Improving traceability to the international prototype of the kilogram

Until a new definition of the kilogram has been adopted, the SI unit of mass remains defined in terms of an artefact, the international prototype of the kilogram which is not readily available for regular recalibration of BIPM prototypes used for the calibration of national prototypes. Since 1889 the working hypothesis has been that the platinum–iridium prototypes are stable mass standards, although mass comparisons indicate that this is not entirely true. In this paper we present a method for improving metrological traceability to the international prototype by modelling the change in mass of prototypes over time and evaluate the model parameters by a weighted least squares adjustment. The method has been applied to comparisons between 18 prototypes performed at the BIPM in the period 1889–2009. The mass values predicted by the model are compared to the mass values assigned by BIPM in the period 1992–2009 and to results of the Extraordinary Calibrations performed at BIPM in 2014 using the international prototype as reference standard.