Measuring Principle - BASI BFM921 User Manual

Electromagnetic Flowmeter BFM921

7.8 Measuring principle

The flowmeter is designed for electrically conductive fluids. Measurement is based on Faraday's law of
induction, according to which a voltage is induced in an electrically conductive body, which passes through
a magnetic field. The following expression is applicable to the voltage:
U = K x B x v x D
where:
U = induced voltage
K = an instrument constant
B = magnetic field strength
v = mean velocity
D = pipe diameter
Thus the induced voltage is proportional to the mean flow velocity, when the field strength is constant.
Inside the electromagnetic flowmeter, the fluid passes through a magnetic field applied perpendicular to the
direction of flow. An electric voltage is induced by the movement of the fluid (which must have a minimum
electrical conductivity). This is proportional to the mean flow velocity and thus to the volume of flow. The
induced voltage signal is picked up by two electrodes, which are in conductive contact with the fluid and
transmitted to a signal converter for a standardized output signal. This method of measurement offers the
following advantages:

No pressure loss through pipe constriction or protruding parts.

Since the magnetic field passes through the entire flow area, the signal represents a mean value over the
pipe cross-section; therefore, only relatively short straight inlet pipes x DN from the electrode axis are
required upstream of the primary head.

Only the tube liner and the electrodes are in contact with the fluid.

Already the original signal produced is an electrical voltage, which is an exact linear function of the
mean flow velocity.

Measurement is independent of the flow profile and other properties of the fluid.
The magnetic field of the primary head is generated by a square wave current fed from the signal converter
to the field coils. This field current alternates between positive and negative values. Alternate positive and
negative flowrate-proportional signal voltages are generated at the same frequency by the effect of the
magnetic field, which is proportional to the current. The positive and negative voltages at the primary head
electrodes are subtracted from one another in the signal converter. Subtraction always takes place when the
field current has reached its stationary value, so that constant interference voltages or external or fault
voltages changing slowly in relation to the measuring cycle are suppressed. Power line interference voltages
coupled in the primary head or in the connecting cables are similarly suppressed.
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