Answer / me

The third most common flowmeter behind differential pressure
and positive displacement flow meters, is the magnetic flow
meter, also technically an electromagnetic flow meter or
more commonly just called a mag meter. A magnetic field is
applied to the metering tube, which results in a potential
difference proportional to the flow velocity perpendicular
to the flux lines. The physical principle at work is
electromagnetic induction. The magnetic flow meter requires
a conducting fluid, for example, water that contains ions,
and an electrical insulating pipe surface, for example, a
rubber-lined steel tube.
Usually electrochemical and other effects at the electrodes
make the potential difference drift up and down, making it
hard to determine the fluid flow induced potential
difference. To mitigate this, the magnetic field is
constantly reversed, cancelling out the static potential
difference. This however impedes the use of permanent
magnets for magnetic flowmeters.

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