Present state of the solid and liquid density standards

The method and devices used for the absolute measurement of the density of solids and liquids are reviewed. A density standard based on a single-crystal silicon sphere has recently been developed at national metrology institutes, where the mass of the 1 kg silicon sphere is compared with that of the mass standard traceable to the prototype of the kilogram, and the volume is determined directly by measuring its diameter using an optical interferometer, so that the unit of the density is directly traceable to the SI units of mass and length. The relative combined standard uncertainty for such a density determination has been reduced to 1 × 10 −7 . Details of this solid density standard, and differential density measurements of solids and liquids by hydrostatic weighing, pressure of flotation, and magnetic suspension methods are described. Recent results on the absolute measurements of the density of water and mercury are briefly summarized.