How to Choose an Electromagnetic Flowmeter
lining, electrode & grounding
The electromagnetic flowmeter (magmeter) is the workhorse for conductive liquid flow in water and wastewater — no moving parts, unaffected by viscosity or density, and a straight length of pipe gives a stable reading. But it only works on conductive fluid, and the wrong lining, electrode material, or grounding produces noise or a dead zero. Get these three right and a magmeter runs for years.
1. How a magmeter works
By Faraday's law of induction, a conducting fluid moving through a magnetic field induces a voltage proportional to velocity (V × B → EMF). There are no obstructing parts, so pressure loss is zero and the reading is independent of viscosity, density, or downstream pressure.
2. The conductive-fluid rule
A magmeter needs the fluid to conduct electricity — practically, conductivity above about 5 µS/cm. That covers water, wastewater, and most slurries, but excludes hydrocarbons, oils, and distilled / deionized water (unless specially conditioned). If your fluid is non-conductive, a magmeter will read zero — choose ultrasonic or another technology instead.
3. Lining selection
- PTFE / PFA. Resists aggressive chemicals and abrasion; for corrosive or gritty streams.
- Natural / soft rubber. Economical workhorse for clean water and wastewater; limited temperature.
- Ceramic / hard rubber. For highly abrasive slurries.
Match lining temperature and pressure ratings to the line.
4. Electrode material
- 316L stainless. General water / wastewater.
- Hastelloy. Chlorides and oxidizing services.
- Titanium. Seawater and brackish water.
- Tantalum / Platinum-Iridium. Severe oxidizing acids.
Electrode choice is a corrosion decision — specify it from your fluid chemistry, not price.
5. Grounding & earthing rings
A magmeter needs a stable reference potential. In plastic or non-conductive pipes, or near variable-frequency drives and stray current, you must use grounding / earthing rings (or grounding electrodes) so the signal is not drowned in noise. Skipping this is the most common cause of an unstable or zero reading.
6. Sizing & output
Design velocity of 1–5 m/s is the sweet spot: too slow invites sedimentation and zero instability, too fast wastes pressure and wears the lining. Look for empty-pipe detection, 4–20 mA / pulse / Modbus output, and a battery option for unmanned rural sites. Full-bore vs insertion depends on pipe size and access.
7. Magmeter vs ultrasonic
Magmeters need a conductive fluid and excel on dirty wastewater. Ultrasonic meters need clean liquid with few bubbles and work on non-conductive fluids and very large pipes. Both are non-intrusive to the flow; for typical water/wastewater, the magmeter is the more robust default.
Related buying guides
Frequently asked questions
Can a magmeter measure oil or hydrocarbons?
No. A magmeter requires the fluid to conduct electricity; oils, hydrocarbons, and distilled/deionized water are non-conductive and will read zero. Use an ultrasonic meter (for clean liquid) or another technology for non-conductive fluids.
Why are grounding rings needed?
Grounding / earthing rings provide a stable reference potential so the induced signal is not overwhelmed by stray current, VFD noise, or the non-conductive pipe wall. Without them on plastic or grounded-metal pipe, readings drift or sit at zero.
Magmeter or ultrasonic?
Choose a magmeter for conductive, dirty water/wastewater (robust, no moving parts). Choose ultrasonic for clean, bubble-free, non-conductive fluids and very large pipes. Both are obstructionless; the fluid conductivity decides it.