Absolute density measurements by dual sinker magnetic levitation densimeter

The use of a magnetic levitation densimeter (MLD) currently represents the most sensitive way of measuring fluid density for a wide range of temperature and pressure conditions. However, due to magnetic force transmission errors, the accuracy of this approach is limited to approximately 100 ppm in density. Here, the authors propose an improved method to eliminate the uncertainty caused by magnetic forces acting on fluids based on the use of dual sinkers and control of the magnetic coupling's levitation height. The technique cuts out almost all force transmission errors and enables a level of density measurement precision better than 1 ppm. A new high-sensitivity MLD system was developed using a novel sinker exchange mechanism with a magnetic coupling. Single-crystal silicon and germanium were selected as the sinker materials because of their outstanding performance in terms of isotropy, stability and universality of thermophysical properties. A number of tests to check the measurement performance of the MLD were conducted by the National Metrology Institute of Japan. The experimental results for n-tridecane are also presented in this paper.