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How
Electromagnetic Flowmeters Work
Measuring
Flow with an Electromagnetic Flow Meter
Electromagnetic flow meters actually measure the velocity
(speed) of fluids in a pipe. Flow is then automatically calculated
in the controller by multiplying the velocity by the cross
sectional area of the pipe.FLOW
(Q) = AREA x VELOCITYSince
the cross sectional area of the pipe can be calculated from
the pipe diameter we need only know the velocity of the fluid
in order to calculate the flow.
FLOWMETER OPERATION
The magnetic flowmeter consists of a non-magnetic pipe
lined with an insulating material. A pair of electromagnetic
coils is situated and a pair of electrodes penetrates the
pipe and its lining (as shown below). Magmeters can detect
the flow of conductive fluids only (greater than .05 microsiemens).
The operation of magnetic flowmeters is based on Faraday's
law of electromagnetic induction.
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As a conductive fluid flows through the magnetic field generated
by the coils, a voltage (E) is developed across the electrodes,
as predicted by Faraday's law. The voltage will be proportional
to the velocity (V) of the liquid. Because the magnetic field
density and the pipe diameter are fixed values, they are converted
into a calibration factor (K) which is labeled on each sensor.
E = K x VThe
magmeter's K factor is determined by a traceable wet calibration
of each flow sensor during manufacture. This K value is valid
for any conductive liquid and is linear over the entire flowmeter
range. Magmeters can measure flow in both directions, as reversing
direction will change the polarity but not the magnitude of
the signal.
FLOMOTION
electromagnetic flowmeters generally consist of two main sections,
the meter itself and a controller unit which interprets the
signal received from the sensor and displays the flow on a
readout. Most models have the option of having the controller
unit separated from the meter itself via a pair of cables.
This allows easy access to the controller for wiring control
and alarm signals and for viewing the display and adjusting
program parameters.
For
a flow meter to give accurate readings, it must be properly
installed. Electromagnetic flowmeters must have a full pipe
in order to read accurately. This is best accomplished by
locating the flow meter in the pipe at a point that will always
be full. It is also important to situate the flow meter in
a straight length of pipe.
SENSOR LOCATION
A rule of thumb is that the meter be located no less than
10 pipe diameters upstream and 5 diameters downstream of a
bend or change in the pipe size. Additionally, the flow meter
should usually be placed ahead of, not after, a valve. Valves
will distort a flow pattern for as many as 25 pipe diameters
downstream of the valve.
GROUNDING
Magmeters require excellent grounding for best accuracy and
special grounding techniques are sometimes needed. For example,
when mounted a sensor in plastic or other non-conductive pipes
a separate grounding ring may be required to provide a sufficient
electrical connection to the liquid. These rings are easily
placed on either side of the magmeter during installation.
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