Toshiba LF470 - Manual (Page 32)

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8. Principle of Operation
The operating principle of the electromagnetic flowmeter is based on Faraday's Law of
electromagnetic induction and it is designed to measure the volumetric flow rate of fluid. An
insulated pipe of diameter D is placed vertically to the direction of a magnetic field with
flux density B (see Figure 8.1). When an electrically conductive fluid flows in the pipe, an
electrode voltage E is induced between a pair of electrodes placed at right angles to the
direction of magnetic field. The electrode voltage E is directly proportional to the average
fluid velocity V.
The following expression is applicable to the voltage.
E = K x B x D x V [ V ] ...........(Eq. 8.1)
Volumetric flow rate Q [m^/s] is;
7T X
E = induced electrode voltage [V]
K = constant
B = magnetic flux density [T]
D = meter pipe diameter [m]
V = fluid velocity [m/s]
Q = X V................... (Eq. 8.2)
Using the Equation 8.1 and 8.2
4
xQ E = K x B x D x
TT X
E = ............ (Eq.8.3)
7T XD
Therefore, volumetric flow rate is directly proportional to the induced voltage.
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Square-Wave Excitation
Figure 8.1 Principle of Operation
The LF470 electromagnetic flowmeter uses the square-wave excitation method, which pro­
vides long-term stable operation. With square-wave excitation, the LF470 offers reliable
measurement without being affected by electrostatic or electromagnetic
interference, or electrochemical polarization between the electrodes and the fluid to be
measured.
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