State-of-the-art mass determination of 28Si spheres for the Avogadro project

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.