If you are calibrating a medium to high accuracy RTD, you will probably use the characterization method of calibration. Characterization is the type of calibration in which the device under test (DUT) resistance is determined at several temperature points and the data are fitted to a mathematical expression.

To calibrate an RTD, perform these steps:

1. Place the reference probe and the DUTs in the temperature source. Make sure they are all placed as close together as possible, in a radial pattern with the reference probe in the center of the circle.
2. Connect the leads to the readout(s), using the proper 2-, 3-, or 4-wire connection.
3. Measure the reference probe and determine the temperature. Ideally, you would use a readout designed for temperature work that can measure the resistance and calculate the temperature from calibration coefficients previously entered into the readout.
4. Measure and record the resistance of the DUT(s). Since the DUTs are resistance thermometers similar to the reference probe, they are measured in a similar manner. It is also good practice to close the process by measuring the reference probe one more time.
5. Fit the data. Data fitting is a process in which you solve a set of simultaneous equations which contain the calibration data to arrive at a set of coefficients unique to the RTD and calibration. There are several software programs available to accomplish this task.