Magnetic flow meter
A pipe section labeled MAG with a pair of inline electromagnetic coils. Magmeters work on conductive fluids and produce a clean linear signal without moving parts.
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How it’s drawn.
An inline pipe section marked as a magnetic meter, working on conductive fluids without moving parts. It produces a flow signal directly, so unlike a plate or venturi it does not need a separate differential-pressure element. Mounting is wafer between flanges on smaller lines or flanged on larger ones.
Variants.
- Wafer mounting. Clamped between flanges on lines from 1 inch up to about 4 inch.
- Flanged body. Standard line connection above 4 inch, lined and unlined options.
- Insertion, probe-style. Single-point measurement in large pipes where a full-bore meter is impractical.
- Sanitary, hygienic. Tri-clamp ends and FDA-compliant liners for food, beverage, and pharma service.
- Pulsed DC excitation. Current standard, low zero drift, immune to most process noise.
- AC excitation. Older technology, simpler signal conditioning, susceptible to slurry noise.
- Liner options. PTFE, PFA, hard rubber, polyurethane, ceramic. Chosen for chemical and abrasion resistance.
- Electrode materials. 316 stainless, Hastelloy C, tantalum, platinum-iridium for aggressive service.
Typical usage.
Water, slurry, conductive chemical service. Won't work on hydrocarbons, non-conductive. Maintenance-free in clean service. In slurry service the electrodes need periodic inspection.
Telling it apart.
- A magnetic meter works only on conductive fluids. It does not read non-conductive hydrocarbons.
- It outputs flow directly, so it does not need the differential-pressure transmitter that an orifice or venturi requires.
- Wafer and flanged versions differ in how the body mounts, usually carried as a note rather than a different symbol.
FAQ.
What conductivity does a magmeter need to work.
The standard minimum is 5 microsiemens per cm, which covers any aqueous solution including tap water, slurries, and most chemicals. Low-conductivity magmeters reach down to about 0.05 microsiemens per cm for demineralized water and dilute acid service.
Why will a magmeter not measure hydrocarbons.
Hydrocarbons are non-conductive. They do not induce a voltage across the electrodes when the fluid moves through the magnetic field. Crude oil, refined products, LNG, and natural gas all need a different primary element such as Coriolis, vortex, ultrasonic, or differential pressure.
Magmeter vs Coriolis. When to choose which.
Magmeters measure volumetric flow on conductive fluids, are line-size-friendly up to 100 inch, and tolerate dirty or slurry service. Coriolis measures mass flow directly on any fluid including hydrocarbons, but is expensive above 6 inch and sensitive to entrained gas. For water, slurry, and aqueous chemicals choose a magmeter. For hydrocarbons or custody transfer choose Coriolis.
What ISA 5.1 tag identifier does a magnetic flow meter use.
The functional tag is FT, flow transmitter or FIT, flow indicating transmitter followed by the loop number. The instrument type field on the I/O list typically reads magnetic flow meter or simply magmeter. The signal class is AI, analog input, 4-20 mA or HART for the transmitter output.
How much straight pipe does a magmeter need upstream and downstream.
Typical installation requires 5 pipe diameters upstream and 3 diameters downstream of any disturbance, elbow, valve, reducer. This is less restrictive than an orifice plate, 10 to 20 diameters and comparable to a Coriolis or vortex meter. Manufacturers publish exact numbers per model in the IOM.