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Flow can be measured by many different principles, and each has a physical requirement the fluid must meet — conductivity, sufficient velocity, a clean acoustic path, a known gas composition. The most efficient selection method is elimination: rule out what the fluid or installation cannot support, then compare what remains.
Questions that eliminate options
- What is the fluid? Liquid, gas, steam, slurry or two-phase. For a liquid, is it electrically conductive?
- How clean is it? Solids, fibres, entrained gas, and any tendency to coat or abrade wetted parts.
- What are the flow range and viscosity? Minimum, normal and maximum flow, the turndown between them, and the Reynolds number at minimum flow.
- What are the process conditions? Pressure and temperature, including cleaning or steaming cycles.
- What is the number for? Volume or mass; control, monitoring, batching, energy accounting or custody transfer. This sets the accuracy needed and often the principle.
- What does the installation allow? Line size, available straight run, orientation, permissible pressure loss, and whether the line can be opened at all.
The main principles and where each fits
Electromagnetic
A conductive liquid moving through a magnetic field generates a voltage proportional to its velocity (Faraday’s law), and the meter reports volumetric flow. With nothing in the bore, pressure loss is negligible and slurries, wastewater and pulps are handled well. The liquid must meet a minimum conductivity — commonly quoted around 5 µS/cm, with some designs rated lower — so hydrocarbons, many organic solvents, gases and steam are excluded, and high-purity water can fall below the limit. The pipe must run full, and liner, electrodes and grounding are chosen for the liquid and the pipe material.
Vortex
A bluff body in the stream sheds vortices at a frequency proportional to velocity, provided the Reynolds number is high enough for the shedding to be regular. Vortex meters suit steam, gases and low-viscosity liquids and have no moving parts. Below a minimum Reynolds number, which depends on the design, shedding becomes irregular, so every vortex meter has a low-flow cutoff; viscous liquids and low velocities are the usual reasons to rule it out. Mass flow of steam or gas needs density compensation — pressure or temperature alone for saturated steam, both for superheated steam and for gases — which some designs integrate.
Coriolis
Fluid flowing through a vibrating tube produces a phase shift proportional to mass flow, and the tube’s resonant frequency gives density — both measured directly and largely independent of flow profile, so straight-run requirements are minimal. Coriolis meters are used where mass, density or high accuracy matter: batching, dosing, blending and transfer of liquids and gases. Cost and weight rise steeply with line size, pressure loss can be significant on viscous fluids, and entrained gas in a liquid degrades the measurement.
Ultrasonic: transit-time and Doppler
Transit-time meters compare how long an ultrasonic pulse takes to travel with and against the flow. They need a reasonably clean fluid, and multi-path designs reduce sensitivity to the flow profile. Clamp-on versions install without opening the pipe — useful for surveys and retrofits — but their accuracy depends on accurate pipe data, wall and lining condition, and acoustic coupling. Doppler meters work the other way round: they measure the frequency shift of sound reflected from particles or bubbles, so they need a fluid that carries them and suit duties where lower accuracy is acceptable.
Differential pressure
An orifice plate, venturi, flow nozzle or averaging pitot tube creates a pressure difference that a DP transmitter measures, and flow is proportional to its square root: Q ∝ √ΔP. The method is well established — orifice plates, nozzles and venturi tubes are standardised in ISO 5167 — and works for liquids, gases and steam at high temperatures and pressures and in large lines. The costs are permanent pressure loss (highest for orifice plates), limited turndown, impulse-line maintenance and straight-run requirements.
Thermal mass
A heated sensor loses heat to the gas stream at a rate that depends on mass flow, so thermal meters read gas mass flow — or standard volumetric flow — without separate pressure and temperature compensation. They are used mainly for gases such as compressed air, combustion air and fuel gas, and they handle low velocities well. The calibration is specific to the gas composition, so variable mixtures and wet or condensing gases are poor applications.
Positive-displacement, turbine and variable-area meters remain valid for particular duties — viscous liquids, clean low-viscosity fluids and local indication respectively — and are worth considering where they fit.
Decision table: principle against fluid
| Principle | Measures | Good fit | Rule it out when |
|---|---|---|---|
| Electromagnetic | Volumetric flow | Conductive liquids: water, wastewater, slurries, pulps, many chemicals | The liquid is below the conductivity limit, or the fluid is a gas or steam; the pipe may run partly full |
| Vortex | Volumetric flow; mass with density (P and/or T) compensation | Steam, gases and low-viscosity liquids at moderate to high velocity | Flow is often low, the liquid is viscous, the flow is two-phase, or pipe vibration is severe |
| Coriolis | Mass flow, density, temperature | Liquids and gases where mass, density or high accuracy matter | The line is large, a liquid carries significant entrained gas, or the pressure-loss budget is tight |
| Ultrasonic transit-time | Volumetric flow | Clean liquids and gases; clamp-on for retrofits and surveys | The fluid carries heavy solids or bubbles, or (clamp-on) the pipe condition is unknown |
| Ultrasonic Doppler | Velocity of particles or bubbles | Slurries and liquids with suspended solids or bubbles | The liquid is clean, or high accuracy is required |
| Differential pressure | Flow computed from ΔP (and density) | Liquids, gases and steam; high temperature and pressure; large lines | Wide turndown is needed, pressure loss is costly, or the fluid would plug impulse lines |
| Thermal mass | Mass flow of gas | Compressed air, combustion air and fuel gas of known composition; low velocities | The fluid is a liquid, the gas is wet or condensing, or its composition varies |
Worked example: the square-root law
DP flow measurement shows why turndown has to be checked rather than assumed. Take a DP transmitter ranged 0–250 mbar for 0–100 % flow, with an error of ±0.25 mbar (±0.1 % of DP span). Because the differential varies with the square of flow, it falls much faster than the flow does:
| Flow | ΔP | ΔP as % of span | Resulting flow error |
|---|---|---|---|
| 100 % | 250 mbar | 100 % | ±0.05 % of reading |
| 50 % | 62.5 mbar | 25 % | ±0.2 % of reading |
| 25 % | 15.6 mbar | 6.25 % | ±0.8 % of reading |
| 10 % | 2.5 mbar | 1 % | ±5 % of reading |
The relative flow error is half the relative DP error: ±0.25 mbar is ±10 % of 2.5 mbar, which becomes about ±5 % of flow at 10 % load. Primary-element uncertainty and density effects come on top. That is why a single DP flow loop is specified for a modest turndown, and wide-range duties use stacked transmitters or a different principle.
Engineering note
Field check — where is the square root taken?
If the transmitter outputs linear DP, 50 % flow reads 8 mA (25 % of DP span), not 12 mA. The square root must be extracted exactly once — in the transmitter or in the control system, never both and never neither. A low-flow cutoff is normally applied as well, because square-root extraction amplifies noise near zero.
Installation decides between valid options
Straight-run requirements vary by principle, by manufacturer and by the upstream disturbance — a single elbow, two elbows out of plane, a partly open valve — and flow conditioners can shorten them. Coriolis meters are largely insensitive to flow profile; DP primary elements follow the upstream lengths in ISO 5167 for the fitting and diameter ratio; vortex, electromagnetic and ultrasonic meters follow the manufacturer’s figures.
- Keep liquid meters full. Install in a rising vertical line or at a low point of a horizontal run. On electromagnetic meters, keep the electrode axis horizontal so bubbles do not settle on the electrodes.
- Put control valves downstream. A valve upstream of the meter disturbs the flow profile and, on liquids, lowers the pressure at the meter, which can cause flashing or cavitation.
- Orient Coriolis tubes for the fluid so liquids do not trap gas and gases do not trap condensate, and mount the meter without pipe stress.
- Plan access. Allow for removal for calibration or cleaning, bypasses where the process cannot stop, and grounding rings where an electromagnetic meter sits in plastic or lined pipe.
Engineering questions
- What is the minimum conductivity for an electromagnetic flowmeter?
- It depends on the design. A figure around 5 µS/cm is commonly quoted for standard meters, and some designs are rated lower. Hydrocarbons, many organic solvents and some high-purity water fall below the limit, so check the datasheet against the lowest conductivity the liquid can reach.
- Which flowmeter is used for steam?
- Vortex meters and differential pressure primary elements such as orifice plates, nozzles and venturis are the most common choices for steam. Mass flow needs density compensation: pressure or temperature alone for saturated steam, both for superheated steam.
- Do Coriolis flowmeters need straight pipe runs?
- Coriolis meters are largely insensitive to flow profile, so straight-run requirements are minimal. Installation still matters: mount the meter without pipe stress, avoid strong external vibration, and orient the tubes so liquids do not trap gas and gases do not trap condensate.
- Can a clamp-on ultrasonic flowmeter replace an inline meter?
- Clamp-on transit-time meters are well suited to surveys, verification and retrofits where the pipe cannot be opened. Their accuracy depends on accurate pipe data, the condition of the wall and lining, and acoustic coupling, so permanent control or accounting duties are often better served by an inline meter.