Smart temperature transmitters, with their flexibility and enhanced accuracies, have become the number one temperature calibration workload for instrumentation professionals. Calibrating a HART smart temperature transmitter requires an accurate temperature simulator or temperature source, mA measurement, and a HART communication tool for calibration. You can use separate tools or a calibrator that integrates all three to perform this task.
Before going to the field: Gather the needed calibration and communication test tools. If testing a RTD transmitter, be sure to bring extra test leads for connections. Testing a 3-wire RTD requires five (5) test leads, three for simulating the RTD sensor and two for measuring the mA signal. If using a separate communicator, you will need its test lead set as well. For thermocouple (TC) calibrations, be sure to have the correct TC test wire type with a mini-connector terminated with the correct TC connector type, (i.e. Type K wires and connector to simulate a Type K thermocouple).
To get the accuracy needed: As a rule of thumb, your mA measurement tool and temperature source calibrator should be at least four times more accurate than the device being tested. To make that determination, the data sheets of both the transmitter and the calibrator being tested need to be considered. Be sure to account for temperature and stability (time) in addition to the accuracy the devices have specified. For more information on determining accuracy and interpreting accuracy, see the "Interpreting Specifications" application note referenced at the end of this article.