The evaluation of measurement systems is done through the Stability, Bias, Linearity, Repeatability and Reproducibility studies.
Stability is the the change in bias over time.
Repeatability is the variation among repeated measurements performed by the same operator, on the same unit, and with the same measurement instrument.
Reproducibility is variation among the average measurements performed by several operators using the same units and the same measurement instrument.
Some uses of the evaluation:
- To accept new measurement equipment (ME)
- To compare multiple ME
- To assess a dubious gauge
- To evaluate a gauge before and after being repaired
A gauge is defined as a measuring instrument for measuring and indicating a quantity.
Note: The selection and accuracy of the measuring device and its relationship to the measurement system uncertainty is critical. Accuracy is the degree of closeness of measurements to that of the actual value. The accuracy of the gauge being used is a critical element and adds to the uncertainty of all calculations. It is a common misunderstanding of users to assume the measuring instrument's reading to be nominal. The accuracy of the measuring device to the tolerance of the measurement being made should be a known factor. The closer the accuracy of the measuring device is to the measurement being made, the larger the uncertainty is for each of the calculations. This is the reason for having a turn down ratio [defined as a measurement term that indicates the range a specific flow meter, or meter type, is able to measure with acceptable accuracy. It is also known as rangeability], i.e.: Test Accuracy Ratio (TAR) or Test Uncertainty Ratio (TUR) for the process.