How to Calibrate an RTDUsing a Dry-Block Calibrator

Kurt ZabriskieFluke Calibration

Fluke – American Fork, UT

• Kurt Zabriskie• 17 Years with Fluke/Hart Scientific

– Western Regional Product Manager - Temperature• Kurt.Zabriskie@flukecal.com

AgendaHow to Calibrate an RTD Using a Dry-Block Calibrator

• Quick Dry-block introduction• Dry-block sources of error• Finding solutions for common errors and problems• Calculating uncertainties• How to calibrate probes with odd shapes and sizes• Temperature range concerns• Liquid-in-glass thermometers• RTD calibration example• Dry-block maintenance• Summary

Dry-Block Introduction

Dry-Blocks/Dry-WellsModerate accuracyFixed hole diameterFixed immersion depthDry and cleanPortableFaster temperature changesInternal reference probe

9100SHandheld Dry-well

9190AUltra-CoolField Metrology Well

914X SeriesField Metrology Wells

9103 9140 9141Dry-Block Calibrators

Dry-Block Sources of Errors

• Immersion effects• Stem conduction• Well contact errors• Display accuracy• Stability• Uniformity (radial & axial)• Loading

Dry-Block – Immersion Effects

• Immersion depth of the UUT is critical

• Dry-blocks are generally calibrated by fully inserting a reference probe

Dry-Block – Immersion Effects

• By adding a reference probe, we can be less concerned with the control probe location

• The UUT and Reference can be moved to different depths

• Immersion depth should still be considered

• (15 X Probe Diameter)

• Many Fluke dry-blocks include a reference Input

Dry-Block – Stem Conduction

• May impact the ability to make a good measurement

• The probe sheath acts as heat sink

• Probe diameter matters, length is less critical

• (15 x probe diameter) a good rule to follow

Heat

Dry-Block – Well Contact Errors

• Fit is important

• Loose fitting probes exhibit erratic behavior

• Wells should be 0.005”-0.010” larger than the diameter of the probe for 0.5” diameter probes and smaller.

• Too snug, and the probe may become stuck

Dry-Block – Display Accuracy

• Generally a one year specification on a spec sheet

• Internal control sensors are designed to be robust

• Display accuracy can be one of the largest contributors to the overall uncertainty

Dry-Block – Stability

• Check dry-block specifications

• Units generally come with optimized proportional bands

• Fine tuning the proportional band may help at specific temperatures

Dry-Block – Axial Uniformity

• Axial Uniformity – Variation in the temperature along the axial length of the insert (block) within the measurement zone

• Generally inherent to the dry-block

• The closer the sensing element is to ambient air, the larger the uncertainties

• Higher temperatures present larger errors

• Look for dry-blocks with a calibrated zone for optimal results

Dry-Block – Radial Uniformity

• Radial Uniformity – Variation in the temperature between different wells of the insert (block) within the measurement zone.

• Mostly inherent to the dry-block design

• Heater placement and profiling is critical during engineering of the dry-block

Dry-Block – Loading

• Loading can impact uncertainties.

• Some models have a specification

• Units without specifications can be evaluated in the field

Calculating Uncertainties

• RSS method is generally used to calculate uncertainties when using a reference

23

22

21 )()()( bbbbtotal

222 )()()( uniformitystabilityrefbtotal

222 )02.0()1.0()05.0(113.0 CCCC

Calculating Uncertainties

• GUM compliant example

-38 °C 0 °C 157 °C 232 °C 420 °C 660 °C

Uncertainty Sources: Type mK mK mK mK mK mK

Process Var. (check std) Norm 3.0 3.0 5.0 5.0 5.0 6.0

UUT Precision (noise) Norm 2.8 2.8 2.8 3.3 4.4 5.6

Ref. Precision (noise) Norm 2.8 2.8 2.8 2.8 2.8 5.6

Ref. Calibration Norm 0.2 0.1 0.6 0.5 0.6 1.1

Ref. Drift Rect 1.7 2.0 3.2 3.8 5.1 6.7

Radial Uniformity Rect 10.0 10.0 30.0 30.0 30.0 30.0

Axial Uniformity Rect 6.3 6.3 6.3 6.3 6.3 6.3

Readout (SPRT) Rect 0.2 0.3 0.4 0.5 0.7 0.9

Readout (UUT) Rect 1.3 1.5 2.5 3.1 4.4 6.3

Insulation Leakage Rect 10. 0 10.0 10.0 10.0 10.0 10.0

UUT Repeatability (TPW) Norm 2.8 3.3 5.3 6.3 8.6 11.2

Total (k=2): 20.6 21.0 40.7 41.5 43.9 47.9

Dry-Block – Odd Shaped Probes

• Custom inserts are an option

• Micro-Baths might be a good option to explore

Temperature Range Concerns

• –100 to 1200 ºC• May need to use multiple

units • It’s ok to switch from one

dry-block to the next Model 9190A:–95 to 140 ºC

Model 9143: 33 to 350 ºC

Model 9150:150 to 1200 ºC

Dry-Block – LIG’s

• Liquid and glass thermometers are not recommended for use in a dry-block calibrator

• Mercury thermometers are on their way out due to environmental concerns

Dry-Block – Maintenance

• Keep those wells clean, Scotch-Brite pads and a gun cleaning kit work nicely

• Avoid dropping inserts or other heavy objects into the well

• Avoid the use of thermo grease

• Recalibrate regularly

• Verify stability

RTD Calibration Example

• Three point RTD Calibration• –95 ºC, 0 ºC, 140 ºC• Utilize the 9190A with the “Process

Option” to measure the UUT • Set the 9190A to each of the three

set points, generally starting with the lowest point

• Allow for plenty of soak time at each temperature (15 minutes)

• Record a resistance at each set point• Utilize at tool such as TableWare to

calculate coefficients

TableWare Software (Model 9933)

When the Super-DAQ is connected to a Fluke Calibration dry-well, fluid bath, or furnace, it can control the temperature source to calibrate up to 40 sensors automatically.

You simply program the setpoint temperatures and their values, select a scan sequence, assign a reference channel, and set the required stability band.

The Super-DAQ monitors the temperature source’s stability through the reference channel, collects the data from the reference probe and the “unit under test” (UUT) once stabilized, and then advances to the next set-point temperature.

After you configure and start the test, you can walk away to work on other things. The Super-DAQ just made your day a whole lot easier!

Automate temperature sensor calibration with the 1586A Super-DAQ

9190A Ultra-Cool Field Metrology Well

1586A Super-DAQ with DAQ-STAQ

Application Note and Video

Application Note: Automating Temperature Sensor Calibration with the 1586A Super-DAQ

Companion Video: Automating Temperature Sensor Calibration with the 1586A Super-DAQ

The application note and video demonstrate the Auto Test function of the 1586A using a 9142 Field Metrology which can be substituted with other Fluke Calibration dry-wells, fluid baths, and furnaces.

Dry-Block – Summary

• Dry-blocks are a great option in many situations

• Highly portable and quick to change temperature

• Sources of error should be considered

• On board references are a great way to combine several instruments into one

• Be sure to contact us any time for help with your specific application

Where Can I Find Out More?• Comparison Calibration Application Notes

– 914X Series Field Metrology Well– 9190A Ultra-Cool Field Metrology Well

• Product information at: www.Transcat.com/Fluke

Limited Time Offer

• Free Temperature Probe with Metrology Well Purchase• Order from Transcat

• Details at: www.transcat.com/Fluke• 800-800-5001

Questions or Comments?Email the moderator, Nicole VanWert:

nvanwert@Transcat.com

Transcat: 800-800-5001www.Transcat.com

For related product information, go to:

www.Transcat.com/Fluke