Beginning a new phase of the dissemination of the kilogram
The new definition of the kilogram, based on the fixed numerical value of the Planck constant, came into force on 20 May 2019. In principle it gives any NMI the possibility to realize the kilogram by using a Kibble or joule balance or by applying the X-ray crystal density technique. Nevertheless, after reviewing the results from the different realization experiments, the CCM decided in 2017 that initially the dissemination of the kilogram should be internationally coordinated, by basing it on the so-called 'consensus value', until the dispersion between values became compatible with the individual realization uncertainties. The consensus value can be seen as an internationally agreed mean kilogram realization and is determined by the CCM Task Group on the Phases for the Dissemination of the Kilogram (CCM-TGPfD-kg). It is based on results of comparisons of kilogram realizations using the different realization methods.
The first key comparison of realizations of the kilogram, CCM.M-K8.2019 has been completed. The objectives of this comparison were to determine the level of agreement between kilogram realizations from different NMIs and to provide information for the calculation of the first CCM consensus value. The CCM started CCM.M-K8.2019 soon after the new definition of the kilogram came into force.
The BIPM was chosen as the pilot laboratory, as it had been for the CCM pilot study in 2016. Seven institutes, including the BIPM, participated with realizations based on Kibble balances, a joule balance and the X-ray crystal density technique. Each participant determined the mass of one or two 1 kg standards under vacuum with their realization method. At the BIPM all mass standards were compared in a vacuum mass comparator with a reference standard. These weighings, together with the mass values determined by the participants, allowed a comparison of the consistency of the individual realizations. The chi-squared test for consistency using the 95 % cut-off criterion was passed, although the two results with the smallest uncertainty were not in agreement with each other (see figure). The weighted mean of the participants' results for a 1 kg mass standard deviates by −0.019 mg from the value based on the BIPM 'as-maintained' mass unit, which is traceable to the International Prototype of the Kilogram (IPK). The results are available from the BIPM Key Comparison Database.
The CCM Task Group on the Phases of the Dissemination of the kilogram has calculated the consensus value as the arithmetic mean of the results of the calibration campaign using the IPK in 2014 , the CCM pilot comparison of kilogram realizations in 2016  and the key comparison reference value of CCM.M-K8.2019 . The consensus value for the mass of the IPK thus determined is 1 kg — 0.002 mg . The standard uncertainty is 0.020 mg. Traceability for the SI unit of mass will be taken from the consensus value of the kilogram commencing 1 February 2021.
Since the consensus value is consistent with the value of 1 kg within its uncertainty, no adjustment of the international mass scale needs to be made. However, NMIs with CMCs below or close to 0.020 mg at the level of 1 kg will need to increase their calibration uncertainty.
The BIPM will continue to provide calibrations of 1 kg Pt-Ir prototypes and stainless steel mass standards for Member States, which will be traceable to the Planck constant through the consensus value.
- M. Stock, P. Barat, R. Davis, A. Picard, M. Milton, Calibration campaign against the International Prototype of the kilogram: Part I: Comparison of the International Prototype with its official copies, Metrologia 52 (2015) 310-316.
- M. Stock, P. Barat, et al., Comparison of future realizations of the kilogram, Metrologia 55 (2018) T1-T7.
- M. Stock, P. Conceição, et al., Report on the CCM key comparison of kilogram realizations CCM.M-K8.2019, Metrologia 57 (2020) Tech. Suppl. 07030.
- CCM Task Group on the Phases for the Dissemination of the kilogram following redefinition, Calculation of the Consensus Value for the Kilogram 2020