Magnetic flowmeters use Faraday’s Law of Electromagnetic Induction to determine the
flow of liquid in a pipe. In a magnetic flowmeter, a magnetic field is
generated and channeled into the liquid flowing through the pipe. Following
Faraday’s Law, flow of a conductive
liquid through the magnetic field will cause a voltage signal to be sensed by
electrodes located on the flow tube walls. When the fluid moves faster, more
voltage is generated. Faraday’s Law states
that the voltage generated is proportional to the movement of the flowing
liquid. The electronic transmitter processes the voltage signal to determine
liquid flow.
A magnetic flow meter (mag flowmeter) is a volumetric flow
meter which does not have any moving parts and is ideal for wastewater
applications or any dirty liquid which is conductive or water based. Magnetic
flowmeters will generally not work with hydrocarbons, distilled water and many
non-aqueous solutions). Magnetic flowmeters are also ideal for applications
where low pressure drop and low maintenance are required.
Principle of Operation
The operation of a
magnetic flowmeter or mag meter is based upon Faraday's Law, which states that
the voltage induced across any conductor as it moves at right angles through a
magnetic field is proportional to the velocity of that conductor.
Faraday's Formula:
E is proportional to V x B x D where:
E = The voltage generated in a conductor
V = The velocity of the conductor
B = The magnetic field strength
D = The length of the conductor
To apply this principle
to flow measurement with a magnetic flowmeter, it is necessary first to state
that the fluid being measured must be electrically conductive for the Faraday
principle to apply. As applied to the
design of magnetic flowmeters, Faraday's
Law indicates that signal voltage (E) is dependent on the average liquid
velocity (V) the magnetic field strength (B) and the length of the conductor
(D) (which in this instance is the distance between the electrodes).
Magnetic flowmeters measure the velocity of conductive
liquids in pipes, such as water, acids, caustic, and slurries.However fluids
with low conductivity, such as de-ionized water, boiler feed water, or
hydrocarbons, can cause the flowmeter to turn off and measure zero flow.
This flowmeter does
not obstruct flow, so it can be applied to clean, sanitary, dirty, corrosive
and abrasive liquids. Magnetic flowmeters can be applied to the flow of liquids
that are conductive, so hydrocarbons and gases cannot be measured with this
technology due to their non-conductive nature and gaseous state respectively.