8 Steps RTD Calibration Procedure

What is RTD?

RTD stands for Resistance Temperature Detector, a sensor measuring temperature by changes in electrical resistance. Commonly used in industrial applications, it provides accurate and stable temperature readings for various processes.

This below Step by Step calibration procedure provides a thorough explanation of how to calibrate a RTD using reference standards.

Step 1: Prepare the Tools Required for RTD Calibration

1. Necessary hand tools.
2. Multifunction process calibrator.
3. Standard Multimeter.
4. Test leads and probes.
5. Soft Cloth for cleaning.
6. Dry Bath temperature calibrator.
7. Standard Reference Thermocouple
8. Digital Standard Thermometer

Step 2: Safety Precautions

1. For more information on general calibration process recommendations and basic safety in the process business, please click this page.
2. Process Industry Calibration Methodology: Essential Safety and General Considerations
3. Request that the panel operator set the controller’s MOS (Maintenance Override Switch) in manual mode for the RTD control loop and the ESD loop.
4. It is necessary to locate the RTD that requires calibration.
5. Verify that the RTD is the correct one and record any important details, like the tag number (e.g. the manufacturer, model number, temperature range, type of sensor, coefficient factor & etc.).
6. Remember that this fundamental approach may need to be modified based on the specific RTD and the process location. Always follow local safety regulations and the manufacturer’s instructions when working with an RTD or any other component of process equipment.
7. Follow all relevant lockout/tagout instructions to prevent an inadvertent start. Make that the RTD and its signal are separated from the operation.
8. Prioritizing safety is crucial when calibrating an RTD. This includes using the proper PPE, removing and inserting the RTD from dry block, and isolating the process area.
9. Always adhere to local safety regulations and guidelines, and make sure that all personnel involved in the calibration process are aware of and comply with the specified safety measures. 
10. Regular safety training and awareness can significantly contribute to a safer work environment during RTD calibration.

Step 3 : Prepare the Calibration Setup for RTD

1. Ensure that the location chosen for RTD calibration, as well as the positioning of calibration tools, is completely without of electromagnetic interference and vibrations. Additionally, ensure the calibration area is well-lit and adequately ventilated.
2. Collect all the necessary tools and supplies essential for the RTD calibration.
3.  Refer to the instrument loop diagram to power off the temperature transmitter connected to the RTD undergoing calibration. Remove any wires or connectors from the nearby junction box or Marshalling panel accordingly.
4. Identify the RTD cable linked to the temperature transmitter, usually marked with a core. This cable might be connected to a separate terminal block or connector on the RTD temperature transmitter.
5. Inspect the removed RTD cable for signs of wear or damage before re-attaching it to the temperature transmitter; replace it if necessary. Label, insulate, and secure the removed cable for later reconnection post-calibration.
6. Carefully extract the RTD from the process equipment and place it in the calibration area. Once the RTD temperature normalizes to room temperature, clean the sensing surface to remove any dust.
7. Get  the temperature calibrator and a suitable thermowell, considering the RTD size and process temperature range. Refer to the instrument data sheet for the measuring range.
8. Ensure that all relevant documentation, including the manufacturer’s manuals for the RTD and calibration tools, is available and up-to-date.
9. Prior to use, check the calibration status and validity of the multifunction process calibrator, standard multimeter, and other calibration tools.
10. Take note of the area’s surrounding temperature and humidity levels as these can have an impact on the accuracy of the calibration.
11. Place the RTD in the dry block calibrator with the appropriate thermowell. Obtain a suitable standard thermocouple with a thermometer insert along side the RTD in the thermowell of the calibrator to compare the temperature in the dry block calibrator.
12. Ensure that the RTD is immersed to the correct depth in the dry bath calibrator, considering the immersion depth specified in the instrument data sheet.
13. Use a multifunction process calibrator and connect the RTD terminals to measure the output resistance in terms of temperature. For a 4-wire RTD, connect all four wires to the multifunction calibrator using probes and leads, following the diagram provided.
14. Upon completing the connections as illustrated in the diagram, the setup is now ready for the calibration of the RTD.

How do you calibrate a RTD sensor?

Step 4 : Calibration of RTD

1. Check the connections and ensure that there are no loose wires or poor connections.
2. Power on the dry block calibrator and set it to the desired temperature for calibration. Consider the type of RTD for calibration, adjusting the dry bath calibrator’s set temperature accordingly, incorporating any range locks.
3. Take precautions not to exceed the temperature range of the RTD; consult the data sheet and nameplate for capacity information. Adjust the dry bath calibrator’s temperature as needed based on this information.
4. Allow sufficient time, typically several minutes, for both the RTD and dry bath calibrator to stabilize at the same temperature.
5. Monitor the multi-function calibrator for the RTD’s measuring temperature indication, observe the dry bath calibrator’s reading, and note the reference thermometer’s indication at the reference temperature.
6. Confirm the stability of the dry bath calibrator and the multifunction process calibrator readings before starting the calibration procedure.
7. Record both temperature readings from the calibrator and corresponding resistance values from the RTD using the multi-process calibrator.
8. Configure the multi-function calibrator to measure temperature instead of resistance by referencing the RTD’s nameplate for the proper coefficient factor.
9. Repeat the calibration process for each calibration point, ensuring enough time for stabilization before recording measurements. If temperature adjustments are made during the process, re-stabilize the system before recording final readings.
10. Repeat the procedure for various reference temperatures, spanning the entire range from 0% to 100% on the RTD’s scale, to comprehensively calibrate it.
11. Calculate the expected resistance from the RTD concerning the known temperature, utilizing the RTD coefficient factor (e.g. 0.00392, 0.00385) and the RTD type (e.g., PT100, PT500).
12. Check for any calibration adjustments in the RTD; typically, RTDs do not require correction. Refer to the manufacturer’s provided accuracy specifications and ensure that the results are within tolerance. If not, consider for replacing the RTD.
13. Before initiating the calibration procedure, carefully review the manufacturer’s instructions, as the process may vary depending on the specific dry block calibrator used.

Step 5: Enhance RTD Calibration with Calculators:

What is RTD formula?

Rt = Rref × [1 + α × (T-Tref)] In this equation, Rt represents RTD resistance at a given temperature (in ohms), Rref represents RTD resistance at the referenced temperature (in ohms), and α is the temperature-resistance coefficient (in ohms per ohm/degree). The ohms are commonly represented as Ω in equations. Refer to the following calculators for improving the accuracy of RTD calibrations.

1. Click here for RTD Temperature Coefficient Calculation to learn about RTD temperature coefficients and their impact on resistance variations.
2. Click here for RTD Calculator: Converting Measured Resistance to Process Temperature to utilize this calculator to convert RTD measured resistance to accurate process temperature readings.
3. Click here for RTD Calculator: Converting Process Temperature to measured Output Resistance to access this calculator to convert process temperature values into corresponding RTD measured output resistance for precise temperature measurements.

Step 6: Recording Calibration and Verifying Linearity

1. Include inputs in both the upscale and downscale temperature directions from the dry block calibrators, corresponding to the output values indicated on the RTD in the multifunction calibrator.
2. Recalibration is necessary if the RTD output value falls outside the tolerance specified by the manufacturer. In case of further deviation from the permitted range, replace the RTD with a new one.
3. If every output value (+/- %) of the RTD is within accepted limits, no additional calibration is required.
4. Fill in the “as found/as left” column of the blank calibration report with the RTD output values.

Step 7: Completion of Calibration:

1. After a successful calibration, affix the calibration label to the RTD.
2. Clean the apparatus, store it securely, and record the calibration data for future reference.
3. Remove the calibrators from their connections.
4. Reinstall the RTD in the processing area.
5. Ensure the workplace is tidy.
6. De-isolate the equipment.
7. After reinstalling the RTD in the processing area, perform a functional check to ensure that the RTD is operating correctly within the control loop.

Step 8: Calibration Report Preparation

The calibration of the RTD is demonstrated in the sample report using a thermometer and a dry bath temperature calibrator, with a standard thermocouple serving as the reference.

For an Excel template used in creating the RTD calibration report, you can access it through the link provided.

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RTD Calibration Procedure: Aligned with NIST and IEC 61508 Standards

To achieve the highest levels of accuracy, reliability, and safety in temperature measurements inside industrial processes, our RTD calibration procedure has been carefully developed in compliance with both NIST and IEC 61508 specifications. 

NIST compliance ensures outstanding precision, whereas IEC 61508 compliance provides a systematic and safety-focused approach throughout the RTD’s lifecycle. This comprehensive calibration method improves temperature sensing capability while also aligning with industry best practices for both accuracy and safety.