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Pressure Calibration Terminology
Source: | Author:SW | Published time: 2025-01-03 | 26 Views | Share:

Accuracy vs. Uncertainty

Accuracy and uncertainty are two of the most common terms used to determine the specification of pressure measuring and controlling devices, however, they are often confused with each other. 
 
According to the International Vocabulary of Metrology (VIM), measurement uncertainty is defined as the "parameter associated with the result of a measurement that characterizes the dispersion of values that could reasonably be attributed to the measurand" or a measure of the possible error in the estimated value as a result of the measurement. However, in day-to-day terms, it is basically the accumulation of all the systematic components that contribute toward the overall error in measurement. The typical components contributing toward an instruments’ measurement uncertainty are the defined uncertainty of the reference instrument, effect of ambient conditions, the intrinsic uncertainty of the instrument itself and the deviation recorded in measurement.
 
Accuracy, on the other hand, is defined in the VIM as the "closeness of agreement between a measured quantity value and a true quantity value of a measurand."  Accuracy is more of a qualitative concept rather than a quantitative measurement. Manufacturers often use this term to represent the standard value of the maximum difference between measured and actual or true values.

So what does it really mean for pressure calibration?

With pressure as a measurand, the uncertainty of the instrument is dependent on the reference calibrator’s uncertainty, the linearity, hysteresis, and repeatability of measurements across measurement cycles, and the compensation for ambient conditions such as atmospheric pressure, temperature, and humidity. This is typically reported at a certain coverage factor. The coverage factor determines the probability density of the stated uncertainty using a numerical factor to derive the expanded uncertainty. The coverage factor is usually symbolized with a letter “k.” For example, k= 2 represents a 95% confidence level in reporting the expanded uncertainty, while k = 3 represents a 99% confidence level. It’s typical in pressure calibration that expanded uncertainty is reported with a confidence level of k=2.
 
Accuracy being a qualitative concept allows for more flexibility in interpretation and may lead to different definitions from different manufacturers. As accuracy is the overall representation of the closeness of values, it often encompasses the contributions of measurement uncertainty, longterm stability, and a guard band over an interval of time. The purpose of this term is to provide the user with an estimation of the overall worst-case specification of their instrument over the stated time interval.