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First official comparison measurement of the realizations of the redefined kilogram

Categories:
  • Fundamentals of Metrology
12.04.2021

On 16 November 2018, the 26th General Conference on Weights and Measures (CGPM) approved a revision of the International System of Units (SI). Since the new SI was introduced on 20 May 2019, all SI units have been based on the fixed numerical values of seven constants of nature. In this new system of units, the kilogram is derived from a fixed numerical value of Planck’s constant [1-4].

The Consultative Committee for Mass and Related Quantities (CCM) performed the first international comparison measurement (key comparison CCM.M-K8) of the realizations of the redefined kilogram to enable international comparability and to allow different realizations of the unit of mass to be recognized in the future. This key comparison was organized by the Bureau International des Poids et Mesures (BIPM). Besides the BIPM, which also took part in the comparison measurement with a Kibble balance, the following six national metrology institutes participated: KRISS (South Korea, Kibble balance), NIM (China, joule balance), NIST (USA, Kibble balance), NMIJ (Japan, 28Si sphere), NRC (Canada, Kibble balance), PTB (Germany, 28Si sphere). Each participating institute determined the mass of one or two 1 kg transfer standards in vacuum by means of its respective realization experiment. The transfer standards used by PTB were a kilogram prototype made of a platinum/iridium alloy and a silicon sphere. The mass of these transfer standards was determined in vacuum by comparing them with the mass of two 28Si spheres named AVO28-S8c and Si28kg01a. These two spheres were manufactured using two different 28Si single crystals and used as primary mass standards [3]. The relative standard uncertainties (k = 1) of the two realizations of the redefined kilogram amounted to 1.6 × 10-8 (AVO28-S8c) and 1.4 × 10-8 (Si28kg01a). For each transfer standard, the relative mass difference between the different realizations was 9 × 10-9.

At the BIPM, all the participants’ transfer standards were compared with a reference standard in vacuum. The differences between the individual realizations and the reference value (which was calculated as the weighted mean from the comparison measurements of the transfer standards carried out at the BIPM) were determined in combination with the mass values obtained by the participants based on their respective realization [5]. Figure 1 shows the agreement of the participants’ results with the reference value. The chi-squared test for consistency (confidence interval: 95 %) was passed. Since the latest determination of Planck’s constant and the Avogadro constant for the revision of the International System of Units (SI) in 2018, the agreement between the results of the experiments with the smallest measurement uncertainties (NIST, NMIJ, NRC, PTB) has improved [6]. Figure 1 also shows the difference between the traceability of the BIPM working standards to Planck’s constant h(IPK) (which is maintained at the BIPM via the international prototype of the kilogram (IPK)) and the reference value. The result of the BIPM working standards (with their traceability to Planck’s constant that is maintained at the BIPM via the international prototype of the kilogram) deviates from the reference value of the realization experiments by 19 µg. After completing this international key comparison of realizations of the new kilogram definition, the CCM Task Group on the Phases for the Dissemination of the kilogram following redefinition (CCM-TGPfD-kg) was able to determine the first consensus value for the dissemination of the unit of mass [5, 7-11]. The next comparison measurement of the realization experiments has already been scheduled. It is expected to start officially in 2021/22.

 

Figure 1: Mass differences between the results of the participants and the reference value (KCRV). The results of the participants were attributed to the 1 kg transfer standards on the basis of the respective realization experiments. The KCRV was calculated as a weighted mean from the comparison measurements of the transfer standards at the BIPM. In addition, the figure also shows the difference between the traceability of the BIPM working standards to Planck’s constant h(IPK) (which is maintained at the BIPM via the international prototype of the kilogram (IPK)) and the reference value. The uncertainty ranges correspond to the expanded measurement uncertainties (k = 2) of this difference [5].

 

Literature:

[1] Resolutions of the 26th meeting of the General Conference on Weights and Measures (CGPM), Versailles, 13-16 November 2018, Opens external link in new windowLink

[2] PTB: Research on the new SI, Opens external link in new windowLink

[3] 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, Opens external link in new windowLink

[4] Bureau International des Poids et Mesures: The International System of Units (SI), 9th SI Brochure, 2019, Opens external link in new windowLink

[5] Stock, M. et al.: Report on the CCM key comparison of kilogram realizations CCM.M-K8.2019, Metrologia 57 (2020) Tech. Suppl. 07030, Opens external link in new windowLink

[6] 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 Opens external link in new windowLink

[7] 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, Opens external link in new windowLink

[8] 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, Opens external link in new windowLink

[9] M. Stock, S. Davidson: Report on the Calculation of the CCM Consensus Value for the Kilogram 2020, Opens external link in new windowLink

[10] 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, Opens external link in new windowLink

[11] Bureau International des Poids et Mesures: Beginning a new phase of the dissemination of the kilogram, Opens external link in new windowLink

 

Contact:

Borys, Michael, FB 1.1, E-Mail: Opens local program for sending emailmichael.borys(at)ptb.de

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