The method and devices for absolute density measurements of silicon
crystals and relative density measurements of solid materials by hydrostatic
weighing are described. A solid density standard based on single-crystal
silicon spheres has been developed, where the mass of the 1 kg sphere is
compared with that of the prototype kilogram through secondary 1 kg standards,
and the volume is determined directly by measuring its diameters by optical
interferometry, so that the unit of density is traceable to the definition
of mass and length in SI units. The relative combined standard
uncertainty of this absolute density measurement is 1.0×10-7. Using
these silicon spheres as a reference standard, relative density measurements
of other solid materials were performed by hydrostatic weighing. Results have
shown that the densities of other silicon crystals and stainless steel samples
have been determined with relative combined standard uncertainties of
1.1×10-7 and 5.8×10-7, respectively. Details on the
evaluation of uncertainty in the density measurement are
presented.