Calibration of RTD and Transmitter

Question….



We have quite a few RTD and Transmitters throughout our facility.



Within our group we have had several conversations around the calibration of these devices.



Calibration Option #1

  • Check PLC readout and record temperature
  • Remove RTD wires and connect to Fluke Process Calibrator and Record RTD Temperature
  • Connect Fluke to Transmitter and Source 10%, 50%, and 90% of scale to PLC
  • Record Readings and make adjustments as needed


Calibration Option #2

  • Place RTD into Dry Block and Set to 10%, 50%, 90% of Scale
  • Record Temperatures from PLC
  • Adjust RTD Transmitters as needed


Option #1 does not account for any issues with the RTD itself should the RTD have resistance loss due and is dependent on Fluke and Transmitter Only



Option #2 calibrates the entire loop but takes considerably more time do to the use of the Dry Block.

  • Is the use of the dry block even necessary? Can the calibration be performed using only the Fluke (or a decade box).
 
RTD's can drift. That's why people do loop calibration with a dry block or oil bath because a loop calibration takes into account all the elements of the loop - sensor, wiring, analog input, receiver configuration and display, as you recognized in your post.

Another approach is have a second set of out-of-service RTD's for which a certified calibration is done just before calibration is due, either in-house or by a calibration service. The in-service RTD's are removed and the newly certified RTD's are placed into service at the time of calibration.

Is a dry block necessary?

How do you know that the sensor is outputting the expected output at any given temperature unless the sensor is tested under known, certifiable conditions? There is no check of the accuracy/uncertainty of the sensor unless it is checked at a known temperature.
 
Question….



We have quite a few RTD and Transmitters throughout our facility.



Within our group we have had several conversations around the calibration of these devices.



Calibration Option #1

  • Check PLC readout and record temperature
  • Remove RTD wires and connect to Fluke Process Calibrator and Record RTD Temperature
  • Connect Fluke to Transmitter and Source 10%, 50%, and 90% of scale to PLC
  • Record Readings and make adjustments as needed


Calibration Option #2

  • Place RTD into Dry Block and Set to 10%, 50%, 90% of Scale
  • Record Temperatures from PLC
  • Adjust RTD Transmitters as needed


Option #1 does not account for any issues with the RTD itself should the RTD have resistance loss due and is dependent on Fluke and Transmitter Only



Option #2 calibrates the entire loop but takes considerably more time do to the use of the Dry Block.

  • Is the use of the dry block even necessary? Can the calibration be performed using only the Fluke (or a decade box).
Hi INST3499,

Using a Decade Box will only test the Head Transmitter / Transmitter and not the RTD itself (i.e. PT100). Checking the RTD with the Fluke Process Calibrator is only checking the loop at one point e.g. "RTD still in process or in air with Fluke Process Calibrator connected to 2,3 or 4 resistance wires". Calibration can be performed using the Fluke Process Calibrator in conjunction with a Dry Block e.g. " RTD sitting in Dry Block with Fluke Process Calibrator connected to 2,3 or 4 resistance wires".

Providing both the Fluke Process Calibrator & Dry Block are Certified regularly this is the best way for checking the RTD as you are referencing off two certified process calibrators. Similarly the probe can be checked & referenced from the relevant RTD tables in conjunction with a Ohm-Meter, e.g. " PT100 Resistance tables with correct alpha number & Ohm-Meter connected to the 2,3 or 4 Resistance wires. If both the Red & White wires read the same resistance reference this resistance from the PT100 tables and find the relevant Temperature @ that resistance".

Calibration Option #1:
  1. Check range of Instrument - Take Note.
  2. Set-Up Dry Block to measure 3 point (0%,50%,100%) or 5 point (0%,25%,50%,75%,100%) calibration.
  3. Place probe into dry block.
  4. Take calibration readings from SCADA/PLC Readout.

If an Offset is Present try below:

Calibration Option #2:
  1. Check range of Instrument - Take Note.
  2. Set-Up Dry Block to measure 3 point (0%,50%,100%) or 5 point (0%,25%,50%,75%,100%) calibration.
  3. Place probe into dry block - Connect RTD to Fluke Process Calibrator.
  4. Take calibration readings.
  5. Connect Current Loop to Fluke Process Calibrator.
  6. Simulate Loop - Check Loop for Offset over 5 Points (4mA,8mA,12mA,16mA,20mA)
  7. Take Note.

This way you are checking the Loop and the RTD separately, Time consuming but if there is an Offset present you can determine if it is in the Loop or the RTD. (Dry Block is needed to check Zero (Lower Range Value)& Span (Upper Range Value of the Transmitter).


Hope this helps.
 
Thank you... I feel the same and I was going crazy thinking I was wrong as others were suggesting proper calibration could be achieved without the use of the Dry Block.

This makes me feel better...now to convince the others... I have the less experience in this matter so that is at times challenging.

Best regards,
 
Thank you... I feel the same and I was going crazy thinking I was wrong as others were suggesting proper calibration could be achieved without the use of the Dry Block.

This makes me feel better...now to convince the others... I have the less experience in this matter so that is at times challenging.

Best regards,
Anytime!
Glad to have helped.
 
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