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FLOW MEASUREMENT

Flow measurement technologies: how flowmeters work and which one to choose

A flowmeter measures how much fluid passes through a pipe per unit of time. No single technology suits every job: each one relies on a different physical principle, with its own strengths and limits. This page covers the nine main technologies — how each one works, where it fits and where it does not — and the criteria for choosing the right one for your application.

What exactly are we measuring?

Volume flow or mass flow

Most flowmeters measure volume flow, in m³/h or l/min: in effect they measure how fast the fluid moves and, since the cross-section of the pipe is known, work out the volume passing through. A few technologies measure mass flow directly, in kg/h — Coriolis in liquids and gases, thermal meters in gases.

The difference matters most with gases and with liquids whose temperature changes: their volume varies with pressure and temperature, their mass does not. When a volumetric meter measures gas or steam, the reading usually needs pressure and temperature compensation to give mass, or volume at standard conditions.

Direct or inferred measurement

Only a few technologies measure the quantity you want directly: a positive displacement meter fills and empties chambers of known volume, and a Coriolis meter in effect weighs the flow. Most measure something else and derive the flow from it: velocity (electromagnetic, ultrasonic, vortex, turbine), the heat carried away (thermal), the pressure drop across a restriction, or the position of a float.

Meters that rely on velocity or differential pressure are affected by the flow profile in the pipe. That is why they usually need a straight run of pipe before and after the meter, so that bends and valves do not distort the flow where it is measured.

The nine flow measurement technologies

For each one: the operating principle in a sketch, where it fits and where it does not, and how many instruments of that technology are in our catalogue.

Coriolis flowmeters

The fluid flows through one or two tubes that vibrate at their natural frequency. The flow twists the tubes slightly, and two sensors pick up the twist as a time difference proportional to mass flow; the vibration frequency also gives the density.

Where it fits
Custody transfer, batching and dosing, liquids whose density or viscosity changes, non-conductive liquids, and installations with no room for a straight run.
Where it does not
Very large pipe sizes, where size and cost climb steeply, gases at low pressure, and steam.
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Electromagnetic flowmeters

Two coils create a magnetic field across the flow. A conductive liquid moving through the field generates a voltage proportional to its velocity (Faraday’s law), which electrodes in the pipe wall pick up. The bore stays completely clear, with no moving parts.

Where it fits
Water, wastewater, sludge, slurries and corrosive liquids, from small to very large pipe sizes, with no pressure loss.
Where it does not
Non-conductive liquids such as oils, fuels and deionised water, as well as gases and steam. The OPTIFLUX 1050 and 1300, for example, need a conductivity of at least 5 μS/cm.
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Ultrasonic flowmeters

Two transducers send ultrasonic pulses to each other, diagonally through the fluid. The pulse travelling with the flow arrives sooner than the one travelling against it; the difference in transit times gives the flow velocity (the transit-time method).

Where it fits
Water and clean liquids in large pipes (the OPTISONIC 3400 goes up to DN 4000), gases and even gas custody transfer (ALTOSONIC V12), with no pressure loss. Clamp-on versions mount on the outside, without cutting the pipe.
Where it does not
Liquids with a lot of bubbles or solids, which scatter the sound, and locations where the flow profile is distorted and the straight run is too short.
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Differential pressure flowmeters

A restriction in the line — an orifice plate, a nozzle or a venturi tube — speeds the fluid up and lowers its pressure. A differential pressure transmitter measures the difference before and after the restriction; the flow is proportional to the square root of that difference.

Where it fits
Liquids, gases and steam, even at high temperatures and pressures; a technology proven over decades and standardised (ISO 5167).
Where it does not
Applications with a wide flow range: because of the square root, a 1:100 range in differential pressure covers only 1:10 in flow. The restriction also causes a permanent pressure loss.
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Variable area flowmeters

The fluid rises through a vertical tapered tube that widens towards the top and lifts a float. The float settles where its weight balances the force of the flow; its position on a scale shows the flow rate.

Where it fits
Local indication without power, small flows of liquids and gases, purge and cooling lines; with a transmitter it also gives a 4–20 mA signal (e.g. H250 M40).
Where it does not
Horizontal pipework, downward flow in the usual designs, and applications that call for high accuracy.
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Vortex flowmeters

A bluff body across the pipe makes the flow shed vortices behind it, alternately from one side and the other. The rate at which the vortices form is proportional to the velocity, and a sensor counts the pressure pulses they cause.

Where it fits
Steam, gases and low-viscosity liquids, even at high temperatures; no moving parts.
Where it does not
Very low velocities, where vortices do not form steadily, and viscous liquids.
Not in our catalogue yet — ask us about availability. See how we supply them Read the article

Thermal mass flowmeters

One sensor in the flow is heated while a second one measures the gas temperature. The more gas flows past, the more heat it carries away; from that the meter works out mass flow directly, with no pressure or temperature compensation.

Where it fits
Compressed air, nitrogen, natural gas and other gases of known composition, even at very low velocities, with little pressure loss.
Where it does not
Liquids and steam, gas mixtures whose composition changes, and gases with condensing moisture or dirt that builds up on the sensor.
Not in our catalogue yet — ask us about availability. See how we supply them Read the article

Positive displacement flowmeters

Two oval gears in a chamber are turned by the flow itself. With every turn they trap a fixed, known volume of liquid and carry it from inlet to outlet; by counting the turns, the meter measures volume directly.

Where it fits
Oils, fuels, resins and other viscous liquids, dosing and fuel billing, and installations with no straight run.
Where it does not
Liquids with solid particles, which wear or jam the gears — a strainer upstream is needed — as well as gases and steam.
Not in our catalogue yet — ask us about availability. See how we supply them Read the article

Turbine flowmeters

A bladed rotor in the middle of the pipe spins at a speed proportional to the flow velocity. A pickup outside the pipe senses each blade going past as a pulse; the pulse frequency gives the flow rate.

Where it fits
Clean, low-viscosity liquids such as water, solvents and light fuels, and clean gases, with a fast response.
Where it does not
Viscous or dirty fluids. The bearings wear over time, and the meter needs a straight run or a flow straightener upstream.
Not in our catalogue yet — ask us about availability. See how we supply them Read the article

At a glance

The typical properties of the nine technologies in one table, and their typical accuracy in one chart.

Flow measurement technologies compared: what they measure, which fluids, typical accuracy, straight run, pressure loss and main limitation
TechnologyMeasuresLiquidsGasesSteamTypical accuracyStraight run, up / down (× D)Pressure lossMain limitation
CoriolisMassYesYesLimited±0.1–0.2%0 / 0MediumSize, weight and cost rise steeply in large lines
ElectromagneticVolumeYesNoNo±0.2–0.5%5 / 2NegligibleConductive liquids only
UltrasonicVolumeYesYesLimited±0.5–2%10 / 5NegligibleSensitivity to the flow profile and to bubbles or solids in the liquid
Differential pressureVolumeYesYesYes±0.5–2%10–40 / 5HighNarrow measuring range and permanent pressure loss
Variable areaVolumeYesYesLimited±1.6–4% *5 / 3MediumVertical mounting with upward flow; lower accuracy
VortexVolumeYesYesYes±0.75–2%15–20 / 5MediumNeeds a minimum flow velocity; not for viscous liquids
Thermal massMassNoYesNo±1–3%10–20 / 5LowGases of known composition
Positive displacementVolumeYesNoNo±0.2–0.5%0 / 0HighMoving parts; clean liquids only
TurbineVolumeYesYesNo±0.5–1%10–20 / 5MediumMoving parts; clean, low-viscosity fluids

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.

Typical accuracy by technology

Typical accuracy of each flow measurement technology, ± % of the reading, on a logarithmic scale. The further left, the more accurate.
  • Coriolis±0.1–0.2%
  • Electromagnetic±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%

How to choose a flowmeter

The right technology follows from the application, not from the catalogue. These are the criteria we look at:

  1. The fluid

    Liquid, gas or steam? Clean, or carrying solids and bubbles? Corrosive, viscous or subject to hygiene requirements? The answer rules out several technologies straight away.

  2. Conductivity

    An electromagnetic flowmeter only measures conductive liquids. Water and most aqueous solutions are conductive; oils, fuels and solvents usually are not.

  3. Accuracy

    Billing and dosing need the best accuracy; for monitoring a process, a simpler technology is often enough. Accuracy is usually stated as ± % of the reading.

  4. Measuring range

    What are the minimum, normal and maximum flows? Some technologies cover a wide range with one meter, others, such as differential pressure, a much narrower one.

  5. Straight run

    How much straight pipe is there before and after the mounting point? If there is no room, look at technologies that need no straight run at all, such as Coriolis and positive displacement.

  6. Pressure loss

    Every obstruction in the flow costs energy at the pump or compressor. Electromagnetic and ultrasonic meters cause almost no pressure loss; an orifice plate causes a permanent one.

  7. Cost

    Not just the price of the meter, but also installation, maintenance and energy over its whole service life. In large pipe sizes the gap between technologies becomes very wide.

Flow measurement technologies: frequently asked questions

Need help choosing a flowmeter?

Tell us about the fluid and the operating conditions, and our engineers will recommend the right technology and the specific instrument.