Calibration Levels and Traceable Measurement Results
When it comes to this topic, the applicable differences are particularly relevant when working with temperature calibrators. Inexperienced users are often not aware that there are key differences between accuracy and measurement uncertainty.
In this blog post, I will explain the differences clearly and concisely.
The accuracy of a temperature calibrator describes the deviation between the actual temperature and the temperature produced by the calibrator.
Here’s a simple example:
There is a standard thermometer in the calibrator which indicates 100.005°C.
The calibrator has an accuracy of ±0.070°C.
This means that the calibrator is allowed to deviate ±0.070°C from the true temperature across its entire temperature range.
Therefore, if the calibrator shows 100.000°C, the value of the standard thermometer must be between 99.930 and 100.070°C. In our example above, the value is thus exactly within the permitted deviation (accuracy).
However, the standard thermometer used in our example also has a measurement error. In addition, it is subject to drift, and the measuring bridge connected to the standard thermometer also has its own measurement error.
As a result, accuracy is not simply the deviation from the standard thermometer, but rather the deviation from the true temperature. Therefore, our specified accuracy includes all factors that cause deviation between the true temperature and the temperature displayed on the calibrator. These are:
- The reproducibility of the temperature difference between the block and the calibration insert (only relevant with internal reference sensor)
- The reproducibility of the reference sensor
- The long-term drift (12 months) of the reference sensor
- The measurement uncertainty of the standard thermometer
- The measurement uncertainty of the measuring bridge in the standard thermometer
- The linearization error of the calibrator
Taking all these factors into account, in our example we can confirm an accuracy (difference between the true temperature and the temperature in the calibrator) of ±0.070°C over the entire temperature range.
If, on the other hand, we want to know the measurement uncertainty of a temperature calibrator, further factors come into play:
- How stable can the calibrator keep the temperature?
- What is the effect if several sensors are plugged into the calibrator at the same time?
- Temperature differences between spaced boreholes in the calibration insert
- Different immersion depths in the calibration insert
In a nutshell: The measurement uncertainty includes additional factors relating to measurement error and tells us how accurately the calibrator can work, taking into account all factors that affect measurement uncertainty.
Further explanations of measurement uncertainty in accordance with official guideline DKD-R 5-4 can be found on this poster.
