From September 2021 to January 2023, the BIPM organized the second CCM key comparison of realizations of the kilogram definition based on the fixed numerical value of the Planck constant, which came into force on 20 May 2019. The objective was to determine the level of agreement between realizations of the kilogram using Kibble and joule balances and the X-ray crystal density (XRCD) method and to provide input for the calculation of the second Consensus Value of the kilogram. The Consensus Value serves as the basis for an internationally coordinated dissemination of the kilogram and is updated after each new key comparison. Its use will continue until satisfactory agreement between realization experiments has been achieved.

The comparison, organized by the BIPM, included nine participants:

• BIPM, France (LNE), Switzerland (METAS), USA (NIST), Canada (NRC) and Türkiye (UME) operating Kibble balances,
• China (NIM) using a joule balance,
• Japan (NMIJ) and Germany (PTB) using ²⁸Si spheres the masses of which had been determined using the XRCD method.

These techniques were used to calibrate 1 kg mass standards in vacuum or in air. Four types of travelling standards were used: five platinum-iridium (Pt-Ir) standards, nine stainless steel standards, a tungsten weight and a natural Si-sphere. The standards were sent to the BIPM where they were compared with each other (in the same environment as at the participants’ laboratories) and with the BIPM Pt-Ir working standards. The latter maintain traceability to the International Prototype of the Kilogram (IPK), the mass of which served as the definition of the unit kilogram until 20 May 2019.

The results of the weighings performed at the BIPM headquarters together with the measurement results communicated by the participants allowed comparison of the values attributed to 1 kg mass standards using the realization experiments of the participants (as depicted in the figure). The final report is available in Metrologia.

The key comparison reference value (KCRV) was calculated as the weighted mean of the results. It has a deviation of – 0.0152 mg with respect to the mass unit maintained by the BIPM working standards (traceable to the IPK), with a standard uncertainty of 0.0074 mg. The chi-squared test for consistency using the 95 % cut-off criterion is passed, however the two results with the smallest uncertainty are not in agreement with each other.

The BIPM Kibble balance participated in this comparison, with a deviation from the KCRV well within its standard uncertainty of 4.1 x 10^–8. The uncertainty was reduced from 5 x 10^–8 in the previous key comparison of kilogram realizations thanks to the refinement achieved on the voltage measurement.

Differences between mass values attributed to 1 kg mass standards using the realization experiment of the participants and the key comparison reference value, with associated standard uncertainty. The difference between mass values based on the BIPM working standards, traceable to the Planck constant through the IPK and those based on the reference value is indicated by the rightmost point.