18.11.1 Definition

According to the International Vocabulary of Basic and General Terms in Metrology (VIM) the measurement uncertainty is an estimate characterizing the range of values within which the true value of a measurand lies.

The measurement uncertainty comprises many components, i.e. the random deviations (errors) and the unknown (and therefore uncorrected) systematic deviations (errors) of all quantities that contribute to the measuring result.

According to ISO 14 253-1 for geometrical deviations (Geometrical Product Specifications (GPS)) for measurement uncertainty the statistical confidence level of 95% applies, if not otherwise specified.

18.11.2 Application

ISO 9001:1994 clause 4.11.1 specifies:

Inspection, measuring and test equipment shall be used in a manner which ensures that the measurement uncertainty is known and is consistent with the required measuring capability.

The standard ISO 14 253-1 defines uncertainty zones at the specification (tolerance) limits. The width of the uncertainty range (zone) is the expanded measurement uncertainty U, see Fig. 18.118. For tolerances of form, orientation, coaxiality and symmetry, and run-out, with the one tolerance limit zero, there is only one uncertainty zone at the other tolerance limit, see Fig. 18.119.

Figure 18.118: Conformance zone, non-conformance zones and uncertainty zones (ranges) with specification zone limits, both ? 0

Figure 18.119: Conformance zone, non-conformance zone and uncertainty zone (range) with one specification limit = 0

The standard specifies the following decision rules for the case where the result of measurement falls within one of the uncertainty zones:

• Repeat the measurement with a measurement...