Magnetic Flow Meters Principle

Electromagnetic flowmeters operate on the basis of Faraday’s law of induction. According to this principle, any change in the magnetic flux linked to an electric circuit causes an electromotive force (or voltage) to be induced in this circuit. Experiments demonstrate that the faster the change in the magnetic flux, the higher induced voltage. The induced voltage is therefore directly proportional to the rate of change of magnetic flux with time.

This principle is used in the electromagnetic flowmeter to measure the volume flow rate of the fluid. The flowmeter typically consists of two electromagnetic coils that are mounted on opposite sides of a non-magnetic measuring tube. Two electrodes are fitted inside the tube to detect the voltage generated by the conducting fluid. Although these electrodes come into contact with the fluid, they do not obstruct its flow. The inside of the measuring tube is lined by an insulating material such as rubber, neoprene, etc. which is resistant to the properties of the fluid. The flowmeter also includes an electronic control unit which calculates the flow rate and displays the output.
 

When current is applied to the coils, they generate an alternating magnetic field across the cross-sectional area of the tube. A fluid flowing through the magnetic field acts as a conductor, and a voltage is induced. The induced voltage is picked up by the electrodes and sent to a transmitter which is either mounted on the flowmeter or connected remotely. The transmitter calculates the volumetric flow rate and displays the output.
 

Calculation of the volumetric flow rate

The induced voltage is calculated using the following equation:

U = B x d x v

Where:
  U = voltage induced by the conducting fluid 
  B = magnetic flux density
  d = distance between the electrodes 
  v = average velocity of the conductor (i.e. fluid flowing in the tube through the magnetic field)

Since the magnetic flux density and the distance between the electrodes remain constant, the induced voltage is directly proportional to the conductor velocity. The value of the velocity is used to calculate the volumetric flow rate as follows:

Q = A x v

Where:
  Q = volumetric flow rate of the fluid
  A = cross-sectional area of the tube
  v = flow velocity of the fluid.