A Look At How Electromagnetic Flow Meters Are Used

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					A Look At How Electromagnetic Flow Meters
Are Used
The modern process industry and utilities have been using electromagnetic flowmeters for about 60
years but the principals behind it's operation go back to the time when electricity was in its infancy. The
famous physicist and inventor, Michael Faraday formulated his law of electromagnetic induction
wherein a moving wire in a magnetic field will develop a voltage across it. In the 1900's, another
inventor, Father Bonaventura Thurlemann found that free ions in water and other conductive fluids also
were affected where two electrodes in a moving flow across a magnetic field, develop a voltage across
them.
Early attempts to create an electromagnetic flow meter using a constant magnetic field had some
success but it wasn't long before it was realised that the resulting measurements drifted with time.
It took a while to work out the physics of why this should be. What actually happens is that the positive
fee ions in the water flow in a pipe with electrodes in the pipe wall, gravitate towards the negative
electrode and the negative ions to the positive electrode. This creates a bifurcated laminar flow regime
and this affects the accuracy of measurement.
The solution was simple but it had to wait for advances in practical electronics to make a pulsed
reversing magnetic field possible. It was also realised that the actual conductivity of the fluid being
measured was not a factor as long as its electrical conductivity was a between 5 to 20 micro-siemens/cm
as a minimum. With the arrival of the practical magnetic flowmeter (or magmeter as they are
sometimes called), its reliability of measurement for aqueous solutions for both small flows in 50mm
(2") diameter pipelines in process plants and very large flows in 1.8m (72") diameter land drainage and
flood relief pipelines, became legendary.
Such mag meters have wide application, in fact anywhere where there is a conductive fluid to be
measured. These applications include process cooling plants, flood relief systems, wastewater
treatment, water treatment plants, processing food and many chemical processing plants.
But why use an electromagnetic flowmeter in place of say an ultrasonic flowmeter which utilises the
Doppler effect of a number transmitter/sensor pairs set flush in a pipe wall instead of electrodes? The
answer is that both can be used for conductive fluids but the ultrasonic flowmeter scores in the non-
conductive fluids such as most petroleum products and many chemicals. The big feature of both types of
meter is the clear pipe bore through the flow meter which then will not restrict or cause blockage
problems of the types of flow meter that intrude into the flow or otherwise modify the flow such as the
orifice flow meter or the vane meter.
There are many types of flowmeter that have been developed for the highly complex processing plants
in industry and that are on the market. There are big overlaps on their suitability for particular
requirements by the plant designer. At the end of the day, flow measurement method selection comes
down to: the fluid to be measured, the range of flows, operating temperatures and pressures, electrical
conductivity of course, type of fluid, plant pipeline arrangement restrictions and cost.
Finally getting back to mag meters, there are two basic types of such magflow installations that are
available; the inline type and the insertion type. The Inline type of electromagnetic flowmeter is the
most accurate; it being contained within a flanged unit with the same diameter as the pipeline. Sizes are
from 50mm diameter up to 600mm diameter for practical weight considerations. The insertion mag
meter is mounted on one side of a pipeline usually within a small branch tee. The integrated unit
contains both electro-magnets and the electrodes. Applications are from 200mm diameter up to about
1,000 mm diameter.

				
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posted:6/28/2011
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