Between January 2014 and January 2015, the BIPM Mass Department carried out a calibration campaign with respect to the International Prototype of the Kilogram (IPK), in anticipation of the planned redefinition of the kilogram. The results have been published in the latest edition of Metrologia. This calibration campaign, named "Extraordinary Calibrations", is an important element of the CCM roadmap towards the redefinition of the kilogram.
The objective of the "extraordinary calibration" campaign was to provide improved traceability to the IPK for those NMIs involved in the determination of the Planck constant, upon which the future definition of the kilogram will be based. Traceability of the Planck constant to the IPK ensures that the future kilogram will be of the same magnitude as the present kilogram.
The previous calibration of the BIPM working standards against the IPK took place during the 3rd Periodic Verification of National Prototypes (3rd PV), 1988-1992. Since then, the mass unit has been maintained by this set of working standards, which also served for the calibration of national prototypes. The results obtained in the latest measurement campaign suggest that the IPK and the official copies have behaved over the period since the 3rd PV as a consistent set of mass standards. However, the results obtained for the set of BIPM working standards indicate that the as-maintained BIPM mass unit (which was traceable to the IPK at the 3rd PV) had drifted away from the IPK over 22 years by 35 µg. These findings will be published in a separate publication.
The CCM reviewed the situation at its 15th meeting (February 2015) and made a recommendation to NMIs on how to manage the consequences of the corrections to the BIPM as-maintained mass unit.