A new method for correcting a force transmission error due to magnetic effects in a magnetic levitation densimeter

The magnetic levitation densimeter is one of the most accurate methods to measure fluid density in a wide range of temperature and pressure. For the existing commercially available densimeter, a relative measurement uncertainty of density is about 0.03%. One of the largest parts of the uncertainty is caused by magnetism of materials around the magnetic coupling in the densimeter, such as a sample fluid under test and a pressure cell. These magnetic effects cause a force transmission error in the magnetic coupling, which is composed of a permanent magnet in the sample fluid and an electromagnet suspended from an electronic balance placed under ambient temperature and pressure. For the existing densimeter, the force transmission error induces an uncertainty of around 0.01% in density. In the present study, the force transmission error is quantitatively investigated by the finite element method (FEM). Reliability of the FEM analysis was confirmed through a comparison with the experimental data measured by using the densimeter established at the National Metrology Institute of Japan (NMIJ). It is found from the FEM results that the density measurement error caused by the force transmission error can be reduced by controlling the permanent magnet at the same vertical position in every measurement state. In addition, the force transmission error linearly changes with the magnetic force acting on the electromagnet when the vertical position of the permanent magnet is constant. On the basis of these FEM results, the authors propose a correction method for the force transmission error by using a dual-sinker type of densimeter.