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Dissemination of the kilogram from the second Consensus Value

The new definition of the kilogram - based on the fixed numerical value of the Planck constant - came into force on 20 May 2019. It underpins the realization of the kilogram by National Metrology Institutes (NMIs) and Designated Institutes (DIs) by means of the Kibble or joule balance, or by applying the X-ray crystal density technique. After reviewing the results from the different realization experiments, the Consultative Committee for Mass and Related Quantities (CCM) decided in 2017 that the dissemination of the kilogram should initially be coordinated internationally, by basing it on the so-called Consensus Value, until such times when the dispersion between values realized by individual NMIs/DIs becomes compatible with their uncertainties[1][2]. The Consensus Value can be seen as an internationally agreed mean of the kilogram realizations, determined by the Task Group on the Phases for the Dissemination of the Kilogram following Redefinition (CCM-TGPfD-kg) of the CCM. It is based on results of comparisons of kilogram realizations and is updated after each new comparison, organized roughly every 2 years.

The first Consensus Value was implemented on 1 February 2021, while the second Consensus Value will come into force on 1 March 2023. The final report of the most recent key comparison used for the 2023 calculation is available in Metrologia.

Subsequent data analysis has led to the determination of the mass of the International Prototype of the Kilogram (IPK) to be 1 kg - 7 μg, with a standard uncertainty of 20 μg. Consequently, NMIs/DIs shall have to reduce the mass value of their national as-maintained mass unit by 7 μg with respect to the mass value based on the IPK or by 5 μg with respect to the first Consensus Value. The adoption of the second Consensus Value requires no further adjustment to the published Calibration and Measurement Capabilities of NMIs/DIs.

The BIPM will continue to provide Member States with calibrations of 1 kg Pt-Ir prototypes and stainless-steel mass standards that are traceable to the Planck constant through the second Consensus Value.

[1] CCM detailed note on the dissemination process after the redefinition of the kilogram

[2] Dissemination from the Consensus Value for the kilogram