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 To assess the performance of the world-wide ensemble of watt balances, it is advisable to compare a common measurement obtained from these devices. The traditional way to compare laboratory standards is by means of a transportable transfer standard. Typically, problems with this approach arise when the transfer standard is less stable than the laboratory standards being compared. Therefore, it would be beneficial to have a macroscopic mass standard which is as stable as possible over time. This would allow all watt balances to be compared using a common reference, at least for a certain period after the redefinition of the kilogram. For that purpose, and following a recommendation of the CCM in 2009, the BIPM Mass Department is working to establish an ensemble of 12 mass standards. The masses are currently being collected and characterized and the ensemble will include artefacts made from these different materials: stainless steel, Pt-Ir and single-crystal silicon. They will be stored under different conditions: vacuum (~1 mPa), air, pure nitrogen or argon (at atmospheric pressure).
The artefacts will be frequently inter-compared and the mass of each individual element compared to the mean mass of the ensemble, which will be calculated by giving each element a statistical weight that reflects its stability. The mean mass will therefore be more stable than any of the individual masses. In addition, by calibrating one (or more) of the mass standards using the available watt balances and other primary realizations of the kilogram (with the help of transfer mass standards), traceability of the whole “ensemble” is assured with respect to the fundamental constants.
Once the ensemble of mass standards is traceable to the fundamental constants, it will be possible to use it to disseminate the SI unit of mass, taking advantage of the high stability of the mean mass value of the ensemble. Using widely-available modern mass comparators, relative uncertainties of a few parts in 109 can be obtained when comparing 1 kg mass artefacts.
We are grateful to the BIPM Time Department for fruitful discussions and invaluable advice.
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Summary
