Instrumentation · Process Measurement
Flow Meter
Technologies
How each flow-metering technology actually works, in plain language, plus the full spec sheet for all eight. Trying to pick one for your application? Start with the companion Flow Meter Field Guide and its interactive selector.
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Meet the technologies
What each meter actually measures and how, in plain language — read this before you open a datasheet.
Coriolis
Fluid runs through a vibrating tube. Mass flow makes the tube twist slightly (the Coriolis effect) — like a garden hose whipping harder the more water runs through it while you shake it. The twist is measured directly as mass flow, and density comes along for free.
Best forCustody transfer, viscous/multi-phase liquids, anywhere you need mass + density in one instrument
Watch outCauses pressure drop; heavy and expensive at large pipe sizes
Magnetic (4-wire)
Applies Faraday's Law: move a conductive liquid through a magnetic field and it generates a tiny voltage proportional to velocity. Two electrodes pick up that voltage.
Best forWater, wastewater, conductive slurries — full bore, no obstruction
Watch outFluid must be electrically conductive — will not read hydrocarbons, gases, or ultra-pure water
Capacitance Magnetic
Same Faraday principle as a standard magmeter, but senses through capacitive (non-contacting) electrodes instead of wetted metal ones — built for fluids too weakly conductive for a standard magmeter to see.
Best forUltra-pure water, WFI, low-conductivity or coating-prone chemicals
Watch outNarrower size range and higher cost than a standard magmeter
Vortex
A bluff body placed in the flow sheds a regular train of vortices, alternating side to side, at a frequency directly proportional to velocity — count the shedding frequency, get the flow rate.
Best forSteam (saturated & superheated), clean gas, high-temperature service — no moving parts
Watch outNeeds a minimum flow velocity to shed reliably; not suited to high-viscosity fluids or solids
Variable Area
A tapered tube with a float inside: as flow increases, the float rises until the gap around it balances drag against gravity. Read the float's position on a scale — the simplest flow meter there is.
Best forLocal visual indication, no power required, low cost, simple utility duties
Watch outMust be mounted vertically; moving parts wear; lower accuracy and turndown than modern technologies
Differential Pressure
Restrict the flow with an orifice plate, venturi, or Pitot tube and measure the pressure drop across it — flow rate is proportional to the square root of that differential pressure.
Best forThe most universal, best-proven technology — works across liquid, gas and steam, cheap primary elements
Watch outCauses permanent pressure loss; narrow turndown (roughly 3:1–5:1) unless paired with a multivariable transmitter
Thermal Mass New
Two sensors sit in the gas stream: one is heated, one reads the ambient process temperature. Flowing gas carries heat away from the heated sensor — more mass flow means more heat carried away. That heat-loss rate converts directly into mass flow, with no separate pressure or temperature compensation needed.
Best forCompressed air, biogas, natural gas, flare/vent gas, low-flow gas metering, utility submetering
Watch outGas only — not for liquids or steam; accuracy is gas-specific, so it must be calibrated (or field-configured) for the actual gas being measured; sensitive to condensation and coating
Positive Displacement New
Fluid fills a chamber of precisely known volume — an oval gear set, a nutating disc, a rotary piston, or a diaphragm bellows — and each fill-and-empty cycle is counted. Multiply the chamber volume by the cycle count and you get total volume directly, without inferring flow from velocity, pressure, or frequency.
Best forViscous liquids (fuels, oils, chemicals) where velocity-based meters lose accuracy; fiscal/custody-grade liquid metering; low-pressure utility gas metering — diaphragm meters are the standard residential/commercial gas meter worldwide
Watch outMoving parts wear and need periodic servicing; requires reasonably clean fluid — solids can jam or damage the mechanism; causes a permanent pressure drop; mechanical cycling limits it at very high flow rates
Note
Thermal Mass and Positive Displacement Flow Meters are the technologies added beyond this guide's original OEM chart — neither is part of every vendor's standard flow-metering lineup, so if either fits your application, confirm which manufacturers in your approved supplier list actually offer one and check specifics on their current datasheets. Positive Displacement is not shown for steam service: moving parts and condensate don't mix — the mechanism wears quickly and steam's phase changes defeat volumetric counting anyway.
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Detailed specifications
Typical figures for reference only — always confirm against the current manufacturer datasheet before sizing or specifying an instrument.
| Spec | Coriolis | Magnetic (4-wire) | Capacitance Magnetic | Vortex | Variable Area | DP Flow | Thermal Mass ★ | Positive Displacement ★ |
|---|
Selecting?
Head back to the Flow Meter Field Guide for the interactive selector and full suitability matrix by process condition.
