Principle Of Operation - Toshiba LF150 Instruction Manual

7. 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 7.1). 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. 7.1)
Volumetric flow rate Q [m
π × D²
4
Q = × V ................(Eq. 7.2)
Using Equation 7.1 and 7.2
E = K × B × D ×
4 × K × B
π × D
E =
Therefore, volumetric flow rate is directly proportional to the induced voltage.
The LF150 flange-type electromagnetic flowmeter uses square-wave excitation method, which
provides a long-term stable operation. With square-wave excitation, the LF150 offers reliable
measurement without being affected by electrostatic or electromagnetic interference, or
electrochemical polarization between the electrode and the fluid to be measured.
3
/s] is:
4
π × D²
× Q
× Q ............(Eq. 7.3)
Figure 7.1 Principle of Operation
—
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 Method
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