Benefits of Temperature System Calibrations
By not including a probe in your calibrations, you may be missing out on a major contributor to inaccuracy.
There seems to be a growing trend within the calibration industry to perform electrical simulation of temperature equipment as a method of calibrating temperature systems. Equipment such as displays, controllers and meters can be checked using electrical signals with voltage for thermocouples and resistance for RTDs and thermistors. There are a number of calibrators currently available that can provide one or all the necessary functions to calibrate these temperature instruments. From a calibration standpoint, the electrical simulation is a quick, clean and inexpensive way to verify the accurate function of these measuring devices. Thus it is an attractive method for both users and calibration laboratories.
However, this type of measurement performed on the display can provide a false sense of control and accuracy of the temperature process. In nearly all temperature measurements, the probe/sensor being used is the major contributor to the uncertainty or inaccuracy of the measurement. Any calibrations that take into account the impact of the temperature sensor and connectors as part of a loop measurement may give an incomplete assessment of the systems function. In fact, the European Association of National Metrology Institutes states that “the user must ensure that both the indicator and the sensor have been calibrated, either separately or as a system, in order to make traceable temperature measurements .” Clearly the electrical simulation of the temperature display, while convenient, is not sufficient to establish measurement traceability to NIST or otherwise within a measured process.
When considering the accuracy of measurements using a temperature display and sensor system, the contribution of the sensor is typically much larger than the display. Consider the scenarios below of an RTD system and a thermocouple system with accuracies of the sensor and display shown separately and then combined using the root-sum-square method.
Clearly, the importance of quantifying tolerance contributed by the probe or sensor is critical in situations where the accuracy of the overall temperature system loop.
InnoCal recommends that the probe/sensor and display/meter elements be tested for all temperature systems including freezers, refrigerators, incubators, ovens and furnaces. This can be done by either testing the probe and meter separately against its own tolerance or as a system with the combined through the root-sum-square method. If out-of-tolerance conditions are found within the temperature system, an electrical simulation of the meter or display is appropriate to isolate the issue which is most often on the temperature probe.
When the accuracy of your temperature equipment matters, be sure to request a full temperature system calibration from your provider!