Mass determinations of a 1 kg silicon sphere for the Avogadro project

For a new determination of the Avogadro constant to play a role in the redefinition of the mass unit, the BIPM and other laboratories must carry out mass comparisons at a very high level of accuracy. We must achieve mass comparison among a 1 kg Si-28 sphere and 1 kg Pt/Ir mass standards to a combined standard uncertainty of 4 mu g. The BIPM has developed a method to reach the target requested by the Avogadro group. The goal of this paper is to report the advances at the BIPM and to describe our new method for linking between weighings in air and in vacuum. The results obtained by classical comparisons in air are compared with those obtained by the new BIPM method.