Accurate measurement of asymmetric flow by means of a multi-electrode induction flowmeter

Induction flowmeters are relatively accurate non-invasive devices for measuring volume flow of electrically conducting liquids in pipes. Whereas under ideal conditions, axially symmetric flow can be measured to arbitrary accuracy, asymmetric flow (e.g. as caused by bends or valves) leads to significant measurement errors. Thus, a relaxation pipe upstream of the flowmeter must be provided to ensure accurate results. The subject of this work is an improved induction flowmeter that is less sensitive to asymmetric flow and that can be used without relaxation pipes. It is shown that the accuracy of an induction flowmeter subject to asymmetric flow is greatly improved by using more than two electrodes and a second magnetic field direction. In experimental set-ups with extremely strong asymmetry, a conventional two electrode flowmeter produced measurement errors of up to 90%. A four electrode flowmeter permitted measurements with an error of about 20%. Satisfactory results with an error of approximately 3% were achieved using six or more electrodes. Additional measurements with more realistic flow profiles caused by bends and orifices proved that a six electrode setup is capable of measurement errors below 0.5% in virtually any flow situation. A flowmeter of that type could be installed downstream of bends, valves and orifices without any relaxation pipe. Furthermore, a sensor of that type can be assembled with standard magnetic field systems and only slightly modified transducers.