An electromagnetic flow meter, or mag meter, measures flow of a conductive liquid in a full pipe from the voltage induced as it crosses a magnetic field.
An electromagnetic flow meter, often called a mag meter, measures the flow of a conductive liquid in a full pipe using Faraday's law of induction. Coils in the meter body create a magnetic field across the pipe. As water moves through the field it acts like a moving conductor, and a small voltage appears across the flow. Two electrodes in the pipe wall pick up this voltage, which is proportional to the mean velocity. The meter multiplies velocity by the bore area to give flow rate. The voltage follows E = k·B·D·v, where B is the field strength, D the inner diameter, v the mean velocity and k a meter constant set at factory calibration. Because the signal comes from the whole cross section, the meter is less sensitive to flow profile than many other types. Mag meters are common in water and wastewater utilities: treatment plant inlets and outlets, pumping stations, district meters, borehole outlets and sludge lines. The bore is open, with no moving parts, so pressure loss is very small and solids pass through. The liquid must conduct electricity. Drinking water and wastewater are well above the usual minimum, often quoted as about 5 µS/cm, but very pure water such as demineralised water or reverse osmosis permeate may be too low. The pipe must run full, or the reading will be wrong. Installers still keep a short straight run, often about 5 diameters upstream and 2 downstream, and earth the meter so stray electrical currents do not disturb the signal. Grease or scale on the electrodes can cause drift,. For example, a 200 mm mag meter on a borehole main reads 0.9 m/s. The bore area is 0.0314 m², so flow is about 0.028 m³/s, or 102 m³ per hour. Strengths are good accuracy, no obstruction and long life. Limits are the need for conductivity, a full pipe and power for the coils.