RADIATION METHODS (PYROMETRY)

×  Measures temperature by detecting thermal radiation from an object without touching the object.

×  Consists of an optic system which accumulates visible infrared energy and focuses the energy to the detector.

×  The detector will convert the energy to electricity, where the electric energy is proportionate to temperature.

×  Radiation emission:

×  where, T is the temperature of the object in Kelvin.

Classes of Pyrometry ×  For temperatures above 650oC, the heat radiations emitted

from the body are of sufficient intensity to be used for measuring the temperature

×  Instruments that employ radiation principles fall into three general classes:

1.  Total radiation pyrometer ×  sensitive to all the radiation that enters the instrument

2.  Selective (or partial) radiation pyrometers ×  Sensitive to radiation of a particular wavelength

3.  Infrared (IR) pyrometer ×  the IR pyrometers employ the infrared portion of the spectrum by using

a thermal detector to measure the temperature on the surface of the body

Total Radiation Pyrometer ×  The total radiation pyrometer receives a controlled sample of the

total radiation of a hot body and focuses it on to a temperature sensitive transducer.

×  The term 'total radiation' includes both visible (light) and invisible (infrared) radiations.

×  Wave lengths of light in the visible range is from 0.3 to 0.72 µm & infrared radiations is from 0.72 to 1000 µm.

×  Require special optical materials for focusing - ordinary glass is unsatisfactory

�  It consists of blackened tube T open at one end to receive the radiations from the object whose temperature is to be measured

�  The other end of the tube has a sighting aperture in which an adjustable eyepiece is usually fitted.

�  The detector disc is usually of blackened platinum sheet/foil and is connected to thermocouple or thermopile junctions or to a resistance thermometer bridge circuit

�  Leads from the detector are led out of the casing to a meter for measuring the thermoelectric emf or the variation the electric resistance of the platinum foil.

Selective Radiation Pyrometer

×  The classical form of optical pyrometer is the disappearing filament optical pyrometer

×  It is most accurate of all radiation pyrometers - ± 50C in the range of 850-12000C

×  However, its use is limited to temperature, greater than about 7000C, since it requires visual brightness match by a human operator.

×  This instrument used to realize the International Practical Temperature Scale above 10640C.

�  In the disappearing filament instrument shown an image of the target is superimposed on the heated filament

�  The tungsten lamp (stable) is previously calibrated, so that when the current through the filament is known, the brightness temperature of the filament is also known.

�  A red filter that passes only a narrow band of wavelengths around 0.65 pm is placed between the observer eye and the tungsten lamp and the target image

�  The observer controls the lamp current until the filament disappears in the superimposed target image.

Thermopile ×  Thermopile is normally used to detect infrared energy

where it normally consists of a few small thermocouples.

×  Thermopiles are used to provide an output in response to temperature as part of a temperature measuring device, such as the infrared thermometer.

×  These small thermocouples are connected serially to increase sensitivity of the pyrometer where the effect of serial connection will increase the sensitivity of sensor as much as n times.

Physical Construction of Thermopile

Examples of Pyrometer Applications

Measurement of exhaust gas temperature in the engine

×  The above diagram shows a typical exhaust gas temperature control using a thermocouple. A DC amplifier is used to amplify the thermocouple output to a suitable value to be connected to a “matching network”.

Temperature measurement in a hot water tank.

×  A platinum resistance thermometer is placed in a liquid filled tank. The resistance change will be changed to voltage through a “bridge network”. A DC amplifier produces enough voltage for the recorder to operate. A stirrer is used to keep the temperature uniform

Telemetry In Temperature Measurement

×  Sometimes temperature measurement from a distance is needed and this is when the telemetry comes in. Voltage from a thermocouple is used to modulate the frequency to high wave radio frequency. This radio wave is transmitted and received by the radio frequency receiver where a demodulator is used to get the initial temperature reading