Magnetic flowmeters small in tube-diameter are known to be easily calibrated using appropriate tanks and clocks, however, for large diameter ones, there arise many difficulties. On the other hand, it is also known that there exists a theoretical relationship between flowrate and output voltage, which, in an ideal case, is represented by the following equation: E=B·D·uzwhere B: flux density, E: output voltage, D: diameter, uz: mean flow velocity in MKS unit In actual cases, however, another coefficient should be factored on the right hand side. This paper discusses the coefficient either theoretically or based on the experiments made by the author.
1) The original relationship is transformed into such a form as E/I=B/I·D·uz where I: exciting current. The left hand side is an easily measurable quantity especially when the exciting source for the flowmeter regarded as an active impedance is ordinary commercial one.
2) Flux density B varys as the source voltage and frequency fluctuate, but the quantity B/I does not. This quantity is measured, compared with that of a standard solenoid with known dimensions.
3) Theoretical value of coefficient is derived in terms of distributions of the flux density and flow velocity for actual cases in which both of the distributions are not necessarily be uniform. Theoretical results show a very good coincidence with experimental data.
4) To avoid undesirable effects of rapid fluctuation of flow during measuring period, output voltage is transduced into pulse rate and integrated by a pulse counter. The result is calibrated by a tank of 100m3 volume for a magnetic flowmeter of 3m3/sec. of full scale with an accuracy of 0.5%.
5) This paper also describes the calibration data using current-meters and/or Pitot's tubes for a magnetic flowmeter actually installed in a penstock of Kamo Hydropower Plant.
