− ３７ −
６Ｆ８Ａ０９２６
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
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
3
/s] is:
Q = × V ................(Eq. 14.2)
Using the Equation 14.1 and 14.2
E = K × B × D × × Q
E = × Q ............(Eq. 14.3)
Therefore, volumetric flow rate is directly proportional to the induced voltage.
Figure 8.1 Principle of Operation
The LF654/LF620F and LF654 uses the square-wave excitation method, which
provides long-term stable operation.
With square-wave excitation, the LF654/LF620F and LF654 offers reliable measurement without being affected by
electrostatic or electromagnetic interference, or electrochemical polarization between the electrodes and the fluid to
be measured.
E = induced electrode voltage [V]
K = constant
B = magnetic flux density [T]
D = meter pipe diameter [m]
V = fluid velocity [m/s]
π
× D²
4
4
π
× D²
4 × K × B
π
× D
Square-Wave Excitation
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