Problems in the theory and design of electromagnetic flowmeters for dielectric liquids - Part 3a: modelling of zero drift due to flux linkage between coil and electrode cables

摘要

This paper investigates theoretically the magnitudes of zero offsets and zero drifts originating from magnetic flux linkage between the coils of the electromagnet and the loop formed by the electrode cables in an electromagnetic flowmeter for dielectric liquids. The dependence of such zero offsets on liquid properties, frequency of operation, etc. is explained. The paper gives the results of measurements of the effective electrical conductivity and permittivity of dielectric liquids 13P180 and Castrol OX-9 (and mixtures of these) in the frequency range 1 Hz-1 kHz. These are used to predict the zero offsets expected in metering these liquids using phase sensitive detection with the flowmeter tube described in Part 1 of the paper. Precautions needed in flowmeter design to minimise instrument zero offsets and zero drifts are explained. The electromagnetic flowmeter theory was developed at Cranfield and the determination of the electrical properties of the dielectric liquids was carried out at QinetiQ.