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First official comparative measurement of the implementation of the new definition of the kilogram
On November 16, 2018, the 26th General Conference on Weights and Measures decided on a revision of the International System of Units (SI). Since the introduction of the new SI on May 20, 2019, all SI units have been based on fixed values of seven fundamental constants. In this new system of units, the kilogram is derived from a fixed value for the Planck constant [1-4].
In order to enable international comparability and future recognition of different implementations of the unit of mass, the Consultative Committee for Mass and Related Quantities (CCM) carried out a first international comparison measurement (Key Comparison CCM.M-K8) of the implementations the redefinition of the kilogram was carried out under the direction of the International Bureau of Weights and Measures (Bureau International des Poids et Mesures, BIPM). In addition to the BIPM, which also took part in the comparative measurement with a kibble scale, the following six national metrology institutes were represented: KRISS (South Korea, kibble scale), NIM (China, joule scale), NIST (USA, kibble scale) ), NMIJ (Japan, 28Si ball), NRC (Canada, kibble balance), PTB (Germany, 28Si sphere). Each participant determined the mass of one or two 1 kg transfer standards in a vacuum with the help of his realization experiment.
PTB used a kilogram prototype made of platinum-iridium and a silicon sphere as transfer standards. The mass of these transfer standards was compared to the mass of two in a vacuum 28Si balls with the designations AVO28-S8c and Si28kg01a, which consist of two different 28Si single crystals produced and used as primary standards of mass  were determined. The relative standard uncertainties (k = 1) of the two realizations of the definition of the kilogram amounted to 1.6 · 10-8 (AVO28-S8c) and 1.4 x 10-8 (Si28kg01a). For the respective transfer standard, there was a relative mass difference between the implementations (AVO28-S8c and Si28kg01a) of 9 · 10-9.
In the BIPM, all transfer standards of the participants were compared in a vacuum with a reference standard. In connection with the mass values determined by the participants on the basis of their realization, the differences between the individual realizations and the reference value, which was calculated as a weighted average from the comparison measurements of the transfer standards at the BIPM, could be determined . Figure 1 shows the agreement of the participants' results with the reference value. The chi-square test for consistency (confidence level 95%) was passed. The agreement of the results of the experiments with the smallest measurement uncertainties (NIST, NMIJ, NRC, PTB) has improved since the last determination of the Planck and Avogadro constants for the revision of the International System of Units (SI) in 2018 . Figure 1 also shows the difference between the traceability of the BIPM working standards to the Planck constant preserved at the BIPM via the International Kilogram Prototype (IPK), H(IPK), and the reference value. The return of the BIPM working standards preserved at the BIPM deviates by 19 µg from the reference value for the implementation experiments. With the completion of this international comparative measurement of realizations of the new definition of the kilogram, the first consensus value for the dissemination of the unit of mass could also be determined by the "CCM Task Group on the Phases for the Dissemination of the Kilogram Following Redefinition" (CCM-TGPfD-kg) [ 5, 7-11]. The next comparative measurement of the implementation experiments is already planned. The official start is expected to be in 2021/22.
Figure 1: Mass differences between the results of the participants, who were assigned to the 1 kg transfer standards on the basis of the respective implementation experiments, and the reference value (KCRV), which was calculated as a weighted mean value from the comparative measurements of the transfer standards at the BIPM. In addition, the difference between the traceability of the BIPM working standards to the Planck constant, preserved at the BIPM via the International Kilogram Prototype (IPK), H(IPK), and the reference value. The uncertainty ranges correspond to the expanded measurement uncertainties (k = 2) this difference .
 Resolutions of the 26th meeting of the General Conference of Weights and Measures (CGPM), Versailles, November 13-16, 2018, link
 PTB: Research on the new SI, link
 Bureau International des Poids et Mesures: The International System of Units (SI), 9th SI Brochure, 2019 - Appendix 2: Mise en pratique for the definition of the kilogram in the SI, link
 Bureau International des Poids et Mesures: The International System of Units (SI), 9th SI Brochure, 2019, link
 Stock, M. et al .: Report on the CCM key comparison of kilogram realizations CCM.M-K8.2019, Metrologia 57 (2020) Tech. Suppl. 07030, link
 Newell, D. B. et al .: The CODATA 2017 values of h, e, k, and NA for the revision of the SI, Metrologia 55 (2018) L13 link
 Consultative Committee for Mass and Related Quantities (CCM): Report of the 16th meeting (18-19 May 2017) to the International Committee for Weights and Measures, link
 Consultative Committee for Mass and Related Quantities: CCM detailed note on the dissemination process after the redefinition of the kilogram, Approved at the 17th CCM meeting, 16-17 May 2019, link
 M. Stock, S. Davidson: Report on the Calculation of the CCM Consensus Value for the Kilogram 2020, link
 CCM Task Group on the Phases for the Dissemination of the Kilogram Following Redefinition (CCM-TGPfD-kg): Calculation of the Consensus Value for the Kilogram 2020, link
 Bureau International des Poids et Mesures: Beginning a new phase of the dissemination of the kilogram, link
Borys, Michael, FB 1.1, email:michael.borys (at) ptb.de
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