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Maintaining and disseminating the kilogram following its redefinition

The expected adoption of a new definition of the kilogram by the 26th General Conference on Weights and Measures (CGPM) in November 2018 will bring to an end the period during which the kilogram was defined as the mass of a material artefact, the International Prototype of the Kilogram. In the future the kilogram will be defined in terms of the Planck constant, guaranteeing long-term stability of the SI mass unit. The new definition opens the possibility for National Metrology Institutes (NMIs) to develop their own realization experiments: a Kibble balance or an x-ray crystal density (XRCD) experiment.

An important change with respect to the current situation will be that after the redefinition, traceability will, in principle, be available from more than one source. The international recognition of mass measurement traceability to a particular realization experiment will be based on successful participation of that realization in international measurement comparisons. A trial comparison with five NMIs was carried out in 2016[1] with a satisfactory result.

To ensure a smooth transition from the present dissemination scheme from one single institute (the BIPM) to the future dissemination scheme from multiple realization experiments, it is planned that dissemination will be internationally coordinated by a task group of the Consultative Committee for Mass and Related Quantities (CCM). This will come into effect after the adoption of the redefinition and during a transition period, which will continue until the robustness of the new approach has been demonstrated. The BIPM, together with co-authors from the CCM, has published a paper which describes the maintenance and the dissemination of the kilogram following its redefinition[2]. An important conclusion is that for NMIs which do not operate a realization experiment, in principle nothing will change: they can request mass calibrations from either the BIPM or an NMI with a realization experiment. However, the long-term stability of the mass unit will be better than with the present artefact-based definition.

  1. Stock M., et al., A comparison of future realizations of the kilogram, Metrologia, 2018, 55, T1-T7.
  2. Stock M., Davidson S., Fang H., Milton M., de Mirandés E., Richard P., Sutton C., Maintaining and disseminating the kilogram following its redefinition, Metrologia, 2017, 54, S99-S107.
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