Investigation of a Hydrostatic Weighing Method for a 1 kg Mass Comparator

Abstract

A mass comparator for the comparison of 1 kg weights was built according to a hydrostatic weighing principle, where the buoyancy in a liquid serves to compensate the force due to gravity. In accordance with the method known for hydrometers or areometers, the immersion depth of a float is measured as a function of the force due to gravity, using a laser interferometer. The substitution principle can thus be quite simply realized at constant load.An essential advantage of this weighing method compared with the mechanical beam balance results from the frictionless and vibration-resistant bearing of the float in the liquid. For achieving a high accuracy with this technique, two prerequisites were important: the reduction of the influence of temperature by adapting the coefficients of expansion of buoyant body and liquid to each other, and the improvement of the wetting property of the liquid by adding a surfactant. The accuracy was further improved by the use of an electromagnetic feedback control to keep the immersion depth constant. By this method, a relative standard deviation of the weighings of better than 5 x lo-' could be achieved.

Fundamental limits to mass comparison by means of a beam balance

“…Another is an equal-arm two-pan knife-edge balance of the National Research Laboratory of Metrology Japan and the third is an experimental equal-arm balance developed at BIPM which uses flexure strips and servo control. Also a hydrostatic balance has been developed a t the Physikalisch Technische Bundesanstalt which promises a relative uncertainty of 10-9 (Probst & Kochsiek 1984). As is well known, there are many problems associated with high-precision weighings such as sorption of water vapour and other condensible gases, air buoyancy, errors occurring on exchanging the masses due to non-centring of the loads on the pans and other mechanical effects resulting from high stress concentrations a t knife edges for example.…”