A comparison of future realizations of the kilogram

The redefinition of the kilogram is scheduled for 2018. This will be achieved by fixing the numerical value of the Planck constant with relation to the current unit of mass, as realized by the International Prototype Kilogram (IPK). The Consultative Committee for Mass and Related Quantities (CCM) has requested that, as one of the essential steps before the redefinition, a comparison of realizations based on watt balance and X-ray crystal density experiments be carried out, since these will become the primary realization experiments in the new SI. This comparison will be undertaken as a 'Pilot Study' and will be carried out during 2016. The aim of the study is to compare the future primary realizations to check their equivalence with each other and with the IPK and to develop and evaluate the protocol which will be used for future comparisons of the realization experiments after the redefinition. This will ensure the continuity of the mass unit across the redefinition and the validity of its dissemination following the redefinition. This paper outlines the basis for the comparison protocol taking into account the results of previous work evaluating transfer methods for primary mass standards. Possible future traceability routes for the kilogram from watt balance or XRCD experiments to end user requirements are also presented.