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Electromagnetic Flowmeter Review
The third most common flowmeter behind differential pressure and positive displacement flow meters, is the Electromagnetic 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.
The electromagnetic flowmeter is similar in
principle to the generator. The rotor of the generator is
replaced by a pipe placed between the poles of a magnet so
that the flow of the fluid in the pipe is normal to the
magnetic field. As the fluid flows through this magnetic
field, an electromotive force is induced in it that will be
mutually normal (perpendicular) to both the magnetic field
and the motion of the fluid. This electromotive force may be
measured with the aid of electrodes attached to the pipe and
connected to a galvanometer or an equivalent. For a given
magnetic field, the induced voltage will be proportional to
the average velocity of the fluid. However, the fluid should
have some degree of electrical conductivity.