Mass determination of silicon spheres used for the Avogadro project

Spheres made of a silicon single crystal with a mass of about 1 kg are used as density standards and in the context of a project for the determination of the Avogadro constant. The status of the mass determination of such silicon spheres at the PTB is presented. Special facilities and procedures used for the mass determination in air are described. Results for a single sphere and the mass difference between two spheres are given with a discussion of the measurement uncertainty and mass stability.