A hydrostatic comparator for the density determination of solid objects

Abstract We describe a hydrostatic comparator optimized for the determination of the density (or equivalently volume) of the sinkers used in magnetic suspension densimeters. The hydrostatic comparator is a modification of the traditional hydrostatic weighing technique for the determination of the density of solid objects: by weighing a density standard as well as the unknown object (with both immersed in a fluid) the density information of the standard is transferred to the unknown, and the density of the hydrostatic bath fluid need not be known. Our instrument makes use of two density standards of single-crystal silicon and a high-density, fluorinated-ether bath fluid; it can weigh objects as large as 100 g four-at-a-time (one or more standards and up to three unknowns). We discuss weighing designs and demonstrate a new method that employs an additional standard to eliminate the need for the usual empty-pan weighings. Uncertainties are reduced by the use of compensation and sensitivity masses. As a demonstration of the instrument’s capabilities, results are presented for a set of 12 objects with masses from 33 g to 99 g and densities from 2.33 g⋅cm –3 –16.7 g⋅cm –3 . The expanded ( k = 2) uncertainty in volume averaged 0.000 041 cm –3 , with relative uncertainties ranging from 1.1 × 10 –6 for a 99 g/42.3 cm 3 silicon object to 7.5 × 10 –6 for a 60 g/7.5 cm 3 gold-plated stainless steel object.