Main text This report describes 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 objectives were 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. Another objective was the determination of the reproducibility of the realization experiments by comparing the new results with those of the first key comparison of kilogram realizations, CCM.M-K8.2019. The comparison was organized by the BIPM and had nine participants. The BIPM, LNE, METAS, NIST, NRC and UME operated Kibble balances, the NIM used a joule balance and the NMIJ and the PTB participated using 28Si spheres, the masses of which were determined with the XRCD method. These realization methods were used to calibrate 1 kg mass standards under vacuum or in air. The standards were sent to the BIPM where they were compared with each other and with BIPM Pt-Ir working standards. The latter were calibrated traceable to the International Prototype of the Kilogram (IPK), the mass of which served as the definition of the kilogram until 20 May 2019. The results of the weighings at the BIPM 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. The level of agreement between mass determinations with the realization experiments and the BIPM as-maintained mass unit, traceable to the Planck constant through the mass of the International Prototype of the Kilogram, could also be deduced. To reach the main text of this paper, click on Final Report. Note that this text is that which appears in Appendix B of the BIPM key comparison database https://www.bipm.org/kcdb/. The final report has been peer-reviewed and approved for publication by the CCM, according to the provisions of the CIPM Mutual Recognition Arrangement (CIPM MRA).

This report describes the first 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 objectives were 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 first "consensus value" of the kilogram. The consensus value will serve as the basis for an internationally coordinated dissemination of the kilogram which will continue until sufficient agreement between realization experiments has been achieved. The comparison was organized by the BIPM and had seven participants. The BIPM, KRISS, NIST and NRC operated Kibble balances, the NIM used a joule balance and the NMIJ and the PTB participated using 28Si spheres, the masses of which were determined with the XRCD method. These realization methods were used to calibrate 1 kg mass standards under vacuum. The standards were sent (in air) to the BIPM where they were compared under vacuum with each other and with BIPM Pt-Ir working standards. The latter were calibrated (in air) traceable to the International Prototype of the Kilogram (IPK), the mass of which served as the definition of the kilogram until 20 May 2019. The results of the weighings at the BIPM together with the measurement results communicated by the participants allowed comparison of the values attributed to mass standards of 1 kg using the participating realization experiments. The level of agreement between mass determinations with the realization experiments and the BIPM as-maintained mass unit, traceable to the Planck constant through the mass of the International Prototype of the Kilogram can also be deduced. Main text To reach the main text of this paper, click on Final Report. Note that this text is that which appears in Appendix B of the BIPM key comparison database kcdb.bipm.org/. The final report has been peer-reviewed and approved for publication by the CCM, according to the provisions of the CIPM Mutual Recognition Arrangement (CIPM MRA).

We describe a project to re-evaluate the transfer of the absolute determination of the local acceleration due to gravity from a nearby site, to the center of mass of the test mass in the NRC Kibble Balance. This re-evaluation includes the characterization of the sensitivity of a relative gravity meter to the fringing magnetic flux density, as well as the attenuation provided by a simple magnetic shield.

We present a summary of the Planck constant determinations using the NRC watt balance, now referred to as the NRC Kibble balance. The summary includes a reanalysis of the four determinations performed in late 2013, as well as three new determinations performed in 2016. We also present a number of improvements and modifications to the experiment resulting in lower noise and an improved uncertainty analysis. As well, we present a systematic error that had been previously unrecognized and we have quantified its correction. The seven determinations, using three different nominal masses and two different materials, are reanalysed in a manner consistent with that used by the CODATA Task Group on Fundamental Constants (TGFC) and includes a comprehensive assessment of correlations. The result is a Planck constant of 6.626 070 133(60)  ×10−34 Js and an inferred value of the Avogadro constant of 6.022 140 772(55)  ×1023 mol−1. These fractional uncertainties of less than 10−8 are the smallest published to date.

The results of the latest determination of the Planck constant at the national research council of Canada (NRC) were published in April 2014 [1]. In December 2014 we received from the international bureau of weights and measures (BIPM) corrected values and uncertainties of three calibrations of NRC’s reference PtIr masses. These corrections arose from a recently discovered difference between the BIPM mass scale and the SI mass [2]. Naturally, these corrections, which were approximately 35 μg in value and resulted in revised calibration uncertainties of about 3 μg, have an impact on the 2014 NRC Planck constant determination. In the 2014 publication we reported a Planck constant value of: = × − = ± × −

A new watt balance is being constructed at National Institute of Standards and Technology (NIST) in preparation for the redefinition of the International System of Units and the realization of mass through an exact value of the Planck constant. We describe the procedures used and give results for the measurements of the local acceleration of gravity in the new watt balance facility.

We have measured Planck's constant and have obtained a value of 6.626 070 34( 12) x 10(-34) J s. To our knowledge this measurement of h has the lowest uncertainty reported to date. This result has been obtained from measurements of four masses of different material and nominal values varying from 1 kg to 250 g. The experimental procedures and the measurement uncertainties are described in detail.

In a watt balance (WB) the gravity on a known mass is used in the determination of the value of the Planck constant with a precision of <2 × 10−41 J s. To attain this precision the gravity value at the centre of a mass within the WB must be known with a relative uncertainty of <1 × 10−8. This necessitates laboratory gravity mapping in three dimensions, the establishment close to the WB of a gravity reference station, and modelling of the ties to the centre-of-mass location. The self-gravity of the WB must be accounted for, and since WB measurements run over several weeks, corrections for tidal, polar motion and atmospheric effects must be included. We describe the gravimetry carried out at the National Research Council of Canada WB laboratory, and include a discussion of past gravity variations in the vicinity of the WB, which suggest that seasonal variation may be of sufficient magnitude to affect results obtained with the balance.

We report on a 2012 comparison of gravity used to determine the Planck constant by the National Institute of Standards and Technology (NIST) and the National Research Council Canada (NRC) watt balances. The results provide verification of the gravity values used in recently published discrepant Planck constant determinations that play a vital role in the redefinition effort of the International System of Units (SI) and set an upper limit of 10 parts in 10(9) on the relative uncertainty contribution of gravity observations to future Planck constant determinations by the NIST and NRC watt balances.

We report on a 2012 absolute gravity comparison performed at the National Institute of Standards and Technology (NIST), Gaithersburg, MD. Unique to this work is the direct comparison of gravitational measurement systems used in the determination of the Planck constant by NIST and the National Research Council Canada (NRC) watt balances. The results will provide verification of the gravity values used in recently published Planck constant determinations that play a vital role in the redefinition effort of the International system of Units (SI).

In 2009 NRC commenced construction of a new laboratory to house the NRC watt balance. We describe the procedures used and give results for the measurements of the acceleration due to gravity in the new laboratory. The data will be linked both to those laboratories participating in the Key Comparison CCM.G-K1 of 2009, and to the Canadian Gravity Standardization Network.