In the internal pipe wall of magnetic flowmeters should be nonconductive to prevent generated electromotive force from short circuiting. Usually inside of metalic pipe is lined by insulating material. The lining material limits applicable temperature range of measured fluid and also reliability of meter. The objectives of this investigation are to eliminate the lining and to improve reliability and performance of magnetic flowmeter.
A potential distribution is formed on the pipe wall by applying electric current proportional to fluid flowrate. If the potential distribution on the wall is kept almost identical to the flow induced potential in the liquid, then no current flows across the boundary between fluid and wall.
The principle of the proposed magnetic flowmeter is to realize a equivalent boundary condition between fluid and metalic pipe wall. The boundary condition is kept equivalent to that of nonconducting pipe wall automatically by the use of electronic servo techniques. Therefore the output signal is exactly same as that of conventional magnetic flowmeters. An experimental model is built and tested using stainless steel pipe (inner diameter=27mm, outer diameter=30mm).
According to the test results of tap water flow measurement, the output is reduced to about 60% of theoretical value without applied potential, however, it can be increased to 99% of theoretical one with wall potential applied. The theoretical value is calculated from measured value of flux density, flow velocity, and pipe diameter.
The output current of the servo amplifier is proportional to flowrate and is about 2 Ampere at flow velocity=0.8m/s for this model.
Theoretical analysis and test results indicate that flowmeter output of the model is highly dependent on liquid conductivity without wall potential. However, no effect of liquid conductivity is observed if the potential is applied. These results suggest that the proposed wall potential distribution is very effective.
In this paper theoretical analysis and experimental results are described.
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