Study Of Density Measurements By Hydrostatic Weighing

Hydrostatic weighing is one of the main methods of density measurements. It is based on the Archimedes' principle, which measures the buoyancy of an object. The method has been used in precise density measurements of solids and liquids since 1700s. The establishment of the primary standard densitometers in our country (NIM) is based on the density of pure water by hydrostatic weighing method. The density of pure water is determined through temperature measuring and the density table of pure water, its expanded uncertainty is 5×10^-6g/cm³ (k = 2). So the density of primary standard densitometers' expanded uncertainty is below 2×10^-5 g/ cm³ (k= 2) and it is impossible to improve the accuracy.The accuracy of the silicon sphere's density determined by absolute measuring method is super than the others, so it requires high accuracy hydrostatic weighing system. In order to improve the measurement accuracy, we have to consider all the factors influencing the measuring results. This paper developed a high-performance hydrostatic weighing system. The system includes a special hanging system which can support three solid samples and the solid samples' movements are controlled by the movements of step motors. Weighing is a key step when we do the experiment. We choose a AX1005 comparator instead other balances and realized weights' automatic loading and unloading through reconstructed it.When measuring the density of solid or liquid by hydrostatic weighing, we found than the influence of liquid surface tension is the biggest part of all the uncertainty parts. So how to minimize the influence produced by the liquid surface tension is very important. Different liquid has different surface tension coefficient, we choose two kinds of liquid as working liquid in order to find if the measuring results have the relationship with it.