Impr Ind Temp Cal 2013

8/13/2019 Impr Ind Temp Cal 2013

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Improving Industrial TemperatureCalibration Accuracy



©2012 Fluke Corporation NPI Team Intro 2Jason Titmas Temperature Calibration Tour 2009 © 2009 Fluke Corporation 2

Improve temperature calibration by considering thecalibration system components:

1. Thermometer Readout

2. Reference Probe3. Temperature Source (Heat Source)

Drywell

Liquid bath

Agenda




1. Readout for the UUT

2. Reference (readout and probe) to compare to (canbe the drywell controller display)

3. Heat Source - Drywell or Liquid Bath

Calibration system components

Your Sensor andReadout

External ReferenceThermometer

Dry-Well’s Thermometer/Controller

Comparison

1502C

Probe

F

Comm

K

Sample Exit

Menu

Enter

Or




Digital Multi Meters

• Adequate in some situations

• Not as accurate or efficient to use

Readout designed for

temperature measurement

• More accurate

• Designed for ease of use

Thermometer Readouts




•Temperature is calculated from the readoutmeasuring the sensor (resistance or voltage)

•The more accurately the readout measures

the sensor, the more accurate the displayedtemperature will be

•The more sensitive the sensor is, the lesseffect measurement errors have on the

displayed temperature

Principles of measurement




S

E E

R

T

Basic Formula for Error

• E T : equivalent temperature error

• E R : readout measurement error

• S : sensor sensitivity


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• A readout measures the resistance of a sensor with anerror of 0.01W

• The sensor has a sensitivity of 0.4 W/°C

• What is the equivalent temperature error of the

readout’s measurement?

Simple Example

C C

E T

W

W 025.0

4.0

01.0



• 100 W PRT: 0.4 W/°C

• 25 W SPRT: 0.1 W/°C

• 10 kW thermistor: 0.04 W/W/°C x resistance

• Type K thermocouple: 0.04 mV/°C• Type S thermocouple: 0.01 mV/°C

Typical Sensitivities



• Accuracy likely varies over the range

• Accuracy spec might have two or more parts, likepercent of reading and percent of scale

• Absolute accuracy of the measurement must becalculated

Readout Specifications




•Current selection

•Filter settings

•2 or 3 wire vs. 4 wire

•Long cables, poor quality•Thermoelectric EMF

•Drift, calibration, environment

Other Readout Issues




•The temperature calculation might not beaccurate

• Sensor drift

• Erroneous characterization coefficients

• Poor calibration

•The temperature of the sensor might bedifferent from the target

• Temperature gradient• Self-heating

Other Sources of Error




•Specified for stability, not accuracy

•Stability is affected by time and use

•Stability must be verified

Reference Probe- Properties




• A probe that passes calibration doesn’t necessarilymean it is “In Tolerance”

• Tolerance is calculated by comparing currentcalibration data with previous calibration data

Is My Probe In Tolerance?




• Periodically measure the resistance at the triple point ofwater (RTPW)

• Maintain a control chart of the values

• If TPW not available, use ice-point, other fixed-point, etc.

Reference Probes – Prevent Out of Tolerance!




SPRTSN1234CONTROLRTPWCHART

date

RTPW

25.532155

25.532186

25.532215

25.53215

25.53216

25.53217

25.53218

25.53219

25.53220

25.53221

25.53222

25.53223

25.53224

25.53225

MAR01_93

APR02_93

OCT20_93

MAR13_94

MAY04_94

OCT19_94

DEC15_94

JAN15_95

RTPW Drift Control Chart




• Drift• Drift is a natural occurrence with resistors (including resistancethermometers) and can increase or decrease with time.

• Usually the drift is minimal and does not adversely affect probeperformance

• Drift is not reversible• Suggested Measurements

• Frequent Triple Point of Water Checks

• Others (Ice point, Gallium)

Reference Probes




• Stress, strain, and shock• Caused by vibration and slight physical impulses, ordropping (shock)

• Introduces mechanical deformations in element withresulting increase in Rtpw

• Changes can be as large as 0.050°

C• Most effects are reversible by annealing

Causes of Drift




Cold quenching

• Caused by rapid cooling of a hot (> 500 °C) probe

• Introduces crystal lattice vacancies with resulting increase in Rtpw

• Changes generally do not exceed 0.025°

C• Most effects are reversible by annealing

Causes of Drift






• Oxidation and strain can be reduced by annealing inhigh-quality PRT probes

• Platinum begins annealing ~400°C

• Typically 480°C or 665 C is used

Probe Annealing




• When a probe is calibrated, check theRTPW:

1. Before sending to calibration lab

2. When probe is returned to calibration lab

3. Periodically during use

• Know probe’s drift before the cal lab tellsyou the bad news

• If possible, update RTPW in readout to

remove drift• Save money, send the probe out for

calibration when it needs it

Summary: Close the Loop




• Drywells (& Metrology Wells)

• Liquid Baths

• Others?

Heat Sources




Drywells work in industrial calibration because:

• Portable

• Fast

• Wide temperature ranges• Relatively inexpensive

• Easy to use

• Performance is typically adequate

Drywell Strengths




Uncertainty contributions for drywells

• Axial and radial uniformity

• Stability

• Display accuracy

• Block loading• Immersion depth

• Environmental conditions

• Procedural variance

• Probe fit and positioning issues

Drywell limitations

H d d t i d ll




To answer this question you need to know:

•How the drywell will be used

•Important sources of error

•How drywell manufacturers’

specifications are written and applied

How do you determine dry-wellaccuracy?

How you use a dry well greatly




Temperature range• Generally errors are greater the further away from ambient

temperature

Will dry-well temperature be measured using anexternal reference or the internal control sensorand display?

• Each method is valid, but an external reference will generallyprovide better uncertainties

How you use a dry-well greatlyaffects performance!




• Metrology wells and drywells are used forcomparison calibrations

• Comparison calibrations require thermal equilibrium

and consistency

• Without thermal equilibrium no comparison can be made

• Equilibrium requires stability

• Consistency allows comparisons to have meaning over

time and between different tests

• Good consistency requires similar loading, low drift, goodhandling practices and verification

What errors are significant?




Errors depend upon mode of use

External Reference

1. Axial Uniformity

2. Radial Uniformity

3. Stem conduction4. Loading Effect (Very Little)

5. Stability

6. Reference temperaturemeasurement

– Reference Probe – Reference Readout

– Hysteresis

Internal con trol sensor andcal ibrated d isplay

1. Axial Uniformity

2. Radial Uniformity

3. Stem conduction

4. Loading effect

5. Stability

6. Reference temperature measurement

– Sensor and display drift – Hysteresis

– Sensor Calibration




Estimate of Significance




Axial Uniformity

Insert

Measureme

nt Zone

ProbeSensors

• The temperature difference between thetop and bottom of the well

• Measurement zone is where axialuniformity is smallest

• EA-10/13 requires 40mm (1.5 inches)

• Hart recommends 60mm (2.25 inches)

• Axial uniformity in the measurement zoneneeds to be known to determineuncertainty

• Hart has a special probe designed to measurethis error

• Axial uniformity error is minimized by• Dual-zone control

• Ensuring probe sensor fits in measurement zone

• Aligning the centers of the sensing elements inthe reference and UUT






Stem Conduction

d

15-20d

• Heat conducted up the sheathof the probe• Causes non-equilibrium between

sensor and source

• This is a function of the sizeand type of material

• Large diameter probes conduct moreheat

• Alumina conducts more than Inconel

• This error is minimized bydeeper immersion

• Fluke suggests 15 times the diameter of

the probe + sensor length

• An 8mm probe should have about150mm

• Metrology wells have the extra depthneeded to minimize this error




• Added Heat Loss due to increased numbers orsize of thermometers, creates a shift in thetemperature gradient in the insert (block) of thedry-well.

• The Temperature Controller cannot completelycompensate for this shift.

• The result can be a temperature error that is

particularly apparent in the Direct Mode.

Block Thermal Loading




Loading Effect

• The number of probes willimpact the amount of heatdrawn from or into the well

• Loading effect is minimizedby well design

• Deeper immersion• Dual-zone control




Temperature Stability

• Stability is temperature variation

with time

• Time frame needs to be

specified

• EA10/13 recommends 30min

• Should be stated with high

confidence

Use of TYPICAL to be avoided !


249.75

249.8

249.85

249.9

249.95

250

250.05

250.1

250.15

250.2

250.25

1 0: 00 :0 0 1 0: 03 :2 0 1 0: 06 :4 0 1 0: 10 :0 0 1 0: 13 :2 0 1 0: 16 :4 0 1 0: 20 :0 0 1 0: 23 :2 0 1 0: 26 :4 0 1 0: 30 :0 0

±0.181°C – 30 min

±0.046°C – 15 min

±0.017°C – 30 min

±0.011°C – 15 min





• Stability is required to reachthermal equilibrium• Probes need time to reach

equilibrium with surroundings

• Multiple measurements rarely

instantaneous

• Temperature stability error

minimised by design• Accurate control with good

resolution• Off the shelf controllers do not

provide exceptional stability


249.75

249.8

249.85

249.9

249.95

250

250.05

250.1

250.15

250.2

250.25

1 0 :0 0: 00 1 0: 03 :2 0 1 0: 06 :4 0 1 0: 10 :0 0 1 0 :1 3: 20 1 0 :1 6: 40 1 0: 20 :0 0 1 0 :2 3: 20 1 0 :2 6: 40 1 0 :3 0: 00

±0.181°C – 30 min

±0.046°C – 15 min

±0.017°C – 30 min

±0.011°C – 15 min




Control Sensor Hysteresis

Control Sensor Hysteresis

0

100

200

300

400

500

600

700

0 100 200 300 400 500 600 700

Set Point Temperature, C

ActualTempera

ture,C

Heating

Cooling

Average Value

Temperature Range

Midpoint

Error Error




est PracticeProbe fit and position

• Fully Immerse Probes.

• Reference and UUT at Same Depth.

• Similar Diameters.

• Similar sheath heat conduction characteristics.

• Snug Fit Into Sleeves.

• Allow Ample Time For Stability.




Liquid Baths

•Liquid baths are moreuniform heat sources

for comparison

measurements

• Always use aReference Thermometer

•Follow same guidelines

as drywells for bestmeasurement

uncertainty



Other heat sources

• Air chamber• Poor uniformity & stability

• Not Recommended

• “In-situ calibration”

heat source• Provides the same test

environment as the test

probe sees.

Documents

Impr Ind Temp Cal 2013