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FLOW MEASUREMENT TECHNOLOGIES

Electromagnetic flowmeters: measuring conductive liquids with nothing in the flow path

An electromagnetic flowmeter measures the flow of any liquid that conducts electricity — water, wastewater, sludge, slurries, acids and alkalis — through a pipe with nothing inside it: no moving parts, no obstruction and no pressure loss. That is why it is the usual choice for water and wastewater, and a common one in chemical plants, food production and mining.

Electromagnetic flowmeters in our catalogue

How an electromagnetic flowmeter works

The principle is Faraday’s law of induction: a conductor moving through a magnetic field develops a voltage across it. In an electromagnetic flowmeter the conductor is the liquid itself. Two coils, one above and one below the measuring tube, create a magnetic field that crosses the pipe at right angles to the flow.

As the liquid flows through the field, a small voltage appears across it, at right angles to both the field and the flow. Two electrodes set into the pipe wall, facing each other, pick it up. The voltage is proportional to the average flow velocity — twice the velocity, twice the voltage (U = k · B · v · D, where B is the field strength and D the inside diameter).

The transmitter multiplies that velocity by the cross-section of the tube to give volume flow. Modern meters switch the field on and off, or reverse it, several times a second; comparing the readings cancels the slow electrochemical voltages that build up on the electrodes, so the zero point stays stable.

Nothing in the measuring tube moves or narrows the flow, so there is no wear and no pressure loss. The reading does not depend on the density, viscosity, temperature or pressure of the liquid; all it needs is a minimum conductivity. It is also less sensitive to the flow profile than most meters that measure velocity — which is why the straight runs it needs are short — and it measures in both directions.

  • The conductive liquid flowing through the measuring tube (in the cross-section, into the page)
  • Field coils: they create the magnetic field across the pipe
  • The magnetic field B, at right angles to the flow
  • Electrodes in the pipe wall, on the horizontal axis: they pick up the induced voltage
  • Transmitter: turns the voltage into a flow reading
  • The voltage U rises in proportion to the flow velocity v

Where an electromagnetic flowmeter fits — and where it does not

A strong choice for

  • Water and wastewater: abstraction, treatment plants, distribution networks, irrigation and cooling water
  • Sludge, slurries and liquids carrying solids: with nothing in the tube to block, they pass freely
  • Acids, alkalis and other corrosive liquids, with liners such as PTFE or PFA and electrodes in resistant materials
  • Large pipes: the cost stays reasonable in large sizes, and our catalogue goes up to DN 3000
  • Pumping mains and long pipelines, where every bit of pressure loss costs energy
  • Hygienic processes — dairy, beverages, filling — with designs approved to EHEDG and 3-A

Think twice when

  • The liquid does not conduct electricity: oils, fuels, solvents and demineralised water. Every meter has a minimum conductivity — 5 μS/cm for the OPTIFLUX 1050 and 1300, for example
  • You are measuring gas or steam: they do not conduct, so the method cannot work
  • The pipe may run partly full: a standard meter needs a full pipe, and partly filled pipes need a special meter such as the TIDALFLUX 2300
  • The liquid carries a lot of gas bubbles, or leaves deposits that can coat the electrodes, such as grease or scale — unless the meter and its electrodes are chosen for it
  • You need mass flow: an electromagnetic flowmeter measures volume, so mass needs a known density — or a Coriolis flowmeter
  • The process is very hot: the liner or the measuring tube sets the limit — +180 °C for the OPTIFLUX 4300 and 5400 in our catalogue, for example

How it compares with the other technologies

Typical accuracy of each flow measurement technology, ± % of the reading, on a logarithmic scale. The further left, the more accurate. The technology of this article is highlighted.
  • Coriolis±0.1–0.2%
  • Electromagnetic (the technology of this article)±0.2–0.5%
  • Ultrasonic±0.5–2%
  • Differential pressure±0.5–2%
  • Variable area±1.6–4% *
  • Vortex±0.75–2%
  • Thermal mass±1–3%
  • Positive displacement±0.2–0.5%
  • Turbine±0.5–1%

Typical ranges for liquids (thermal mass: gases), ± % of the reading; for variable area, the accuracy class (*). The accuracy of a specific instrument depends on the model, the size and the installation — we confirm it for your application.

 All technologies

Installing an electromagnetic flowmeter

An electromagnetic flowmeter needs far less straight pipe than most meters that measure velocity. A typical requirement is 5 D upstream and 2 D downstream, counted from the nearest bend, valve or reducer (D = nominal diameter). Some models need even less: the WATERFLUX 3070 and 3400 are specified with 0 D on both sides, while the OPTIFLUX 6400 asks for 2 D downstream.

Above all, the measuring tube must stay full of liquid, and the electrodes must stay wetted and free of gas bubbles and deposits. The position of the meter and the angle of its electrodes are chosen with that in mind — and because the signal is very small, the liquid has to be properly earthed:

Straight runsTypically at least 5 D of straight pipe before the meter and 2 D after it, counted from the nearest bend, valve or reducer.
A full pipeInstall the meter in a rising line or at a low point, never at the highest point of the line, where air collects, or in a downward line that may run empty.
Electrodes on the horizontal axisIn a horizontal line, turn the meter so that the electrodes lie on the horizontal axis: gas bubbles rise to the top and solids settle at the bottom, both away from the electrodes.
EarthingThe liquid, the meter and the pipe must be at the same potential. With plastic or internally lined pipes, fit an earthing ring at each flange and connect both to the meter and to earth.
  • Fit control and shut-off valves after the meter, not before it, so that the tube stays full and the flow reaches the meter undisturbed.
  • Do not install the meter on the suction side of a pump, where vacuum can damage the liner; on the discharge side, keep some distance from the pump outlet.
  • Choose the size by flow velocity rather than by the pipe: typically around 1–3 m/s. When the flow is low, a meter smaller than the pipe, with reducers on either side, gives a better reading.
  • Make sure the gaskets do not protrude into the pipe. For liquids with solids, prefer a vertical line with upward flow: the liner wears evenly and no sediment builds up.

Electromagnetic flowmeters by industry

Water & WastewaterAbstraction, treatment plants, sludge and dosing lines, and district metering of drinking water with approvals for potable water and for billing (for example the battery-ready WATERFLUX 3070, MI-001 / OIML R49). For sewers that run partly full, the TIDALFLUX 2300.ChemicalAcids, alkalis and other aggressive liquids, with PTFE or PFA liners or ceramic measuring tubes (for example the OPTIFLUX 4100 and 5100), and approvals for hazardous areas such as ATEX and IECEx.Food & BeverageMilk, juices, beer and other beverages, CIP lines and filling machines, with hygienic designs and approvals such as EHEDG and 3-A (OPTIFLUX 6400), and dedicated meters for volumetric filling (BATCHFLUX 5500).Mining, Minerals & MetalsOre slurries, tailings and process water, where abrasion is the main enemy: wear-resistant liners such as polyurethane or rubber, or ceramic measuring tubes.Power & EnergyCooling water and water treatment in power plants and, for nuclear plants, meters with nuclear qualification on request (POWERFLUX series).MarineSeawater cooling and ballast water systems on board, with class approvals such as DNV, ABS and Lloyd’s Register (for example the OPTIFLUX 4100).

Electromagnetic flowmeters: frequently asked questions

Ask about electromagnetic flowmeters

Tell us the liquid (and its conductivity, if you know it), the pipe size, the flow range, the pressure and the temperature, and our engineers will suggest the right meter, with price and delivery time.