International intercomparison of silicon density standards

In order to obtain a more accurate value for the Avogadro constant N_A the reliability of density values obtained by different measuring techniques for silicon single crystals was checked by density determinations with four silicon spheres at PTB, IMGC, and NRLM. The diameters of the spheres (two of PTB and two of NRLM) ranged from 90 mm to 94 mm. All the mass measurements (at PTB, IMGC, and NRLM) were carried out in air. At IMGC and NRLM the volume was determined by interferometric measurements, whereas at PTB the volume was obtained from hydrostatic weighings using two Zerodur volume standards. In addition, at PTB the densities of the four spheres were cross-checked by flotation measurements. The maximum relative density difference was found to be 1×10^-6, whereas the typical values were 2×10^-7