Errors in electromagnetic flow meter with magnetic particles

Abstract

Magnetic admixtures change the magnitude and distribution of the magnetic field and virtual current in active fluid volume of electromagnetic flow meter, and measurement error appears. The value of error depends on the concentration, permeability and electric conductivity of magnetic admixtures and on the form of particles. The principles of error formation are investigated assuming that in the fluid with uniform properties there may be a small concentration of spherical particles, whose magnetic and electric properties are different from that of the fluid. An ideal electromagnetic flow meter with a rectangular duct and infinitely conductive large electrodes is assumed. The flat velocity profile is supposed. Measurement error is evaluated using the analogy between electrostatic, magnetic and electric current fields. The main causes of measurement error are examined. There is a deviation of densities of virtual current and magnetic flux inside particles, a deviation of mean values of magnetic flux and virtual current densities outside particles and a distortion of magnetic and virtual current fields in surroundings of particles. The magnetic particles with electric conductivity considerably greater than fluid conductivity are especially dangerous. A relative error in this case may exceed volume concentration of particles six times. For non-magnetic particles measurement error is absent independent of their conductivity.