Magnetic Flowmeter – Principle and Applications of the Mag Flowmeter

A magnetic flowmeter is sometimes referred to as a mag flowmeter. It is used to measure the flow rate of liquids through a pipe or tube. Such meters are commonly used in the management of large scale water projects like waste water reclamation or land irrigation schemes.

The magnetic flowmeter is unique from most flowmeters in that it is the translation of Faraday's Law of Induction from theory into fact. Faraday's Law of Induction explains that when an electrical conductor is moved through a magnetic field at right angles to the field, an electromotive force (emf) or voltage is produced in the conductor. The generated voltage is directly proportional to the velocity of the conductor.

How Does it Work?

In the case of a large pipe used for waste water reclamation or an irrigation scheme, the magnetic flowmeter would consist of alternating current coils positioned diametrically opposite each other on the outside of the pipe. These would generate a uniform parallel magnetic field. The water passing through is the electrical conductor.

A voltage is generated between two electrodes placed on a diameter of the tube or pipe at right angles to the the magnetic field and flow direction. The resulting alternating current voltage is measured by sensors and used to indicate the volumetric rate of flow. The discharge, proportional to the velocity and pipe area, is therefore directly proportional to the voltage because the flow tube area is constant.

How do You Use Magnetic Flowmeters

Magnetic flowmeters calculate the velocity of conductive liquids in pipes, such as water, acids and slurries, and also they can calculate properly when the electrical conductivity of the liquid is higher than approximately 5mS/cm. You should be careful because using magnetic flow meters on liquids with little conductivity, like de-ionized water, boiler feed water, or hydrocarbons, can result in the flow meter turning off and measuring zero flow. Since this type of flow meter doesn’t block flow it can be applied to clean sanitary, dirty, corrosive, and harsh fluids.

Limitations and Hazards of Using Magnetic Flowmeters

Magnetic flowmeters can only be used to measure the flow rate of liquids that are conductive.

You shouldn’t operate a magnetic flow meter near its electrical conductivity limit, because the flow meter can turn off. Also you should provide allowance for altering composition and operating circumstances that can change the electrical conductivity of the liquid.

Be sure to size magnetic flowmeters to run higher than the velocity at which solids clear up, in order to steer clear of filling the pipe with solids that can influence the measurement and potentially halt flow.

Summary

Magnetic flowmeters are popular where large quantities of conductive liquids are moving. Like a mass flowmeter, They have no moving parts so are relatively maintenance free and not prone to break down or stop working. They are easy to install and in some cases are portable.