Abstract
A novel magnetic flowmenter has been proposed to eliminate insulating liner and to improve reliability and performance of magnetic flowmeters. The proposed magnetic flowmeter keeps the boundary condition between fluid and metallic pipe wall identical to that of non-conducting pipe wall by applying electric current automatically to form potential distribution on the wall. Subsequently, the output signal is the same as that of conventional magnetic flowmeters.
The feasibility and the effectiveness of the proposed flowmeter have been clarified by the theory and the experiments in previous papers, where the optimum position of the electrodes to feed electric currents and the optimum servo gain of the control cicuit were given by the theoretical analysis of the static performance of wall and fiuid potential.
The present paper describes the dynamic characteristics of the flowmeter. When a usual rectangular wave excitation is adopted, the effect of an electric double layer at the boundary of fiuid and metallic pipe wall cannot be neglected. Since it is difficult to analyze the distributed physics of the layer, a lumped equivalent circuit model which consists of parallel capacitance and resistance with series resistance is constructed and overall dynamics of the flowmeter is studied through computer simulations. The simulated output signal coincides with the experimental one for the various fiuid conductivity ranging from about 100 to 5000S/cm, when the parameters of the model are adequately chosen. Also, the simulation shows that both the capacitance of the layer and the conductivity of the fluid affect the waveform of the output signal.
Therefore, feedback circuit of potential formation is revised to eliminate these effects and to make the flowmeter output equal to conventional magnetic flowmeter. The signal for the feedback is sampled and held at the each end of the half periods of the rectangular wave excitation, where above effects become negligibly small as is demonstrated by the simulation. Both results of the simulation and the experiment of the new circuit configuration show good performance and the suppression of the effects of the capacitance and the conductivity.
