Induction Flowmeter

Abstract

According to Faraday's law of induction an electric field will be induced in a medium moving relative to a magnetic field. This induced electric field is proportional to the intensity of the magnetic field and to the relative velocity of the moving medium. When applied to media of arbitrary electrical conductivity and dielectric constant, it is seen that any potential difference that arises in connection with the induced electric field is attenuated by the shunting effect of the permittivity of free space. For most media this shunting effect is negligible; for media of extremely low conductivity, however, the effect is appreciable. Finally the effect of the electrical properties of the containing pipe is investigated.

Magnetic Meters: Effects of Electrical Resistance in Tissues on Flow Measurements, and an Improved Calibration for Square-Wave Circuits

“…The conductivity of vessel walls relative to that of the moving fluid determines the amplitude of signal that will be recorded from a given flow (1,8,9). Our measurements suggest that blood vessels exceed normal blood in specific resistance (the reciprocal of specific conductance) by a factor less than 2, contrary to some other observations (9).…”

“…The equation for magnetic meters, based on Faraday's law, is E=HLV « 10- 8 , where E is voltage, H is magnetic field strength in gauss, L is internal diameter of the conduit in centimeters, V is average velocity of the moving conductor in centimeters per second, and lO" 8 is the factor converting abvolts to volts. Magnetic permeability in air, blood, saline, or tissues need not be considered because it is very close to unity.…”

Electro-Magnetic Flowmeters Using Eddy Currents

“…(12) and (13) Driving coils are approximated by the circular currents of infinitesimally small area, and the magnitudes are deThe good agreement between theoretical and experimental results concludes the feasibility of the presenting configuration of the electro-magnetic flowmeter in the normal operational range. Since the driving signal correlates to the flow signal, it supplies the function of self calibration to the flowmeter in this range.…”

“…(4),...,(7) in the above volume integral, and some manipulations give the output signals between electrodes in the following separated formulas: UO = UA + UV (11) UA = + At) at dT atvat (12) u7 iwo(c Eo6)ff(OD>(Ũ V = |a (i + at ) (V X B) dT (13) UA depends on eddy currents induced by vector potential of the driving field, and Uv, on flow velocity field. The latter component coincides the output flow signal of the conventional electromagnetic flowmeter [12].…”

“…Operational principle of electromagnetic flowmeters are given by Maxwell's equations, providing that permeability of the fluid is equal to pc of vacuum and the flow velocity is much smaller than the speed of light. The condition for immediate dielectric relaxation [1], [12], thlat is negligible displacement current, is wc/Ic < 1, where w denotes ( tOD ) umi+at) u(1m + atn) cdS where integration surface S consists of electrode surfaces Si, S2 and outer pipe surface S3. When the flow conduit pipe is grounded and has higher conductivity, the integral surface can be reduced to Si and S2, and potentials Um and U are constant there.…”