Instrumentation · Process Measurement

Flow Meter
Field Guide

A plain-language reference for choosing the right flow-metering technology by process condition — built for engineers who are new to instrumentation selection.

§1

How to use this guide

STEP 01

Pick your quick shortlist

Use the picker below — tell it whether you're dealing with a liquid, gas, or steam line and get a short list of technologies worth considering.

STEP 02

Check the suitability matrix

Cross-reference your specific process condition (dirty, corrosive, cryogenic, low flow, etc.) against each technology in §3.

STEP 03

Read the technology card

Once you have 1–2 candidates, read how the meter actually works in §4, then confirm hard limits in the spec table in §5.

§2

Interactive selector

Pick the fluid phase, then answer what you know about the service — the shortlist below re-ranks live. Leave anything unanswered at its default; you can still narrow it down with just two or three answers.

What's in the pipe?

Liquid Gas Steam

§3

Suitability matrix

Read down a column to see what one technology is good for, or across a row to compare every technology against one process condition.

■ Designed for this service ○ Workable under certain conditions — check with the manufacturer – Not typically used

Liquids

Process condition	Coriolis	Magnetic (4-wire)	Capacitance Magnetic	Vortex	Variable Area	DP Flow	Positive Displacement ★

Gas & Steam

Process condition	Coriolis	Vortex	Variable Area	DP Flow	Thermal Mass ★	Positive Displacement ★

★ Added to this guide beyond the original OEM chart it was built from. Thermal Mass is gas-only, so it does not appear in the liquids matrix. Positive Displacement is not shown for steam — see the note in §4.

§4

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.

§5

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 ★

Flow MeterField Guide