Calibrator Uncertainty

When someone mentions calibrator uncertainty, they are talking about what we call accuracy in our specifications. To be strict about it, our accuracy specifications are inclusive of most or all errors. In scientific applications, you might see these all broken out.

Typical examples are errors from linearity, hysteresis, ambient effects and others. In most cases all of these errors are combined into a comprehensive specification. That makes it easier for a user to determine the applicability of the calibrator for a given purpose. This number would then be the total uncertainty of the calibrator for a given calibration.

However, if the calibrator is to be used to make a measurement, you might also have to consider the uncertainty of the primary element or sensor. Let’s take the example of the Martel BETA PTC-8001 being used as a thermocouple thermometer to measure a process temperature. Here, you need to also consider the uncertainty of the thermocouple and thermocouple extension wire to make a determination of the total uncertainty.

In calibrating a transmitter, the sensor is not used, so there is no need to consider sensor error in that case. For resistance sensors (RTDs), you would have to consider an error (lead wire resistance) if you are using only a 2 wire connection. For 3 and 4 wire connections, the lead wire error is too small to be of consideration since we have a compensating measurement.

For calibrating a thermocouple instrument, you must use the correct type of thermocouple extension wire for the cold junction compensation to work correctly. There is a very small error even with the thermocouple extension wire, but it is too small to be of any practical concern. The length of the thermocouple extension wire should be kept short. We typically recommend no more than 1 meter.

We’ll have more on uncertainty in transmitter calibrations in the next post.