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How to Calibrate an RTDor Platinum Resistance Thermometer

©2010 Fluke Corporation.

Common Calibration Techniques

Introduction• Thermometers are transducers• Exhibit a change in a characteristic which is

proportional to the change in temperatureF d t ll lib ti i h t i ti f thi• Fundamentally, calibration is characterization of this relationship

PRTs - Characteristics

• Medium to high accuracy• High purity or doped high purity

platinum sensorWi d thi fil• Wire wound or thin film construction

• Many different types and styles a y d e e ypes a d s y esof assembly

• Very common in industryPhoto courtesy of RTD Co.

PRTs - Characteristics

• Temperature range (-260 to 1000 °C)

• Stable over time and temperatureW ll d fi d th ti ll• Well defined mathematically

• Relatively linear with low sensitivity• Relatively easy to measure &

Photo courtesy of Burns Engineering

• Relatively easy to measure & calibrate

• Available in many configurations

Instruments-Standards-Apparatus

• Reference Probe

• Readout for the reference

• Readout for the UUT (unit• Readout for the UUT (unit under test)

• Temperature source• Temperature source

Reference Probes - Types

• SPRTs • PRTs– Standardized– -200 to 1000 °C range– 0.25, 2.5, 25.5 Ohm versions

– Not standardized– -200 to 1000 °C range– 100 ohm typical0.25, 2.5, 25.5 Ohm versions

– Highly stable & accurate– Typical uncertainties from

0.001 to 0.010 °C

– Quite stable and accurate– Typical uncertainties from

0.010 to 0.025 °C

– Expensive & fragile – Less expensive & less fragile than SPRTs

Readouts - General Requirements

• DMMs provide moderate results• Readouts designed for temperature measurement

provide better results• Readouts designed for temperature calibration

provide best resultsprovide best results• Switch or multiplexer increases efficiency of

measurement system

Readouts - PRTs

E th t th i t i i t f• Ensure that the resistance range is appropriate for temperature range of interest

– 25 SPRTs and PRTs from 4 to 85 (-200 °C to 660 (°C)

– 100 PRTs from 18 to 340 (-200 °C to 660 °C)

Readouts – excitation current

E th t th d t i i th• Ensure that the readout is using the proper source current - too much source current will cause excessive self-heating and calibration errorsg

– Range changes in DMMs cause inconsistent self-heating

• SPRTs -1 mA (25 ), 5 mA (2.5 ), 14 mA (0.25 )

• PRTs - 1 mA is recommended

Temp Sources - Requirements

• Stability & uniformity consistent with desired uncertainty (10:1 recommended by NVLAP)

• Temperature range appropriate for range desired• Temperature sources designed for temperature

calibration provide best resultscalibration provide best results

Temperature Sources - Types

• Dry Wells • Calibration Baths• Dry Wells– Moderate accuracy– Fixed hole diameter

Fi d i i d h

• Calibration Baths– High accuracy– Flexible with immersion depth

and probe diameter– Fixed immersion depth– Dry and clean– Portable

and probe diameter– Can be messy– Not usually portable

Slower temperature changes– Faster temperature changes

– Internal reference probe

– Slower temperature changes– Requires external reference

probe

Temperature Sources - Types

• Dry-blocks for higher temperatures (above ~500 °C)

• LN2 comparison device or variable cryostat for lower temperatures (below -100 °C)

Procedures - PRTs

Ch t i ti T l T ti• Characterization– UUT resistance is

measured at several

• Tolerance Testing– UUT resistance is

measured at several temperature points over a range and the data is fitted to a mathematical formula (mathematical model)

temperature points and the data is compared to defined values at those temperatures No fitting is(mathematical model). temperatures. No fitting is performed.

Procedures - Characterization

M t h t PRT lib ti• Most common approach to PRT calibration• The resistance vs. temperature relationship is re-

determined with each calibrationdetermined with each calibration• Usually, calibration coefficients and an interpolation

table are provided as a product of the calibration

Procedures - Characterization

C• Connect to readout• Insert the reference and UUT(s)

into the temperature source ininto the temperature source in close proximity

• Measure the reference probeMeasure the reference probe and determine the temperature

• Measure and record the UUT resistance

• Fit the data

Procedures - Characterization

P b l tProbe placement• Circular pattern with reference in center

• Sufficient immersion (20*) x (probe diameter) + (sensor length)i.e., 20 x 3/16” + 1” = 3.75” + 1” = 4.75”

* Note: 15X and 20X are often used with some small uncertainty due to immersion error. When practical, immersion of 30X essentially eliminates all error from this component.

Procedures - Characterization

C ti t d tConnection to readout• Proper 2, 3, or 4 wire configuration

• Insure that connections are tight

Procedures - Characterization

R f b tReference probe measurement• Measure temperature directly

• Measure resistance and convert to temperature (linear interpolation)

Procedures - Characterization

Single DMM MethodSingle DMM Method

Procedures - Characterization

D l DMM M th dDual DMM Method

Procedures - Characterization

Thermometer Readout Method

Procedures - Characterization

D t FittiData Fitting• PRTs

– ITS-90– Callendar Van Dusen– polynomialspolynomials

ITS-90 Equations

Resistance Ratio

TPW

9090 R

)R(T)W(T

ITS-90 reference function (above zero)

9

1i

i90

i090r 481)15.754(TCC)(TW

Deviation from ITS-90 reference function)(W)W(T)W(T T

)(W)W(T)W(T 90r9090 T

http://www.bipm.org/en/publications/its-90.html

ITS-90 Characterization

Deviations from the reference function are characterized using calibration coefficients (a,b,c) and the deviation functions below:

• Above zero (0 °C to 660.323 °C)

32

• Below zero (range 5: –38 8344 °C to 29 7646 °C)

390

2909090 )1)(W(T)1)(W(T)1)(W(T)W(T cba

Below zero (range 5: 38.8344 C to 29.7646 C)

))ln(W(T)1)(W(T)1)(W(T)W(T 9090490490 ba

290590590 )1)(W(T)1)(W(T)W(T ba

Example Data Set

4 points + Rtpw Temperature Measured

0.010°C 99.96653 156.599°C 160.89476231.928°C 189.16982

°C300.000°C 214.15407419.527°C 256.72668

Calculate Coefficients using ITS-90 Deviation Functions

S t f tiSet of equations

W a W b WT T T1 1 121 1 ( ) ( )

W a W b WT T T2 2 221 1 ( ) ( )

W a W b WT T T3 3 321 1 ( ) ( )

Solving for a and b using example data: W a W b WT T T4 4 4

21 1 ( ) ( )

a = -5.3581671E-04b = 2.0307049E-05

Procedures - Characterization

PRTs: PolynomialsPRTs: Polynomials

typical expressions take the form:

t R R R R a b c d e2 3 4

Electrical Properties of Platinum Thermometers

• Callendar-Van Dusen equation describes the R vs t relationship of platinum (C=0 when t<0)

10011)( 320

tt

tCBtAtRtR C

100

1100

1)( 0tttRtR C

Precision PRT (α=392) Industrial PRT (α=385)A 3.985 X 10-3 °C-1 3.908 X 10-3 °C-1

B 5 870 X 10-7 °C-2 5 775 X 10-7 °C-2B -5.870 X 10 7 C 2 -5.775 X 10 7 C 2

C -4.000 X 10-12 °C-4 -4.183 X 10-12 °C-4

Procedures - Tolerance Testing

• Typical approach for medium to low accuracy

d i d t i l li tiand industrial applications

• Resistance at temperature T is compared to definedT is compared to defined (table) values

• Usually DIN IEC-751 orUsually, DIN, IEC 751, or ASTM 1137 defined equations are used

Procedures - Tolerance Testing

• ASTM 1137 class A= ±[0.13 + 0.0017|t|]°C

@ 100 °C = [0.13 + 0.0017|100|] = ±0.30 °C

• ASTM 1137 class B[0 2 0 0042| |]°C= ±[0.25 + 0.0042|t|]°C

@ 100 °C = [0.25 + 0.0042|100|] = ±0.67 °C

Example System for Industrial Tolerance Testing

B i t• Basic system– Fluke-744 Documenting Process

Calibrator– Hart Field Dry-Welly

• 9103 for -25°C to 140°C• 9141 for 50°C to 650°C

• Approximate system uncertainty ±0 6°C ith 9141 ±0 4°C ith±0.6°C with 9141 or ±0.4°C with 9103 (rss method)

– 744 PRT measurement accuracy ±0.3°C

– 9141 source accuracy ±0.5°C @ 400°C

– 9103 source accuracy ±0.25°C

Example System for Precision PRTs

• Basic SystemBasic System– 1529 CHUB-E4 Readout– 5626 Secondary SPRT– Deep Immersion Compact

BathBath– 9938 MET/TEMP II

Software• Approximate uncertainty ±0 03°C±0.03°C

– 1529 CHUB ±0.012°C @ 200°C

– 5626 SPRT ±0.009°C @ °420°C

– 6331 combined stability and uniformity ±0.03°C @ 300°C

Example System for SPRTs

• Basic equipment– 1590 SuperThermometer– 5681 Quartz SPRT– 9114 Freeze-Point Furnace– 590X Fixed Point Cells– 9938 MET/TEMP II

Software• Approximate uncertainty ±3mK to ±5mK

– 1590 ±1.5mK– 5681 SPRT ±1mK– 5906 Zinc Point ±1mK

Where can I find out more?

• Application Note• Fluke Hart Scientific Division Catalog• http://www.bipm.org/en/publications/its-90.html

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©2010 Fluke Corporation.