In order to meet the requirements for adequate testing and assessment of product:
- general requirements, terms and definitions of metrological characteristics of instrumentation need to be specified and
- procedures for metrological confirmation that instrumentation indeed fulfills these requirements need to be implemented.
GPS standards such as ISO 14978 2002 are intended to ease the communication between manufacturer/supplier and customer/user by faciltating the specification phase of GPS measuring equipment e.g., micrometers, dial gauges, calipers, surface plates, height gauges, gauge blocks, while not necessarily excluding more complicated equipment employed to measurement geometrical characteristics. ______________________________________________________________________________
Sources of errors, accuracy and uncertainty of measurement
A list of typical sources of error & uncertainty in the GPS context is given in [ISO/TS 14253-2:1999 §7]
Measurement errors are quantified in terms of measurement accuracy. Calibration of in principle each element of a measurement system (instrument, method, operator etc) with metrological traceability to absolute measurement references is essential in ensuring quality-assured measurement which in turn assures products with fit-for-purpose quality.
The GPS standards refer to a “Procedure for Uncertainty Management – PUMA” to be applied when quantifying a measurand, be it a GPS characteristic of a workpiece or a metrological characteristic of a GPS measuring equipment [ISO/TS 14253-2:1999].
PUMA management refers here to choosing a measurement system with the ‘right’ or ‘fit for purpose’ measurement capability for the task at hand.
Note: Uncertainty management means more than just choosing an appropriate “principle of measurement on the basis of experience and available measurement instruments present in the company” [ISO/TS 14253-2:1999 §6.3]. Above all, the demands on the measurement system have to be matched with the conformity assessment task at hand, such as adapting measurement capability to process capability.
The target uncertainty (UT in GPS notation) is a specification limit on the measurement capability of the measurement system employed.
- ISO 10012 Metrological confirmation for measuring equipment & processes
- ISO/TS 14253-2:1999 Calibration of measurement equipment and in product verification
- ISO 14978 2002 Geometrical Product Specification (GPS) —General concepts and requirements for GPS measuring equipment