Electromagnetic flow meters, commonly called mag flow meters, are the most widely used flow meter type in New Zealand water, irrigation, and industrial applications. Despite their ubiquity, the technology behind them is not widely understood.
Mag flow meters work on Faraday's law of electromagnetic induction: when a conductive fluid moves through a magnetic field, a voltage is induced that is proportional to the velocity of the fluid.
In a mag flow meter: magnetic coils on either side of the pipe generate a magnetic field across the pipe bore; electrodes mounted in the pipe wall detect the voltage induced as the conductive fluid flows through the field; and the transmitter converts the measured voltage to a flow velocity, and from there to a volumetric flow rate based on the known pipe cross-sectional area. Because the measurement happens entirely through the fluid and the pipe wall, with no physical contact between the measurement mechanism and the flowing fluid, mag meters have no moving parts.
The liner, an insulating sleeve that prevents the induced voltage from being short-circuited through the pipe wall. Common materials: rubber (EPDM, neoprene), PTFE, and PFA. Liner choice affects chemical compatibility and temperature rating.
The electrodes, two or more electrodes in the pipe wall that measure the induced voltage. Stainless steel is standard; Hastelloy and titanium available for aggressive fluids.
The transmitter, receives the millivolt signal from the electrodes and converts it to usable outputs (4–20mA, pulse, digital communication). Can be integral or remote-mounted.
EPDM rubber for standard water and wastewater. PTFE or PFA for chemical applications. Ceramic for abrasive slurries.
Stainless steel for standard water and wastewater. Hastelloy or titanium for aggressive fluids.
IP67 for general outdoor and wet locations. IP68 for submerged or regularly flooded installations.
Every 5 years for Canterbury water take applications. Annually for trade waste discharge metering.
Mag meters work for any conductive water, including dirty water, water with suspended solids, and wastewater. Not suitable for ultra-pure/deionised water or non-conductive fluids.
±0.5% or better under good installation conditions. Accuracy can be affected by insufficient straight pipe run, partially full pipe, electrode fouling, and electrical interference.
15–20 years or more with correct installation and regular maintenance. No moving parts to wear out, main tasks are electrode cleaning and periodic transmitter checks.
Yes, flow should be upward for vertical installations. For horizontal, position electrodes at 3 and 9 o'clock to avoid gas bubble interference at the top and sediment fouling at the bottom.
Canterbury-based flow meter specialists. Blue Tick accredited. IANZ-calibrated equipment.
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