The CCM.G-K2.2017 comparison was organised for the purpose of determination of the degree of equivalence of the national standards for free-fall acceleration measurement. The comparison was held in the Changping Campus of National Institute of Metrology China (NIM), from October to November in 2017. This is the first time that such a comparison is organized outside of the Europe continent and establishes a new global comparison sites in China [1, 2]. This comparison is also the largest ever organized with the participation of 13 instruments. Dr. Shuqing Wu, Dr. Jinyang Feng and Mrs. Chunjian Li from the NIM were in charge of the local organization of the comparison and of the elaboration of the results. NIM was the Pilot Laboratory under the leadership of Dr. Shuqing Wu. The comparison steering committee (SC) is composed of Prof. Olivier Francis (LU), Dr. Vojtech Pálinkáš (VÚGTK/RIGTC), Dr. Derek van Westrum (NOAA-NGS), Dr. Reinhard Falk (BKG) and Dr. Shuqing Wu (NIM). The SC is supported and consulted by the CCM-WGG Chair, Prof. Alessandro Germak (INRIM). The comparison was organized in accordance with the CIPM MRA-D-05 of the Consultative Committee on Mass and Related Quantities (CCM).

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The determination of the local acceleration of gravity at the mass position is an important aspect of the joule balance. Since a simultaneous measurement of the absolute gravity at the mass position with the joule balance in place is impossible, seven gravity reference sites are chosen in the joule balance laboratory. Absolute and relative gravity measurements at the reference sites were performed between 2015 and 2019, and varioust transfer methods were used to obtain an absolute gravity value at the mass position. The self-attraction effects of joule balance apparatus are corrected by FEA simulation, and calculation accuracy is also verified by experiment. To obtain real-time local gravitational acceleration values during the joule balance experiment, geophysical effects, such as atmospheric pressure, Earth tides, and polar motion are considered. The absolute gravity results exhibit good stability from 2015 to 2019, and the final gravitational uncertainty contribution in the joule balance is 6.1 µGal.

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