Types of Flow Meters: How to Choose the Right One (2026 Guide)
The main types of flow meters — magnetic, Coriolis, vortex, ultrasonic, turbine, DP — how each works, what fluids they suit, and how they wire into a PLC.
The main types of flow meters are magnetic (mag), Coriolis, vortex, ultrasonic, turbine, differential pressure (DP/orifice), positive displacement (PD), and thermal mass. Each works on a different physical principle, handles a different range of fluids, and connects to a PLC via either a 4–20 mA analog signal, a pulse/frequency output, or a HART digital overlay. Selecting the wrong type costs you accuracy, maintenance headaches, and money — so this guide walks through every major technology before showing you exactly how each one wires back to a controller.
How Flow Measurement Works: Volumetric vs Mass Flow
Before comparing meter types it helps to be clear on what is actually being measured, because the distinction drives every selection decision.
Volumetric flow measures the volume of fluid passing a point per unit time — typically expressed in litres per minute (L/min), cubic metres per hour (m³/h), or US gallons per minute (GPM). The reading changes if temperature or pressure changes the density of the fluid, even if the actual mass throughput stays constant.
Mass flow measures the mass of fluid passing a point per unit time — typically kilograms per hour (kg/h) or pounds per hour (lb/h). Because mass is independent of temperature and pressure, mass flow is the preferred measurement wherever custody transfer, chemical reactions, or batch recipes demand absolute accuracy.
Most meter types measure volumetric flow and rely on a separate density input (or a known, stable density) to infer mass flow. Coriolis and thermal mass meters measure mass flow directly, without any assumptions about fluid density.
A derived quantity you will encounter constantly in PLC programs is totalized flow: the running sum of all flow over time, used for batch control, billing, and material balance. More on how to build that in the PLC in the final section.
Magnetic Flow Meters (Mag Meters)
How they work. A magnetic flow meter — often called a magmeter or electromagnetic flow meter — applies Faraday's law of electromagnetic induction. Two electrodes mounted on opposite sides of the pipe measure the voltage induced when a conductive fluid moves through a magnetic field generated by coils in the meter body. The induced voltage is directly proportional to average fluid velocity, and multiplying by the known pipe cross-section gives volumetric flow rate.
What makes them suitable. Mag meters have no moving parts and nothing obstructing the bore, giving them very low pressure drop and virtually zero maintenance once installed. They are immune to changes in viscosity, density, and temperature (within rated ranges).
Fluid requirements — the key constraint. The fluid must be electrically conductive, with a minimum conductivity typically in the range of 5–20 µS/cm depending on the manufacturer. This means mag meters work for:
- Water (municipal, process, wastewater)
- Acids, bases, and most aqueous chemical solutions
- Slurries and pulp
They do not work for hydrocarbons, gases, de-ionised water below the minimum conductivity threshold, or steam.
Typical accuracy. ±0.2 % to ±0.5 % of reading, which is excellent for a volumetric meter.
PLC connection. 4–20 mA analog output is standard; most modern transmitters also offer HART and pulse/frequency outputs. See the types of industrial sensors article for how analog sensor wiring maps to PLC input cards.
Coriolis Flow Meters
How they work. A Coriolis meter drives one or two vibrating tubes at their natural resonant frequency. When fluid flows through the tubes, the Coriolis effect causes a phase shift in the vibration — the inlet side and outlet side of the tube are no longer perfectly in phase. The magnitude of that phase shift is directly proportional to mass flow rate. A second measurement of tube resonant frequency gives fluid density, from which volumetric flow and temperature-compensated totals can also be derived.
Strengths. Coriolis is the most accurate flow measurement technology available for liquids and slurries:
- Mass flow accuracy: ±0.05 % to ±0.1 % of reading, which qualifies for custody-transfer applications
- Simultaneously measures mass flow, density, temperature, and volumetric flow
- No moving parts, no inlet/outlet straight-run requirements
- Works with any fluid — including non-conductive hydrocarbons, viscous fluids, and two-phase mixtures (with limitations)
Limitations. Coriolis meters are the most expensive option per unit. Large line sizes (above DN 150 / 6 inches) become very heavy and costly. They are not the right choice for gases at low pressure (where mass is low and the Coriolis force is tiny relative to vibration noise), and they struggle with aerated or entrained-gas fluids.
PLC connection. 4–20 mA for flow rate, separate 4–20 mA or frequency output for density, plus HART for configuration and diagnostics.
Vortex Flow Meters
How they work. When flow passes a bluff body (a shedder bar) inserted in the pipe, it generates alternating vortices — a pattern called a von Kármán vortex street. The frequency of vortex shedding is linearly proportional to fluid velocity (described by the Strouhal number). A sensor detects the pressure pulses or stresses caused by each vortex and converts the frequency to flow rate.
Strengths.
- Suitable for liquids, gases, and steam — vortex meters are one of the few types that handle all three
- No moving parts
- Good accuracy (±0.5 % to ±1 % of reading for liquids; ±1 % to ±1.5 % for gas/steam)
- Compact form factor
Limitations. Vortex meters require a minimum flow velocity to sustain vortex shedding. Below that threshold (the meter's low-flow cutoff), the output drops to zero — so they are not suitable for applications with highly variable or very low flow. They also need adequate straight-pipe runs upstream and downstream, and vibration in the pipework can corrupt the shedder signal.
PLC connection. Pulse/frequency output is most common (each pulse represents a fixed volume). A 4–20 mA transmitter version is also available.
Ultrasonic Flow Meters
Ultrasonic meters come in two fundamentally different variants with different applications.
Transit-Time Ultrasonic
How they work. Two transducers mounted at an angle to the pipe axis alternately act as transmitter and receiver. The meter measures the difference in transit time of an ultrasonic pulse traveling with the flow versus against it. That time difference is proportional to average fluid velocity.
Best for: Clean, single-phase liquids and gases. Transit-time meters are highly accurate (±0.5 % to ±1 % of reading), have no pressure drop, and can be supplied as clamp-on devices that attach to the outside of an existing pipe — making them ideal for retrofit measurement and non-invasive applications.
Limitation: Suspended particles, bubbles, or a dirty pipe interior attenuate the ultrasonic signal and degrade accuracy. Clamp-on accuracy is lower than inline (flanged) versions.
Doppler Ultrasonic
How they work. A Doppler meter transmits an ultrasonic beam into the fluid and measures the frequency shift of the signal reflected back by particles or bubbles in the flow. The frequency shift is proportional to the velocity of those reflectors (and therefore the fluid).
Best for: Dirty or aerated liquids — slurries, wastewater, paper pulp — where transit-time would fail because reflectors are required.
Limitation: Doppler meters are less accurate than transit-time (typically ±2 % to ±5 %) and are not suitable for clean liquids.
PLC connection. 4–20 mA is standard for both types; most transmitters also offer pulse and HART outputs.
Turbine Flow Meters
How they work. A multi-blade rotor is suspended in the flow path. Fluid velocity spins the rotor, and the rotational speed — detected magnetically or with a Hall-effect sensor — is proportional to volumetric flow rate.
Strengths.
- High accuracy (±0.1 % to ±0.5 % of reading) for clean, low-viscosity liquids and gases
- Wide turndown ratio (typically 10:1 or better)
- Pulse output maps naturally to totalization
Limitations. Moving parts mean wear — turbine meters need periodic bearing inspection and recalibration in demanding services. They are not suitable for high-viscosity fluids (which slow the rotor), dirty or abrasive fluids (which wear bearings), or low-flow conditions. Swirl in the upstream piping distorts readings, so straight-run requirements are strict.
PLC connection. Pulse/frequency output is the native signal; each pulse represents a fixed volume (the meter factor, in pulses per litre or pulses per unit). HART-enabled transmitters add 4–20 mA and diagnostics.
Differential Pressure (DP) Flow Meters — Orifice Plates
How they work. A restriction (most commonly an orifice plate, but also a Venturi tube, flow nozzle, or pitot tube) creates a pressure drop as fluid accelerates through it. By Bernoulli's principle, the pressure difference across the restriction is proportional to the square of the flow velocity. A separate pressure transmitter measures the differential pressure (DP), and the flow rate is calculated using the known geometry (the orifice bore size and the pipe ID).
Strengths.
- Extremely well-understood technology — decades of installation data
- Handles liquids, gases, and steam in virtually any line size
- Orifice plates are inexpensive and easy to replace
- Can be applied to very large pipe sizes where other meter types are not available or are prohibitively expensive
Limitations. The square-root relationship between DP and flow means turndown is inherently limited (typically 3:1 to 4:1 for acceptable accuracy) unless multiple DP ranges are used. Permanent pressure drop (energy loss) is significant — Venturi and flow nozzle geometries recover more pressure. Accuracy is moderate (±0.5 % to ±2 % of reading, depending on installation quality and calibration). Orifice plates foul and wear.
PLC connection. The DP transmitter sends a 4–20 mA signal proportional to differential pressure. The PLC (or the transmitter itself) applies the square-root extraction to produce a flow-rate signal. See the water treatment PLC programming guide for a worked example of DP flow calculation in a process control loop.
Positive Displacement (PD) Flow Meters
How they work. A PD meter captures a fixed, known volume of fluid in a mechanical chamber — a gear, piston, diaphragm, or helical rotor — and counts how many times that chamber fills and empties. Because the displaced volume per cycle is mechanically fixed, totalized volume is highly accurate independent of flow rate fluctuations.
Strengths.
- Excellent low-flow accuracy and high turndown (up to 100:1 in some designs)
- No straight-pipe run requirements
- Well-suited to viscous fluids (oils, syrups, adhesives) that cause problems for velocity-based meters
- Custody-transfer approved for many fluid types
Limitations. Moving parts wear, especially with abrasive fluids. PD meters cause a significant pressure drop. They are not suitable for gases at high flow rates (where the mechanical forces become impractical) or for dirty/slurry flows that would jam the mechanism.
PLC connection. Pulse output (one pulse per chamber cycle) feeding a high-speed counter or pulse input card on the PLC.
Thermal Mass Flow Meters
How they work. Thermal mass meters measure mass flow by exploiting heat transfer. The most common design has two RTD sensors in the flow stream — one heated, one unheated — and measures the power required to maintain a fixed temperature differential between them. That power is proportional to mass flow rate (because more mass carries more heat away per unit time). An alternative design measures the temperature shift downstream of a fixed heater element.
Best for: Gases — particularly compressed air, nitrogen, natural gas, biogas, and other pure or known-composition gases at low-to-medium mass flow rates.
Limitations. Thermal mass meters are calibrated for a specific gas or gas mixture. A different gas composition changes the thermal properties and invalidates the calibration. They are generally not suitable for liquids (where the thermal mass is much higher and measurement ranges become impractical) and do not perform well with saturated or wet gases.
PLC connection. 4–20 mA analog output; some models offer a MODBUS RTU serial output for direct digital integration.
Flow Meter Selection Table
| Type | Best Fluids | Mass or Vol. | Typical Accuracy | Moving Parts | Relative Cost | PLC Output |
|---|---|---|---|---|---|---|
| Magnetic (mag) | Conductive liquids, slurries | Volumetric | ±0.2 – 0.5 % | None | Medium | 4–20 mA / Pulse / HART |
| Coriolis | Any liquid; dense gas | Mass (direct) | ±0.05 – 0.1 % | None (tubes vibrate) | High–Very High | 4–20 mA / HART |
| Vortex | Liquid, gas, steam | Volumetric | ±0.5 – 1.5 % | None | Medium | Pulse / 4–20 mA |
| Ultrasonic (transit-time) | Clean liquid/gas | Volumetric | ±0.5 – 1 % | None | Medium–High | 4–20 mA / Pulse |
| Ultrasonic (Doppler) | Dirty/aerated liquid | Volumetric | ±2 – 5 % | None | Medium | 4–20 mA |
| Turbine | Clean, low-viscosity liquid/gas | Volumetric | ±0.1 – 0.5 % | Yes (rotor) | Low–Medium | Pulse / Freq |
| DP / Orifice | Liquid, gas, steam | Volumetric | ±0.5 – 2 % | None (plate) | Low | 4–20 mA (DP xmtr) |
| Positive displacement | Viscous liquids, oils | Volumetric | ±0.1 – 0.5 % | Yes | Medium | Pulse |
| Thermal mass | Gases | Mass (direct) | ±1 – 2 % | None | Medium | 4–20 mA / Serial |
How to Choose the Right Flow Meter
Work through these five filters in order to arrive at a short list before comparing quotes.
1. What is the fluid? This is the hardest constraint. Mag meters fail on non-conductive fluids. Turbine meters fail on viscous or dirty fluids. Thermal mass meters fail on liquids. If the fluid is conductive water or a water-based solution, the mag meter is usually the default first choice. For hydrocarbons, Coriolis or turbine are the starting points.
2. Is the fluid liquid, gas, or steam? Vortex and DP meters handle all three. Most others are liquid-only or gas-only. Steam measurement almost always uses vortex or DP with a temperature and pressure compensator.
3. Do you need mass flow or volumetric flow? If recipe batching, custody transfer, or a chemical reaction demands mass accuracy, look first at Coriolis (liquids) or thermal mass (gases) to avoid a separate density measurement loop.
4. What are the flow range and turndown requirements? Identify the minimum, normal, and maximum expected flow rates. Calculate the turndown ratio (max/min). PD meters and Coriolis handle wide turndowns. DP and vortex meters struggle at low flow.
5. What are the process conditions? Check temperature, pressure, pipe size, and available straight runs. Large line sizes favour DP or ultrasonic. Tight pipe layouts favour Coriolis (no straight-run requirement) or clamp-on ultrasonic. High-pressure steam favours vortex or DP with a Venturi tube.
Cost guidance: DP/orifice is the lowest installed cost for large pipes. Turbine offers good accuracy at moderate cost for clean liquids. Mag meters deliver reliable accuracy with low maintenance for water services. Coriolis has the highest purchase cost but eliminates downstream density measurement and often reduces calibration frequency.
How Flow Meters Connect to a PLC
Understanding how the signal leaves the field instrument and arrives in the PLC is essential for both panel design and programming. The three dominant interface methods are described below.
4–20 mA Analog Input
The vast majority of process flow transmitters output a 4–20 mA current loop signal. The transmitter is wired to an analog input (AI) channel on the PLC I/O card:
- 4 mA represents 0 % of the calibrated range (zero flow, or the low end of the span)
- 20 mA represents 100 % of the calibrated range (full-scale flow)
- The PLC converts the raw ADC count to engineering units using a linear scale block
A 2-wire transmitter is powered from the PLC's loop supply through the same pair of conductors. A 4-wire transmitter has separate power and signal wiring. For a detailed walkthrough of how analog input cards work across different PLC platforms, see the PLC programming basics and fundamentals guide.
Pulse / Frequency Input (Totalizer)
Turbine, PD, and vortex meters often output a pulse train where each pulse represents a fixed volume increment. The PLC receives this on a high-speed counter (HSC) input or a dedicated pulse input channel rated for the meter's maximum pulse frequency.
Key parameters you need from the datasheet:
- K-factor (or meter factor): pulses per unit volume, e.g. 100 pulses/litre
- Maximum output frequency at full-scale flow (determines which PLC input to use)
- Output type: NPN open-collector, PNP, or NAMUR
Totalizing flow in the PLC. Ladder logic for a pulse totalizer is straightforward:
HSC input → High-Speed Counter block → Raw count register
Raw count ÷ K-factor → Volume in litres (REAL division)
Volume × density → Mass (if density is known)
Reset counter on batch complete or on shift change
Most modern PLC platforms (Allen-Bradley, Siemens, Mitsubishi, etc.) have a built-in HSC function block. Configure the block for single-phase rising-edge counting, set the preset to zero (free-running), and read the accumulated count at whatever interval suits your application. Divide by the K-factor in floating-point to get engineering-unit totals without accumulating rounding error.
Rate calculation from pulse input. To derive instantaneous flow rate from a pulse output, measure the time between pulses (period) or count pulses over a fixed gate time, then convert:
Flow rate = (pulses counted in gate_time) ÷ (K-factor × gate_time)
HART (Highway Addressable Remote Transducer)
HART superimposes a digital signal on the same 4–20 mA loop at 1200 baud. The 4–20 mA signal continues to carry the primary variable (flow rate) while HART carries:
- Secondary, tertiary, and quaternary process variables (e.g. density, temperature, totals)
- Transmitter diagnostics and status
- Remote configuration (range, damping, units)
A HART-capable AI card or a HART multiplexer allows the PLC or asset management system to read all variables digitally. For flow meters with multiple outputs (like Coriolis), HART is the most efficient way to pull mass flow, density, and temperature into the PLC without additional analog channels.
Practical tip: Even if you never use HART communication in your PLC program, specify HART-enabled transmitters. It gives commissioning engineers and maintenance technicians the ability to configure and test the meter from a handheld communicator without breaking the live loop.
For related instrumentation topics, see the companion articles on level measurement — which covers float, differential pressure, ultrasonic, and radar level sensors — and pressure transmitter selection and wiring.
Frequently Asked Questions
What are the main types of flow meters?
The main types of flow meters are magnetic (mag), Coriolis, vortex, ultrasonic (transit-time and Doppler), turbine, differential pressure (orifice plate), positive displacement, and thermal mass. Each operates on a different physical principle and is suited to different fluids, flow ranges, and accuracy requirements.
Which flow meter is most accurate?
Coriolis flow meters are the most accurate, with mass flow accuracy of ±0.05 % to ±0.1 % of reading. They are the standard choice for custody transfer of liquids. Turbine and positive displacement meters achieve ±0.1 % to ±0.5 % accuracy for clean liquids. Magnetic meters typically achieve ±0.2 % to ±0.5 % for conductive liquids. Differential pressure meters are the least accurate of the mainstream types, typically ±0.5 % to ±2 % depending on installation quality.
How do you choose a flow meter?
Start with the fluid: is it conductive (enables mag meter), non-conductive (eliminates mag), gas or steam (vortex or DP), viscous (positive displacement), or dirty/abrasive (Coriolis or Doppler ultrasonic)? Then evaluate whether you need mass or volumetric flow, the required turndown ratio, available straight-pipe runs, line size, and budget. Most selection decisions narrow to two or three candidates after applying the fluid constraint.
How does a flow meter connect to a PLC?
A flow meter transmitter connects to a PLC via one of three methods: (1) 4–20 mA analog input — the most common, where the PLC scales the current signal to engineering units using a linear conversion; (2) pulse/frequency input — used with turbine, PD, and vortex meters, where each pulse represents a fixed volume and the PLC counts pulses on a high-speed counter input to totalize flow; (3) HART — a digital protocol superimposed on the 4–20 mA loop that allows the PLC to read secondary variables (density, temperature, totals) and transmitter diagnostics without additional wiring.
