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EUROMET Project 509: A comparison of platinum-iridium kilogram mass standards.

Davidson, S (2013) EUROMET Project 509: A comparison of platinum-iridium kilogram mass standards. NPL Report. ENG 47

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The kilogram is unique among the base SI units in being the only one defined by an artefact rather than with relation to naturally occurring physical constants [1]. This imposes a unique set of problems relating to the maintenance and traceability of the mass scale worldwide. All mass measurements must be ultimately traceable to the International Prototype of the Kilogram (K) held at the BIPM (Bureau International des Poids et Mesures). This traceability is achieved via copies of the Prototype held by National Measurement Institutes (NMIs) and maintained by the periodic re-verification of these copies by the BIPM [2]. A major problem with maintaining this traceability is the inherent instability of the kilogram artefacts. The national copies of K accrete surface contamination over time which leads to an increase in their mass values. Without reference to K it is difficult to assess the magnitude of this mass gain and the traceability of measurements is compromised to some extent.

The purpose of this project was to compare the values of the national copies of the prototype of 18 European NMIs. The comparison was piloted by NPL (UK) who circulated two platinum-iridium transfer standards using a petal type comparison, each petal consisting of three or four participants. While the results obtained can be used to demonstrate equivalence between the participants, the main objective of the work was to calculate the relative mass gains of the national copies, to analyse this data, and to draw conclusions about the relative stabilities of the kilograms. Each participating NMI uses an algorithm to predict the mass gain of their national standard. The results gained from this comparison have been used to evaluate the accuracy of these algorithms and also to recommend a best fit algorithm for the modelling of mass gain of national standard kilograms.

Item Type: Report/Guide (NPL Report)
NPL Report No.: ENG 47
Keywords: kilogram, primary standard, stability, platinum-iridium
Subjects: Engineering Measurements
Engineering Measurements > Mass, Force, Pressure
Last Modified: 02 Feb 2018 13:14
URI: http://eprintspublications.npl.co.uk/id/eprint/6104

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