Comparison of density difference measurements at PTB and NMIJ

Silicon single crystals are used as density standards and to determine the Avogadro constant. Density comparisons by the flotation method can check the homogeneity of the silicon crystals and determine the absolute density of silicon samples by comparison to primary density standards. Results of the two flotation apparatuses at NMIJ (Japan) and PTB (Germany) were compared with each other and with density determinations from other methods. For this purpose, three small silicon samples from the crystals WASO 04 and NRLM4 were measured in both institutes. These measurements show that the flotation method is able to detect and measure density differences with uncertainties of 0.02·10^-6ρ. Additionally, the density of the 1-kg silicon sphere AVO#1 was determined by flotation comparisons to primary standards of the institutes and compared with density values obtained by different methods. With one exception, all results are consistent within the combined uncertainties, the largest relative density difference being only 0.18·10^-6.