An electromagnetic (magnetic) flowmeter measures the flow of a conductive liquid using Faraday’s law of electromagnetic induction: a conductor moving through a magnetic field generates a voltage proportional to its velocity.
Principle
Coils around the pipe generate a magnetic field; electrodes in contact with the liquid pick up the induced voltage, which is proportional to flow velocity and therefore to volume flow. With a known pipe diameter, velocity converts directly to flow rate.
Key advantages
- No moving parts and no obstruction in the flow path — negligible pressure loss.
- Unaffected by viscosity, density or temperature within limits.
- Handles slurries and aggressive liquids with the right liner and electrodes.
Selection criteria
- Conductivity — the liquid must be conductive (typically >5 µS/cm); it will not measure hydrocarbons.
- Liner and electrode material — PTFE, rubber, PFA liners; Hastelloy, titanium or platinum electrodes against the chemical.
- Line size and flow range.
- Output and communication — 4-20 mA, pulse, HART or fieldbus.
- Environment — IP/NEMA rating and grounding requirements.
Sourcing tips
Match line size, liner, electrode and output; verify the conductivity of the medium. Correct grounding is essential — a poorly earthed magmeter drifts regardless of its specification.
Liner selection — the decision that decides service life
On a magnetic flowmeter, the measuring principle is settled and the electronics are largely equivalent between reputable makers. What determines whether the meter lasts five years or fifteen is the liner and the electrodes. Both are chosen against the medium, its temperature and its abrasiveness.
| Liner | Good for | Avoid |
|---|---|---|
| Soft rubber / neoprene | Water, mild aqueous solutions, low-temperature service | Solvents, oils, concentrated acids |
| Hard rubber / ebonite | Water, sewage, abrasive slurries, higher temperatures than soft rubber | Oxidising chemicals, solvents |
| PTFE | Aggressive acids and alkalis, food and pharmaceutical service | High vacuum, abrasive slurries where the liner can be scored |
| PFA / FEP | Same as PTFE where some vacuum resistance or flexibility is needed | Severe abrasion |
| Ceramic | Highly abrasive slurries, high temperature, long life in mining and minerals | Anything with mechanical shock or rapid thermal cycling |
Electrode material follows the same logic. 316L covers water and dilute aqueous solutions; Hastelloy C covers oxidising acids and seawater; titanium covers chlorides and seawater; tantalum covers strong acids but not hydrofluoric acid or strong caustic; tungsten carbide covers abrasive slurries. Choosing an exotic electrode for a mild duty wastes money, and choosing a mild electrode for an aggressive duty is how a meter fails in its first month.
The installation conditions that cause the most problems
Most “the meter reads wrong” complaints are installation issues rather than instrument faults. Four conditions account for the majority:
- The pipe is not completely full. The measurement assumes a full bore. A meter at the high point of a system, or discharging to atmosphere, will read high. Move it to a low point or a rising section.
- Poor grounding. The signal is a millivolt-level voltage referenced to the medium. In an ungrounded or plastic-lined pipe, grounding rings or a grounding electrode are mandatory, not optional.
- Insufficient straight run. Fittings, valves and pumps create swirl that distorts the flow profile. Allow the maker’s stated upstream and downstream lengths, and fit the meter upstream of control valves rather than downstream.
- Conductivity below the minimum. Magnetic flowmeters need a conductive medium, typically above roughly 5 µS/cm. Deionised water, some solvents and most hydrocarbons are outside the measuring principle entirely — no amount of configuration will help.
What to write on an enquiry
A flowmeter enquiry that produces an accurate quotation the same day carries seven facts. Missing any one of them usually changes the recommendation:
- Medium and concentration — the liquid itself, plus the concentration if it is a solution.
- Temperature range, including any CIP or cleaning cycle at higher temperature.
- Pressure range, including surge or water-hammer conditions.
- Flow range — minimum and maximum, plus the normal operating point. The turndown requirement decides the size.
- Line size and connection standard — flange standard, rating and facing, or a clamp/thread standard.
- Conductivity, if it is unusual. If in doubt, say so and it will be checked.
- Output and communication — 4–20 mA, pulse, HART, Modbus, and whether a totaliser or batch function is needed.
Questions we are asked about magnetic flowmeters
Can a magnetic flowmeter measure a non-conductive liquid? No. Below roughly 5 µS/cm the signal becomes unusable. For hydrocarbons and pure solvents the alternatives are positive displacement, Coriolis or ultrasonic meters, depending on the accuracy required.
How often should the electrodes be cleaned? It depends entirely on the medium. A coating on the electrodes is the most common cause of a slow drift in reading. Where a coating forms, an electrode-cleaning function or a periodic manual clean is part of normal maintenance.
Should I size for the maximum flow? Size for a velocity that keeps the reading accurate across your operating range — too large a bore and low flows become unreadable, too small a bore and pressure drop and liner wear increase. Give the supplier the true minimum and maximum, not just the maximum.
Is a full-bore meter always the best choice? Not always. Insertion and clamp-on variants exist for large pipes where the cost of a full-bore meter is prohibitive. They trade accuracy and turndown for cost, which is a legitimate trade if the application allows it.
References we have recently quoted in this category
33111 MSK061C-0600-NN-M1-UG1-NNNN A93570280 A11431374 3625100205 89609069 96064-33 98240-696 FBM217 C170S09 A93575050 C20466-30 9806087 C2P18-30001 98650-379 QX102910 8zZ1-E100C 2117383 20277300 47657069001 DNR-18 51L223 73145 9138232-460 98076-30 A93575540 A93570930 70051 LT-3050 A51128974 C202285 A08992674 96064-36 A11840674 QX103230 A93570270 A04092974 72651 CV700U2-T01 A11405874
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