How does a flowmeter work?

Flowmeters operate on a variety of measuring principles, each suited to different media and applications.

• Displacement: A rotor or piston counts the volume per cycle (positive displacement)
• Electromagnetic (Faraday): A conductive liquid passing through a magnetic field generates a voltage proportional to flow velocity
• Vortex: Vortices shed behind a bluff body are counted; the frequency is directly proportional to the flow rate
• Thermal: The heat loss of a heated sensor depends on the mass flow rate of the gas
• Ultrasonic (transit-time): The difference in travel time of sound waves sent with and against the flow direction gives the average flow velocity
• Coriolis: A vibrating measuring tube shows a phase shift during flow; this shift is directly proportional to the mass flow rate

Each principle produces a measurement signal – typically 4–20 mA, pulse or digital via Modbus/IO-Link – which your PLC or SCADA system reads for control, alarming or totalisation.

Types of flowmeters

The table below helps you quickly identify the right direction based on medium, accuracy and installation requirements.

Accuracy figures are indicative; actual values are type- and medium-specific.

Ultrasonic flowmeter

An ultrasonic flowmeter uses two or more transducers that send sound waves both with and against the flow direction through the medium. The transit-time difference is directly proportional to the average flow velocity and delivers – in combination with the pipe diameter – the volumetric flow rate.

Choose an ultrasonic flowmeter when:

• you want no process interruption (clamp-on mounting on the outside of the pipe)
• you are working with clean, homogeneously filled liquids such as (drinking) water or cooling water
• you need a temporary measurement during commissioning or troubleshooting
• you want to measure larger pipe diameters without pressure drop

Please note: with media containing glycol or other additives, acoustic coupling in clamp-on versions may be unreliable, which negatively affects measurement accuracy. In these cases, choose an inline ultrasonic flowmeter or consider an alternative measuring principle such as electromagnetic or Coriolis. For gas measurement, special ultrasonic flowmeters are available with pressure and temperature compensation and multiple measurement paths. Typical applications include HVAC energy measurement, water installations and compressed air monitoring.

Coriolis flowmeter

A Coriolis flowmeter measures mass flow directly via the vibration behaviour of the measuring tube. When medium flows through the vibrating tube, a phase shift occurs. This shift is directly proportional to the mass flow rate – independent of density, viscosity or temperature.

Choose a Coriolis flowmeter when:

• you need mass flow rather than volumetric flow
• density determination or concentration monitoring is required
• accuracy requirements are high (up to 0.1%)
• you are working with custody transfer or batch dosing

The Coriolis flowmeter is suitable for both liquids and gases and delivers density and temperature as additional process values alongside mass flow. Allow sufficient installation space and minimise vibrations in the pipe support structure for a stable measurement.

Flowmeter for water

For water installations, the electromagnetic flowmeter is widely used: no moving parts, very low pressure drop and accurate measurement even with varying viscosity and light contamination. Ensure a fully filled pipe, correct earthing and sufficient straight inlet/outlet runs during installation.

Select based on your measurement objective:

• Volumetric flow – water ? electromagnetic flowmeter (conductive medium required)
• Mass flow or mix concentration ? Coriolis flowmeter
• Clamp-on, no process interruption, clean water ? ultrasonic flowmeter
• Local visual indication, low budget ? rotameter

The final choice depends on DN, required accuracy, medium properties and the desired output signal (4–20 mA, pulse or digital). For additional pressure monitoring, combine a flowmeter with pressure sensors or gauges.

Mechanical flowmeters

Mechanical flowmeters are reliable, simple in construction and cost-effective for standard applications.

Rotameter (variable area flowmeter): Provides direct visual flow indication without auxiliary power. Suitable for clean liquids and gases at low to medium pressure. Vertical mounting required; plan for reading distance.

Paddlewheel and turbine flowmeters: Suitable for clean media such as water and light oil. Good price-to-accuracy ratio, but sensitive to contamination and require straight pipe inlet runs.

Oval gear (positive displacement): Measures accurately at variable viscosity and is therefore well suited for dosing and batch applications. A bypass is recommended for maintenance without process interruption.

Electronic flowmeters

Electronic flowmeters have no or few moving parts and are therefore low-maintenance and suitable for demanding processes.

Electromagnetic flowmeter: Measures conductive liquids such as water, aqueous solutions and slurries. No pressure drop, high accuracy and long service life. This is why this type is widely used in the water and process industry.

Vortex flowmeter: Reliable for steam, dry gases and clean liquids. Wide turndown ratio and robust construction. Sufficient straight pipe runs are required for a stable measurement signal.

Thermal flowmeter: Measures mass flow of compressed air and industrial gases directly, without pressure or temperature correction. Ideal for leak detection, consumption allocation per machine and energy management of compressor systems.

Differential pressure flowmeter: Pitot tube, orifice plate or venturi for applications requiring high accuracy and proven technology. Suitable for large pipe diameters and high pressures.

Flowmeter selection

Start your selection with the following questions:

• Medium: conductive or non-conductive? Clean or with particles? Liquid or gas?
• Measurement objective: volumetric flow, mass flow or standard volume flow (Nm³/h)?
• Process conditions: pressure, temperature, viscosity, gas fraction
• Installation: DN, available straight inlet/outlet runs, full pipe condition?
• Accuracy and certification: hygienic, ATEX, custody transfer?
• Output signal: 4–20 mA, pulse, IO-Link, Modbus?

Combine your flowmeter with level meters or pressure sensors for complete process instrumentation. Not sure which type is right for your application? Our specialists will help you select the correct flowmeter based on your process data – request technical advice directly via our contact page.

Frequently asked questions about flowmeters

What is the difference between a flowmeter and a water meter?

A water meter is designed for drinking water registration and billing. It measures volume only via a mechanical register with an optional pulse output. A flowmeter is a broader instrument category covering water, gases and process fluids, using a range of measuring principles (electromagnetic, Coriolis, vortex, ultrasonic). In addition to totalisation, a flowmeter also delivers the current flow rate and process data via 4–20 mA, pulse or digital protocols such as Modbus or IO-Link. Flowmeters are used for process control, energy measurement and leak detection – water meters for consumption registration and billing.

What is a flowmeter for air or gas?

For compressed air and industrial gases, a thermal mass flowmeter is the preferred choice: it delivers mass flow and standard volume flow (Nm³/h) directly, without additional pressure or temperature correction. Vortex flowmeters are suitable for steam and dry gases across a wide flow range. Ultrasonic flowmeters are non-contact and applicable for larger pipe diameters or retrofit projects. Connect the measurement signal via 4–20 mA, pulse or IO-Link/Modbus to your PLC for consumption monitoring and leak detection.

How do I measure flow without interrupting the process?

In that case, choose a clamp-on ultrasonic flowmeter. The transducers are mounted on the outside of the existing pipe without drilling or disconnecting. This type is suitable for clean, fully filled pipes with homogeneous liquids such as water. Note: with media containing glycol or additives, measurement may become unreliable – discuss an inline alternative in that case. For a permanent installation with higher accuracy requirements, an inline flowmeter (electromagnetic or Coriolis) is recommended.