Electromagnetic Flow Meters, by Trevor Forster
In each issue of our fLowdown newsletter we review a particular flow metering technique, its benefits, shortfalls and the applications to which it is best suited. This article extract from our newsletter focuses on electromagnetic flow meters.
Electromagnetic Flow Meters
Informed opinion indicates that electromagnetic flow meters are the third fastest growing sector after meters based upon ultrasonic flow meter and Coriolis technologies. The fundamental operating principle of electromagnetic flow meters is extremely simple – put a moving conductor in a magnetic field and this will induce a voltage at ninety degrees to the field and the movement … Flemings right hand rule.
In the illustrated electromagnetic flow meter, coils on the top and bottom of the pipe provide the magnetic field and electrodes placed at 90° to these provide the electrodes to measure the induced voltage in the moving conductor on the third axis (the fluid). So much for the basic theory, in 1839 Faraday famously attempted to measure the velocity of the UK’s Thames River using the earth’s magnetic field and a pair of electrodes across the river. One problem with constant magnetic fields are that they produce a dc voltage which in turn causes polarisation on the electrodes which can affect results over time.
Electromagnetic flow meters use an alternating voltage which negates these long term errors. Of course the fluid must be conductive for electromagnetic flow meters to work so a large number of liquids are excluded. As a consequence this shortfall has spurred developments being done using ultrasonic and Coriolis alternative technologies. Modern electromagnetic meters can work with very low conductivities and more research is being done to increase the sensitivity even further. Obviously the higher the fluid conductivity the easier the measurement is to make as the relative output voltage will be higher for a given fluid velocity. These meters can be extremely accurate and operate over a wide choice of pipe sizes and flow turndowns.
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