Principle Of Operation - Toshiba LF620F Instruction Manual

14. 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 14.1). 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 × B × D × V [V] ........ (Eq. 14.1)
Volumetric flow rate Q [m
π × D²
Q =
Using the Equation 14.1 and 14.2
E = K × B × D ×
4 × K × B
E =
Therefore, volumetric flow rate is directly proportional to the induced voltage.
D
The LF620F and LF622F electromagnetic flowmeter uses the square-wave excitation method, which
provides long-term stable operation. With square-wave excitation, the LF620F and LF622F 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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3
/s] is:
× V ................(Eq. 14.2)
4
4
× Q
π × D²
× Q ............(Eq. 14.3)
π × D
E
B
Figure 14.1 Principle of Operation
－ 149 －
6 F 8 A 0 9 2 8
E = induced electrode voltage [V]
K = constant
B = magnetic flux density [T]
D = meter pipe diameter [m]
V = fluid velocity [m/s]
Square-Wave Excitation