Transcat and Ron Ainsworth of Fluke Calibration detail the steps and considerations necessary for the calibration of an RTD with a Dry Block Calibrator.

1. How to Calibrate an RTD Using a
Dry-block Calibrator
Presenter: Ron Ainsworth, Fluke Calibration

2. Fluke – American Fork, UT
Ron Ainsworth
17 Years with Fluke Calibration
Process Calibration Tools Business Manager
ron.ainsworth@fluke.com
Phone: 801.847.1139

3. Agenda
How 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

4. • Process tools
calibrate the
transmitter, not the
sensor
• Sensors will drift
o RTDs
o Thermocouples
o Thermistors
• Largest source of
error
• Don’t forget to
calibrate the
sensor!
Overview of Temp Sensor Calibration

5. Temperature sensor verification
• Fully immerse the unit
under test (UUT) into the
dry-well insert
• The calibrated
temperature zone of a dry-
well is where the probe is
fully inserted
Dry-block
Temp
RTD Probe
resistance
Indicated Temp
(example)
Indicated Error
(Example)
Class A
Tolerance
Class B
Tolerance
0 °C 100.39 1.0 °C +1°C ±0.15 °C ±0.3 °C
100 °C 138.51 100 °C 0 °C ±.35 °C ±0.8 °C
200 °C 175.49 199 °C –1°C ±0.55 °C ±1.3 °C
300 °C 211.33 298 °C –2 °C ±0.75 °C ± 1.8 °C

6. Dry-Block Introduction
Dry-Blocks/Dry-Wells
 Moderate accuracy
 Fixed hole diameter
 Fixed immersion depth
 Dry and clean
 Portable
 Faster temperature changes
 Internal reference probe
9100S
Handheld Dry-well
9190A
Ultra-Cool
Field Metrology Well
914X Series
Field Metrology Wells
9103 9140 9141
Dry-Block Calibrators

7. Dry-Block Sources of Errors
• Immersion effects
• Stem conduction
• Well contact errors
• Display accuracy
• Stability
• Uniformity (radial & axial)
• Loading

8. 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

9. Dry-Block – Stem Conduction
• May impact the ability to make a good measurement
• The probe sheath acts as heat sink
• Probe diameter / length matter in a drywell.
• (15 x probe diameter + Sensor Length) is a good rule
to follow
Heat

10. Dry-Block – Immersion and
Gradient Effects
• Immersion depth of
the UUT is critical
• Dry-blocks are
generally calibrated
by fully inserting a
reference probe

11. Dry-Block – Immersion and
Gradient 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

12. 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

13. Dry-Block – Stability
• Check dry-block
specifications
• Units generally come
with optimized
proportional bands
• Fine tuning the
proportional band
may help at specific
temperatures

14. 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

15. 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

16. Dry-Block – Loading
• Loading can impact
uncertainties.
• Some models have a
specification
• Units without
specifications can be
evaluated in the
field

17. Calculating Uncertainties
• RSS method is generally used to calculate
uncertainties when using a reference
2
3
2
2
2
1 )()()( bbbbtotal 
222
)()()( uniformitystabilityrefbtotal 
222
)02.0()1.0()05.0(113.0 CCCC 

18. Metrology Well Uncertainty Budget
Example 1 – External reference
©2010 Fluke Corporation. 18
from: Understanding the uncertainties associated with the use of Metrology Wells (lit code: 2510068)

19. Metrology Well Uncertainty Budget
Example 2 Without external ref.
from: Understanding the uncertainties associated with the use of Metrology Wells (lit code: 2510068)

20. Dry-Block – Odd Shaped Probes
• Custom inserts are an
option
• Micro-Baths might be a
good option to explore

21. 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

22. 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

23. 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

24. 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

25. Tolerance Testing
• Typical approach for
medium to low accuracy
and industrial applications
• Resistance at temperature T
is compared to defined
(table) values
• Usually, DIN, IEC-751, or
ASTM 1137 defined
equations are used

26. TableWare Software (Model 9933)

27. 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 set point 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

28. 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.

29. 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

30. Special limited-time offer!
• Gift with purchase – up to $1300 value
• Go to www.transcat.com/deals for more
details

31. Questions or Comments?
Email Nicole VanWert-Quinzi
nvanwert@Transcat.com
Transcat: 800-800-5001
www.Transcat.com
For related product information, go to:
www.Transcat.com/Fluke