Failure to detect a change in the measurand, due to the limits mentioned above, constitutes a measurement error known as resolution error and is an element in the evaluation of measurement uncertainty. Correctly quantifying the resolution error of a measurement requires a two-step analysis:
- a priori, by assessing before measurements are made the limits to resolution due to physical limitations, display structure, and known instrument resolution errors;
- in the field, by observing during the measurement the presence of instabilities, discontinuities in the display, and others, which may indicate the presence of undocumented limits to the resolution.
The resolution error of an instrument is the resolution that the instrument would have under optimal conditions of use. In the evaluation of the latter it is considered irrelevant external factors due to the measurand, the environment or the operator; in this sense the resolution of an instrument represents the uncertainty of the reading of the same, not to be confused with the uncertainty of instrumental measurement (which must also take into account the other metrological parameters).
A correct evaluation of the instrumental resolution would require a specific analysis by a specialized laboratory, which ensures:
- the optimal control of the boundary conditions;
- adequate sample instrumentation;
- a good knowledge of the working principle of the instruments;
- must have their ”own” measurement uncertainty lower by at least one order of magnitude, compared to the expected resolution of the instrument under examination.