No abstract

No abstract

The Joint Committee for Guides in Metrology, Working Group 1, JCGM-WG1, is currently revising the ‘Guide to the Expression of Uncertainty in Measurement’. In this communication, the motivation for undertaking such a revision is given and the main changes with respect to the current, 2008 edition are outlined.

The main objective of the certification of a reference material is to determine its property values including their uncertainty. In the previous parts of this series, the basis for the evaluation of measurement uncertainty of the property values has been examined, including the conversion of homogeneity and stability study data into standard uncertainties. In this final part, the determination of the property values and the modelling of the certification process is discussed. It is noted that the characterisation of a reference material can be modelled in some cases using analysis of variance statistics, but a more generally applicable model can be developed based on χ 2 -fitting. Furthermore, it is concluded that there is an advantage in using absolute standard uncertainties instead of relative ones when modelling the certification process.

A generic method for the preparation of primary gas mixtures is described. This description forms the basis for a detailed analysis of all sources of uncertainty. Expressions are derived for the combined standard uncertainty in a single weighing cycle as well as for a transferred mass of gas. The expressions are rigorous, as they take into account all aspects, including dependencies between variables introduced by the preparation process. The strengths of these dependencies are evaluated in a generalized way, in order to find out whether the rigorous expressions can be simplified without limiting their validity under practical conditions. The model can be simplified by assessing the covariance between results of weighing cycles, as it can be demonstrated that this covariance will never become significant compared with the uncertainty in the result of a weighing.