Magnetic Flow Meter – Principle and Application

What is Magnetic Flow Meter

A magnetic flow meter (mag meter, electromagnetic flowmeter) is a transducer that measures fluid flow by the voltage induced across the liquid by its flow through a magnetic field.

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.

In a practical sense, only liquid flows are measurable using this technique. Gases must be super-heated into a plasma state before conducting electricity, so magnetic flowmeters cannot be used with most industrial gas flow streams.

Consider water flowing through a pipe, with a magnetic field passing perpendicularly through the pipe:

The direction of liquid flow cuts perpendicularly through the lines of magnetic flux, generating a voltage along an axis perpendicular to both.

Metal electrodes opposite each other in the pipe wall intercept this voltage, making it readable to an electronic circuit.

A few conditions must be met for this formula to successfully infer volumetric flow rate from induced voltage:

• The liquid must be a reasonably good conductor of electricity (note: it is okay if the conducting fluid contains some non-conducting solids; the conductive fluid surrounding the non-conducting solid matter still provides electrical continuity between the electrodes necessary for induction)
• The pipe must be completely filled with liquid to ensure contact with both probes as well as to ensure flow across the entire cross-section of the pipe
• The flow tube must be properly grounded to avoid errors caused by stray electric currents in the liquid

The first condition is met by careful consideration of the process liquid prior to installation. Magnetic flowmeter manufacturers will specify the minimum conductivity value of the liquid to be measured.

The second and third conditions are met by the correct installation of the magnetic flow tube in the pipe. The installation must be done in such a way as to guarantee full flooding of the flow tube (no gas pockets).

Typical Industries Where Magnetic Flow Meters are applied

Magnetic Flow Meter Selection

• Is the fluid conductive or water-based?
• Is the fluid or slurry abrasive?
• Minimum and maximum flow rate for the flow meter?
• Minimum and maximum process pressure?
• Minimum and maximum process temperature?
• Is the fluid chemically compatible with the flow meter wetted parts?
• What is the size of the pipe?
• Is the pipe always full?
• Do you require an integral display or remote display?
• Do you require an analog output?