Measurement uncertainty in the performance verification of indicating measuring instruments

This paper is concerned with measurement uncertainty in the performance verification of the metrological characteristics of indicating measuring instruments to specified tolerances, often called maximum permissible errors (MPE). Performance verification differs from other types of calibrations in that the measurement does not necessarily result in an assigned quantity value. When a measurement involves assigning a quantity value, as is typical with the calibration of material measures or inspection of features on commercial workpieces, the measurand is different than in performance verification. The research literature and published standards and practice for measurement uncertainty typically only address the measurement uncertainty associated with assigned quantity values. When these general approaches to measurement uncertainty are applied to performance verification as well, the measurement uncertainty is not properly estimated and therefore incorrect practice is wide spread in the calibration industry. The purpose of this paper is to clarify the measurand in performance verification and to develop an associated general measurement uncertainty model. Examples are presented that highlight some cases where a measurand associated with performance verification results in a very different measurement uncertainty than when the measurand is associated with the assignment of a quantity value. Some issues for future work are also identified, particularly for consideration in the standardization of specifications for indicating measuring instruments.