How to Calibrate a Temperature Transmitter

Calibrating a temperature transmitter involves simulating a temperature probe input using an accurate simulator or source and measuring the mA output. To make adjustments if the calibration is out of tolerance, you will also need a HART Communicator. Most process calibrators will have the ability to simulate the temperature sources, measure the mA output and communicate via HART protocol. This procedure is for calibrating a 4-20 mA HART temperature transmitter in the field.

HART temperature transmitter calibration procedure for in the field.

Disconnect the temperature sensor wiring at the temperature transmitter’s terminal board and connect the process calibrator’s temperature source. Be sure to make the correct connections and configure the process calibrator to the appropriate type of sensor for the transmitter (PT100 RTD, Type K thermocouple, etc.).
Connect the Process Calibrator’s mA meter in series with the loop (this method ensures accuracy compared to using the test diode terminal).
Configure the process meter to read the simulated temperature value and the measured mA output.
Test the temperature transmitter’s calibration over five points upscale and downscale. 0% >25% > 50%>75%>100%>75%>50%>25%>0% . Verify that the mA output is within the specified error tolerance (e.g +/- 1% or +/- 0.16mA).
Record the calibration results ‘As Found’.
If the transmitter’s calibration is within tolerance, there is no need to perform any adjustments.
If the transmitter’s calibration is outside of tolerance, first check the loop of any connection issues that may be affecting the loop current. Then, determine whether a digital-to-analogue or sensor trim is required.
1. Connect the HART communicator in parallel.
2. If the temperature displayed doesn’t match the temperature applied, perform a sensor trim.
3. If the mA output displayed doesn’t match the process meter’s reading, perform a digital-to-analogue trim.
After making the adjustments, test the pressure transmitter again to confirm that it is within tolerance and record the calibration results ‘As Left’.
Disconnect the calibration equipment, re-instate the sensor and loop wiring and return to service.

What about the RTD or thermocouple sensor calibration?

How you verify the sensor will depend on your site’s requirements, the criticality of the equipment, or whether you are commissioning a new installation or just performing maintenance. When performing maintenance on a low-criticality service, a two-point verification of the sensor may suffice. This can be a check against a temperature calibration standard to confirm it is within tolerance at ambient and at zero degrees Celsius by submersing the sensor in an ice bath. As for commissioning or if the type of service calls for more stringent testing, a dry-block calibrator with a reference probe may be required to verify the sensor’s calibration over more points.

Calibrating a temperature transmitter on a bench.

In some scenarios, a bench calibration is required. For instance, if a new temperature transmitter is being commissioned. Or, if you are required to use a dry-block calibrator to verify the sensor and it is not practical to take one into the field. The procedure for calibrating a temperature transmitter on a bench is similar to calibration in the field. The main difference is that loop power will need to be supplied to the transmitter. This can be achieved using a workshop power supply. However, most process calibrators these days have loop power and measure functionality. A HART communicator is also required for setting the parameters up in the transmitter, such as LRV, URV, damping, tag number, etc. To effectively communicate with the transmitter, a 250 ohm resistor is required in the series with the loop (some calibrators will have this built into them as well). When commissioning a new installation, sensor and transmitter included, it is convenient to verify them both together with the sensor connected to the transmitter. This involves performing the 5-point upscale and downscale calibration using the dry-block with the process meter measuring the mA output of the transmitter.

How often do temperature transmitters need to be calibrated?

There’s no clear-cut answer to this, ultimately is up to the end user as there are many factors to consider. Things to consider when determining the calibration interval:

The criticality of the service.
How harsh are the process and ambient conditions?
Regulatory requirements of the industry.
Does the transmitter form part of a safety system? If so, calibration intervals will be set to meet SIL requirements.
Stability over time. If a temperature transmitter’s calibration history shows it to be very stable, calibration intervals can be extended.
Commercial or contractual obligations, is the temperature transmitter being used in a custody transfer application?

Should I verify the readings back to the DCS/SCADA?

It’s good practice to verify each calibration point pack to the screen or display where the operator can view it. This confirms that the whole loop is accurate and that there are no issues with the analogue input card. It’s generally not required to record this. Keep in mind, we are verifying the calibration of the transmitter against a reference standard. Comparing a 4-20mA temperature transmitter’s output on a HART 475 to the DCS or SCADA display is not calibration.

What other checks should I do whilst calibrating the temperature transmitter?

Perform some visual checks on the condition of the transmitter, sensor, and thermowell mounting and cables. If you are working in a newly commissioned plant, it’s a good idea to check the transmitter’s parameters against your documentation for discrepancies. If discrepancies are found, do not make changes to the transmitter’s parameters to match the documentation without due diligence. Check first with engineering, Supervision, Technical Authorities, etc. to confirm. Changes could have been made during commissioning that was warranted, however, updates to the documentation could be lagging or redlines could have been lost during the handover process. If your pressure transmitter is installed in a hazardous area, you could nest your calibration scope by completing a hazardous area inspection check sheet.

Summary

This post presented a very basic procedure for calibrating a 4-20 mA HART temperature transmitter in the field. Different sites and industries are likely to have slight variations depending on their requirements and calibration equipment available. The steps listed only consider the act of performing the calibration and it is assumed that safe systems of work are followed.