Streaming current noise generation in electromagnetic flowmeters measuring conducting fluids

摘要

This paper provides a model of streaming current noise generated by the interaction of turbulence with the charge distribution which occurs in the liquid, close to the surface of the insulating liner of an electromagnetic flowmeter, as a consequence of charge species being absorbed by the lining material itself. Such noise is present whether the electromagnetic field of the flowmeter is excited or not. It is shown in this paper that this noise can be detected in potable water at a conductivity of order 5 mS/m, although at such levels it is often masked by the first stage amplifier noise, but that it becomes more significant when measurements are made in de-ionised water where the conductivity may be as low as 5 μS/m. The predicted spectral characteristic of the noise and the dependencies of the noise on the conductivity of the fluid, the pipe size and flow velocity provided by the model are all shown to be in good agreement with experimental data. The model is valid for clean aqueous fluids, where there is no noise generated by particle impact or significant electrochemical reaction between the fluid and the sensing electrodes. It can be used to predict the signal to noise ratios in electromagnetic flowmeters using different excitation waveforms and frequencies or different electrode structures.