CHAPTER 4 – Electromagnetic flowmeter
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Publisher Summary
This chapter focuses on the electromagnetic flowmeter. When a magnetic field is applied at right angles to the direction of motion of a conducting fluid, a potential difference is set up at right angles to both the flow direction and the magnetic field. There are several different types of electromagnetic flowmeters differing from each other in the way the magnetic field is produced. However, there are basically two main types: the D.C. and A.C. flowmeters. The D.C. electromagnetic flowmeter employs either a permanent magnet or a direct current energized magnet. The signal picked up at the electrodes is an exact indication of the direction and rate of flow and is typically of the order 10–100 μν. The main disadvantage of the D.C. electromagnetic flowmeter is polarization of the electrodes. A drawback of the D.C. instrument is its susceptibility to interfering potentials and stray thermoelectric and electrochemical effects. The A.C. flowmeters are subdivided depending on the shape of the A.C. voltage that energizes the magnet. They can be of the following types: (1) sine-wave excitation type, (2) gated sine-wave type, (3) square wave, (4) saw-tooth wave, (5) trapezoidal type, and (6) pulsed wave type. The advantage of A.C. excitation of the magnet is that electrode polarization is avoided because of thermoelectric and electromechanical effects. Also, it is easier to amplify A.C. signals than D.C. signals. Consequently, they can be used to measure very low flows. The operation of A.C. flowmeters also has its disadvantages, the main one being electrical pickup by the electrodes caused by the alternating field energizing the magnet.