To determine a new value of the Avogadro constant with a relative combined standard uncertainty of 2 × 10⁻⁸, the mass determination of a 1 kg ²⁸Si sphere is crucial and should be determined with the highest level of accuracy. In the next two years the laboratories involved in the International Avogadro Project should be able to determine the mass of a 1 kg silicon sphere under vacuum with a combined standard uncertainty within 5 µg. To obtain such a target it is essential to gain experience and to promote cooperation in mass measurement on silicon spheres among the laboratories involved. For this purpose an international comparison has been performed to evaluate the weighing procedure and to reveal the difficulties encountered in the mass determination of a silicon sphere.

This particular comparison, which is the first international mass comparison under vacuum conditions, has demonstrated that the reference value for the mass of a 1 kg silicon sphere can be determined traceable to the International Prototype with a standard uncertainty of 4.0 µg.