that a second operation can begin before the first operation is completed. [Eng] 3. Not synchronous with the line frequency as applied to rotating a.e. machinery.
asynchronous transmission 1. Transmission in which each information character, or sometimes each word or small block, is individually synchronized, usually by using start and stop elements. The gap between each character (or word) is not of a necessarily fixed length. (Compare with synchronol/s transmission.) Often called "start-stop transmission." 2. A data transmission mode in which the timing is self-determined and not controlled by an external clock.
ATM Asynchronous transfer mode; type of packet switching that transmits fixed-length units of data, and being asynchronous the recurrence of cells does not depend on the bit rate of the transmission system, only on the source requirements (packets include address of their destinations). Provides a very fast and efficient transfer mode for multimedia applications (up to ten thousand text pages per second), allowing, for example, real-time video transfer and groupware slide projection. Also, abbreviation for atmospheres, a unit of pressure measurement.
atmometer A generic name for any instrument that measures evaporation rates. Also
knJwn as an "atmidometer," "evaporimeter," or "evaporation gauge."
atmospheric air Air under the prevailing atmospheric conditions.
atmospheric communication Sending signals in the form of modulated light through the atmosphere, without the use of fiber optics to contain and direct the beam.
atmospheric corrosion Corrosion that occurs naturally due to exposure to climatic conditions. Cprro;;ion rates vary by global location because ofvariatiohs in average temperature, humidity, rainfall; because of airborne substances such as sea spray, dust, and pollen; and because of airborne pollutants such as sulfur dioxide, chlorine compounds, fly ash, and other combustion products.
atmospheric monochromator A monochromator in which the optical path is through air. This is the standard type of monochromator used for visible and infrared wavelengths transmitted by air.
atmospheric pressure The barometric reading of pressure exerted by the atmosphere. At sea level it is 14.7 lb per sq in. or 29.92 in. 6f mercury.
atomic mass unit A unit for expressing atomic weights and other small masses. It equals exactly 1/12 the mass of the carbon 12 nuclide.
atomic number An integer that designates the position of an element in the periodic table of the elements. It equals the number of protons in the nucleus and the number of electrons in the electrically neutral atom.
atomic weight The weight of a single atom of any given chemical element. It is usually taken as the weighted average of the weights of the naturally occurring nuclides, expressed in atomic mass units.
atomization Mechanically producing fine droplets or mist from a bulk liquid or molten substance.
atomizer A device by means of which a liquid is reduced to a very fine spray.
atom probe An instrument that consists of a field-ion microscope with a probe hole in its screen that opens into a mass spectrometer. It is used to identify a single atom or molecule on a metal surface.
ATRAC Adaptive transform acoustic coding; coding method to create minidisks using a varying number of bits per sample depending upon "critical" frequencies encountered.
attached equipment The auxiliary equipment that must be located on the valve or actuator. [ANSI/ISA-75.05.01-2000]
32
I attachment plug / authority having jurisdiction
attachment plug A connecting device for a flexible cord that, when it is inserted into a receptacle, establishes supply circuit connections between the flexible cord and the recep
~'. tacle. . attemperation Regulating the temperature of
a substance, for instance, by passing superheated steam through a heat exchanger or by injecting water mist into it to regulate final steam temperature.
attemperator A mechanical device used for maintaining and controlling the temperature of superheated steam. [ANSI/ISA-77.441995]
.attemperator (direct contact type) A mechanical device in which the steam and the cooling medium (water) are mixed. [ANSI/ISA77.44-1995; ANSI/ISA-77.44-1995]
attenuate To weaken or make thinner-for example, to reduce the intensity of sound or ultrasonic waves by passing them through an absorbing medium.
attenuation 1. A decrease in signal magnitude between two points or between two frequencies. 2. The reciprocal of gain, when the gain is less than one. [ISA-RP55.1-1075 (RI983)] Note: Attenuation may be expressed as a "dimensionless ratio" or "scalar
" ratio" or expressed in decibels as "20 times the 10glO of that ratio." [ANSI/ISA-51.1-1979 (RI993)] 3. The loss of amplitude in a signal as it is transmitted through a conductor. The opposite of "gain."
attenuator 1. An optical device that reduces the intensity of a beam of light passing through it. 2. An electrical component that reduces the amplitude of a signal in a controlled manner.
attitude 1. The relative orientation of a vehicle or object as represented by its angles of inclination to three orthogonal reference axes. [ISA-37.1-1975 (RI982)]2. The position of an object in space as determined by the angles between its axes and a selected set of planes.
attitude error The error caused by the orientation of the transducer relative to the direction in which gravity acts upon it. [ISA-37.11975 (RI982)] See acceleration error.
~. . attribute 1. A property or characteristic of an entity. For instance, value and status are
; attributes of an output parameter. [ISATR50.02, Part 9-2000] 2. A characteristic quality of a data type, data structure, element of a data model, or system. In object-oriented programming, an attribute is some piece of information that describes a characteristic of the object.
attribute sampling A type of sampling inspection in which an entire production lot is accepted or rejected if the number of items in a statistical sample has at least one characteristic (attribute) that does not meet specifications.
auctioneering device See signal selector. audible device A device that calls attention
by emitting a sound when abnormal process condition~ 9ccur. An audible device may also call attention t<Ja return.to.Dormal conditions. [ANSI/ISA-18.1-1979 (RI992)]
audible device follower See auxiliary output. audio Pertaining to audible sound-usually
taken as referring to sound frequencies in the range 20 to 20,000 Hz.
audio analyzer An instrument that displays audio signal voltage or power as a function of the signal's frequencies.
audio frequencies Frequencies that can be heard by the human ear, usually between 15 and 20,000 cps (cycles per second).
audiometer An instrument used to measure the ability of people to hear sounds. It consists of an oscillator, amplifier, and attenuator, and may be adapted to generate pure tones, speech,'or bone~conductedvibrations.
audit The action of corroborating the evidence regarding the authenticity and validity of data or procedures.
auger 1. A wood-boring tool that consists of a shank with a T-shaped handle. 2. A feeding device that consists primarily of a set of spiral blades mounted on a central shaft or fastened together to make a spiral rotating assembly. The auger may rotate in a tube, trough, or housing to move powdered, granular, or semisolid material axially. In some applications, the auger may be constructed of two counter-spiraled zlUgers, which feed material toward the midpoint or outward from the midpoint of the axis, depending on the direction of rotation.
AUI Access unit interface; attachment unit interface; twisted-pair telephone wire IEEE standard for Ethernet.
authoring system Software that helps developers design interactive courseware easily, without the painstaking detail required in computer programming.
authoring tools Software capabilities that make it possible to create applications without involving oneself in the tedious details of programming.
authority having jurisdiction The organization, office, or individual that has the responsibility and authority to approve equipment, installations, or procedures. Note: The term
33
auto answer / automatic/manual station
I
I ! I
I II
. ! I,
"authority having jurisdiction" is used broadly since jurisdiction and approval agencies vary, as do their responsibilities. Where public safety is primary, the authority that has jurisdiction may be federal, state/ provincial, local, other regional department, or an individual such as an inspector from a labor or health department, electrical inspector, or others with statutory authority. An insurance inspection agency, rating bureau,
.or other. insurance company representative may be the authority with jurisdiction. An owner or his designated agent may also assume the role. At government-owned installations, the commanding officer, departmental official, or designated agent may be the authority with jurisdiction. [ISA12.01.01-1999; ANSI/ISA-RP12.6-1995]
auto answer A modem that can automatically answer incoming telephone calls from computers and provide data to that system.
autoclave An airtight vessel for heating its contents and sometimes agitating them. It usually uses high-pressure steam to process, sterilize, or perform cooking steps using moist or dry heat.
autocollimator A telescopic sight that includes a light source and a partially reflecting mirror, focused to infinity. It is used to measure small angular motion and check alignment.
autocorrelation In a time series, the relationship between values of a variable taken at certain times in the series and values of the same variable taken at other, usually earlier times.
auto dial A modem capable of connecting to a telephone system and dialing a number. Modem and communications software that performs proper procedures so computers may exchange data.
autoexec.bat The name of the file in MS-DOS that contains the commands to be executed when the computer is booted.
autogenous ignition temperature (AIT) The minimum uniform temperature that is required to initiate or cause the self-sustained combustion of a solid, liquid, or gaseous substance, independent of any other ignition source. [ANSI/ISA-12.01.01-1999] Formerly known as "auto-ignition temperature."
auto-manual station Synonym for control station. [ANSI/ISA-5.1-1984 (R1992)]
automate To convert a procedure, a process, or equipment into an automatic operation.
automatic 1. Self-acting or operating by its own mechanism when actuated by some imper
sonal influence, as, for example, a change in current strength, pressure, temperature, or mechanical configuration. [ISA-12.01.011999] 2. A machine that operates automatically. 3. Functioning without intervention by a human operator under specified conditions, as of a process or device.
automatically programmed tools A numerical language.
automatic control The type of control in which there is no direct human action on the controlling device.
automatic control engineering The branch of science and technology that deals with the design and use of automatic control devices and systems.
automatic control panel A panel of indicator lights and switches that display an indication of process conditions and from which an operator can control the operation of the process.
automatic control system See control system, automatic.
automatic controller Any device that measures the value of a process variable and generates a sign'll or some controlling action to ensure that the value corresponds with a reference value, or set point.
automatic error correction A technique for detecting and correcting errors that occur in data transmission or data handling. Such correction usually requires the use of special codes or automatic retransmission, which detects and corrects errors occurring in transmission. The degree of correction depends upon the coding and equipment configuration.
automatic frequency control A device or circuit designed to maintain the frequency of an oscillator within a preselected band of frequencies. In a FM radio receiver, the circuitry that senses frequency drift and automatically controls an internal oscillator to compensate for the drift.
automatic gain control An auxiliary circuit that adjusts the gain of the main circuit in a predetermined manner when the value of a selected input signal varies.
automatic lighter A means for igniting fuel without manual intervention. Usually applied to liquid, gaseous, or pulverized fuel.
automatic/manual station A device that enables an operator to select an automatic signal or a manual signal as the input to a controlling element. The automatic signal is normally the output of a controller, while the
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I automatic pilot / auxiliary output (auxiliary contact)
i I j \
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manual signal is the output of a manually operated device. [ANSI/ISA-51.1-1979]
automatic pilot An automatic control system that is adapted to maintain an aircraft in stable, level flight or to execute selected maneuvers.
automatic reset See reset. automatic send/receive (ASR) A teletype
writer .unit with keyboard, printer, paper tape, readerftransmitter, and paper tape punch. This combination of units may be based on line or off line and, in some cases, on line and off line simultaneously.
automatic test equipment (ATE) Equipment that analyzes the response of an instrument under test to a well-defined electrical stimulus in order to evaluate its performance with minimum human intervention.
automatic test program generator (ATPG) Computer software and/or hardware that automatically generates programs for automatic test equipment based on device characteristics and test parameters.
automatic tracking The action of a control system to automatically track a set point or the process variable without any other corrective mechanisms. [A SI/ISA-77.13.011999]
automatic utility translator (AUTRAN) A process control language and system offered by Control Data Corporation.
automatic zero- and full-scale calibration Zero and sensitivity stabilization by servos for the purposes of comparing demodulated zero- and full-scale signals with zero- and full-scale references.
automation 1. The implementation of processes by automatic means. 2. The theory, art, or technique of making a process more automatic. 3. The investigation, design, development, and application of methods for rendering processes automatic, self-moving, or self-controlling. 4. The conversion of a procedure, a process, or equipment to automatic operation.
autonomous system A system with no inputs.
autoradiography A technique for producing a radiographic image by using the ionizing radiation produced by radioactive decay of atoms within the test object itself.
autoranging The automatic selection of an appropriate range setting so as to measure the value of a signal applied to an instrument's input.
auto-tracking antenna A receiving antenna that always points to the transmitting site,
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automatically tracking all movements of the vehicle being telemetered.
autotransformer A type of transformer in which certain portions of the windings are shared by the primary and secondary circuits.
auto restart The capability to perform automatic initialization functions so as to resume operations after an equipment or power failure.
auto tuning A technique within a controller that analyzes the effects of a change in the set point of a closed toOp or iIi the control'output of an open loop and adJusts or recommends tuning parameters based upon that analysis. The change is necessary because it allows the tuner to learn the magnitude and period of process response, which it is uses to calculate new parameters. See self-adaptive tuning.
auto-tuning controller A controller feature that calculates PID settings based on calculations using measured process dynamics and combining those with the parameters of a PID controller. Depending on who manufactured the controller, open or closed loop tuning may be used. Calculations may be based on transient responses, frequency responses, or parametric models. .
auto-zero An automatic internal correction for offsets and/or drift at zero signal input.
auto-zero logic module A component of a digital controller whose function is primarily to establish an arbitrary zero-reference value for each individual measurement.
auxiliary contact See auxiliary output. auxiliary device 1. Generally, any device that
is separate from a main device but is necessary or desirable for the effective operation of the system. 2. Specifically, any device used in conjunction with an instrument to extend its range, increase its accuracy, otherwise assist in making a measurement, or perform a function not directly involved in making the measurement.
auxiliary location A location for panel instruments that is somewhere other than the control room.
auxiliary means A device or subsystem, usually placed ahead of the primary detector, that alters the magnitude of the measured quantity to make it more suitable for the primary detector without changing the nature of the measured quantity.
auxiliary output (auxiliary contact) 1. An output signal that is operated by a single alarm point or group of points so it can be used with a remote device. [ANSI/ISAS18.1-1979 (R1992)] 2. A secondary output.
I auxiliary output, audible device follo·w"er / average resolution
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auxiliary output, audible device follower (hom relay contact) An auxiliary output that operates while the common alarm audible device operates. [ANSI/ISA-S18.1-1979 (R1992)]
auxiliary output, field contact follower An auxiliary output that operates while the field contact indicates an abnormal process condition. [ANSI/ISA-S18.1-1979 (R1992)]
auxiliary output, lamp follower An auxiliary output that operates while the visual display lamps indicate an alarm, silenced, or acknowledged state. [ANSI/ISA-S18.1-1979 (R1992)]
auxiliary output, reflash An auxiliary output that operates when anyone of a group of alarm points indicates an abnormal process condition. The output usually returns to norJTIal briefly when each alarm point changes to an abnormal process condition and returns to normal when all alarm points in the group indicate normal process conditions. [ANSI/ISA-S18.1-1979 (R1992)]
auxiliary panelLA panel that is not in the main control room. The front of an auxiliary panel is normally accessible to an operator, but the rear is normally accessible only by maintenance personnel. 2. Located at an auxiliary location.
auxiliary storage A storage device in addition to the main storage of a computer, for example, magnetic tape, disk, magnetic drum, or core. Auxiliary storage usually holds much larger amounts of information than the main storage, but the information is accessible less rapidly. Contrast with main storage.
availability The ratio of time in which a system is operating correctly to the total hours of scheduled operation. Before 1962, availability was calculated as the value of MTBF -0
(MTBF + MTTR). After 1962, it was defined by military specifications. as MTTF -0- (MTTF + MTTR), which is a calculation of those times, not a tested value. Very much dependent on environmental conditions, the MTBF of electronic equipment will drop by about half for every lOoC increase. All other ambient factors will have a similar impact.
availability factor The fraction of the time during which the unit is in operable condition.
available capability The portion of the production capability that can be attained but is not committed to current or future production. [ISA-95.00.01-2000]
available draft The draft that may be utilized to cause the flow of air for combustion or the flow of prod ucts of combustion.
available energy Energy that theoretically can be converted into mechanical power.
available heat In a thermodynamic working fluid, the amount of heat that could be transformed into mechanical work under ideal conditions by reducing the temperature of the working fluid to the lowest temperature available for heat discard.
available power" . AI).attribute of a linear source of electric pmver. It is defined as V /4R, where V is the open-circuit rmsrms rms voltage of the power source, and R is the resistive component of the internal impedance of the power source.
available power gain An attribute of a linear transducer that is defined as the ratio of power available from the output terminals of the transd~lcerJ.o the power available from the input circuit.under specified conditions of input termination.
available work The capacity of a fluid or body to do work if applied to an ideal engine.
avalanche. The production of a large number of ions by cascade action in which a single charged particle, accelerated by a strong electric field, collides with neutral gas molecules and ionizes them.
avalanche photodiode (APD) A photodiode that is designed to take advantage of avalanche multiplication of photocurrent. As the reverse-bias voltage approaches the breakdown voltage, hole- electron pairs created by absorbed photons acquire sufficient energy to create additional hole-electron pairs when they collide with substrate atoms, producing a multiplication effect.
average outgoing quality limit The average percentage of defective units that remain undetected in all lots tha t pass final inspection. It is a measure of the ability of sampling inspection to limit the probability of shipping defective product. Here, a defective unit is considered to be one that contains at least one attribute that does not meet specifications.
average-position action A type of control system action in which the final control element is positioned in either of two fixed positions. The average time at each position is determined from some function of the measured value of the controlled variable.
average resolution The reciprocal of the total number of output steps over the unit range multiplied by 100 and expressed in % VR.
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I averaging pitot tube I azimuth circle�
[ISA-37.6-1982 (R1995); ISA-37.12-1982� (R1995)]�
averaging pitot tube An adaptation of the pitot tube in which a multiple-ported pitot tube spans the process tube. Total pressure is measured as a composite of the pressures on several ports facing upstream, while static pressure is measured using one or more ports facing downstream. The device works best for clean liquids, vapors, and gases, but
., can be used for streams that contain suspended solids or viscous contaminants if the purging flow is supplied to the measuring
I tube. AVI Audio video interleaved; digital file for
ma t by Microsoft developed for dynamic graphics.
AWG American Wire Gauge; United States standard system used for designating the size of electrical conductors; gauge numbers are inverse to size.
axial In the direction parallel to the shaft centerline.
axial fan Consists of a propeller or disk type of wheel within a cylinder in which the air is
. discharged parallel to the axis of the wheel. axial-flow Describing a machine such as a ,. pump or compressor in which the general .; direction of fluid flow is parallel to the axis . of its rotating shaft. axial hydraulic thrust In single-stage and
multiple-stage pumps, the axial component ,: of the summation of all unbalanced impeller 1 forces. axial runout For a rotating member, the total
amount that a specific surface deviates from a plane that is perpendicular to the axis of rotation in one complete revolution. It is usually expressed in 0.001 in., or in some other suitable unit of measure, taken at a specific radial distance from the axis of rotation.
axle A rod, shaft, or other supporting mem�ber that carries wheels and either transmits� rotating motion to the wheels or allows the� wheels to rotate freely about it.�
azimuth angle An angular measurement in a� horizontal plane about some arbitrary center� point, in which true north or some other arbi�trary direction is used as a reference direc�tion (0).�
azimuth circle A ring scale graduated from 0� to 360 and used with a compass, radar plan� position indicator, direction finder, or other� device to indicate compass direction, relati\'e� bearing, or azimuth angle.�
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