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HOW MAGS WORK

 


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 VELOCITY

Since 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 5 microsiemens). The operation of magnetic flowmeters is based on Faraday's law of electromagnetic induction.


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 V

The 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 3 pipe diameters upstream and 2 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 mounting 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.