Electromagnetic flowmeter M910
Operation manual
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Appendix A 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.