The definition of the kilogram in the International System of Units (SI) is expected to be revised in 2018. The present definition of the kilogram, the mass of the International Prototype of the Kilogram (IPK), adopted in 1889, would then be replaced by a definition based on a fixed numerical value of the Planck constant. The Consultative Committee for Mass and Related Quantities has requested that, as one of the essential steps before the redefinition, a comparison of kilogram realizations based on future realization methods, Kibble(9) balances and x-ray crystal density (XRCD) experiments, be organized. This comparison was carried out during 2016 in the form of a 'Pilot Study'. One aim of the study was to determine the uniformity of mass dissemination after the redefinition by comparing mass calibrations based on different future realization experiments. Another aim was to test the continuity of the mass unit across the redefinition by comparing mass calibrations based on Kibble balances and XRCD experiments with those based on the IPK. This paper describes the organization of the comparison and presents its results.

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In 2014 the Bureau International des Poids et Mesures (BIPM) carried out a calibration campaign using the international prototype of the kilogram (IPK). This is the second part in a series of publications describing the results of that campaign. As reported (Metrologia 52 310–6), following the comparisons between the IPK and its official copies, it was found that the BIPM ‘as-maintained mass unit’ was offset by 35 μg from the mass of the IPK in 2014. We report here the results of an investigation into this offset that has considered all data available from internal BIPM mass comparisons carried out between 1992 and 2014. This has enabled us to model the evolution of the offset in the as-maintained mass unit and to identify some possible reasons why it has developed. We also report how the model has been used to estimate corrections to all 1 kg mass calibration certificates issued by the BIPM during this period.

The very first definition of the kilogram was in terms of a constant of nature, although this idea could not be fully realized at the end of the 18th century. Instead the kilogram was defined by an artefact whose mass was made to approximate as closely as possible a physical constant with unit kg m-3-the maximum density of distilled water at atmospheric pressure. For the next two centuries, mass comparators improved greatly as did the materials from which artefacts could be constructed. These improvements put tighter constraints on the realization of a non-artefact definition of the kilogram. However, it is now expected that the goal of redefining the kilogram in terms of fundamental constants will be achieved in 2018. We present a history of the kilogram with emphasis on continuity of this unit of mass each time it has been redefined and the stability of a unit defined by the mass of an artefact.

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 K, ?> 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 K ?> 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 K ?> as reference standard.

New results are reported from an ongoing international research effort to accurately determine the Avogadro constant by counting the atoms in an isotopically enriched silicon crystal. The surfaces of two Si-28-enriched spheres were decontaminated and reworked in order to produce an outer surface without metal contamination and improved sphericity. New measurements were then made on these two reconditioned spheres using improved methods and apparatuses. When combined with other recently refined parameter measurements, the Avogadro constant derived from these new results has a value of N-A = 6.022 140 76(12) x 10(23) mol(-1). The x-ray crystal density method has thus achieved the target relative standard uncertainty of 2.0 x 10(-8) necessary for the realization of the definition of the new kilogram.

This report presents the results of the first phase of the campaign of calibration carried out with respect to the international prototype of the kilogram (IPK) in anticipation of the redefinition of the kilogram (Extraordinary Calibrations). The definition of the kilogram was realized according to the procedure outlined in the 8th Edition of the SI Brochure. Thus the IPK and its six official copies have been cleaned and washed following the BIPM procedure.The mass comparisons carried out during this campaign showed a very good repeatability. The pooled standard deviation of repeated weighings of the prototypes was 0.4 mu g. The effect of cleaning and washing of the IPK was to remove a mass of 16.8 mu g. The effect of cleaning and washing of the six official copies was found to be very similar, giving an average mass removed from the seven prototypes of 15 mu g with a standard deviation of 2 mu g.The differences in mass between the IPK and the official copies have changed by an average of 1 mu g since the 3rd Periodic Verification of National Prototypes of the Kilogram (1988-1992). These results do not confirm the trend for the masses of the six official copies to diverge from the mass of the IPK that was observed during the 2nd and 3rd Periodic Verifications.All BIPM working standards and the prototypes reserved for special use have been calibrated with respect to the IPK as part of this campaign. All of them were found to have lower masses than when they were calibrated during the 3rd Periodic Verification. As a consequence, the BIPM 'as-maintained' mass unit in 2014 has been found to be offset by 35 mu g with respect to the IPK. This result will be analyzed in a further publication.

For a new determination of the Avogadro constant to play a role in the future redefinition and realization of the mass unit, the mass measurements involved need to be carried out at a very high level of accuracy. From a mass comparison among two 1 kg Si-28 spheres and the 1 kg platinum-iridium (Pt/Ir) mass standards, the mass of the spheres must be determined with a combined standard uncertainty of less than 5 mu g. The BIPM, the PTB and the NMIJ have carried out such a state-of-the-art mass comparison in air and under vacuum in order to reach this target set by the International Avogadro Coordination (IAC). The results obtained for the spheres AVO28-S5 and AVO28-S8 involved in the comparison have demonstrated that by using air buoyancy artefacts and sorption artefacts it is possible to achieve a relative uncertainty of 4.1 x 10(-9). The reference value for each sphere has been determined, taking into account the traceability of the masses to the International Prototype of the kilogram, K, in due consideration of the correlations among 17 standards used directly or indirectly in this comparison.

A cleaning method using ultraviolet activated ozone has been developed at the National Physical Laboratory (NPL). A comparison to test the effectiveness of this cleaning method with nettoyage-lavage cleaning on platinum-iridium artefacts and kilogram mass standards was undertaken at the Bureau International des Poids et Mesures (BIPM). Three cleaning trials were performed to compare the effectiveness of the two methods at removing natural, medium and heavy levels of surface contamination.Both the ultraviolet light/ozone method and the nettoyage-lavage method successfully removed both natural levels and medium levels of surface contamination. However, in the medium contamination trial nettoyage-lavage cleaning removed additional mass from the Pt/Ir mass standard under test that could not be accounted for in terms of surface contamination.In the heavy contamination trial nettoyage-lavage cleaning successfully removed most of the contamination from the surface of the contaminated Pt/Ir mass standard. The ultraviolet light/ozone method removed about two thirds of the surface contamination and therefore for heavily contaminated mass standards it is advisable to pre-clean them with a solvent before ultraviolet/ozone cleaning.

LNE and BIPM compared their pressure standards equipped with 20 cm2 effective area piston?cylinder units in the absolute pressure range from 80 kPa to 110 kPa. The pressure standards, the method for calculating the reference values and the comparison results are presented. The results of the comparison can be considered as satisfactory as the deviations from the reference value are inside the estimated combined uncertainty. 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 EUROMET, according to the provisions of the CIPM Mutual Recognition Arrangement (MRA).

LNE and BIPM compared their pressure standards equipped with 20 cm 2 effective area piston-cylinder units in the absolute pressure range of 80 kPa to 110 kPa. The pressure standards, the method for calculating the reference values and the comparison results are presented. The results of the comparison can be considered as satisfactory as the deviations from the reference value are inside the estimated combined uncertainty.