How to choose a flow meter - which principle fits?

Which measuring principles are there?

Flow meters measure the quantity of a liquid or gaseous medium per unit of time. The three most common principles are the variable area meter (mechanical), the electromagnetic method (EMF) and ultrasonic measurement. Each has its own strengths regarding medium, accuracy and installation.

The variable area flow meter (rotameter) shows the flow directly via the height of a float in a tapered tube - robust, low-cost and without auxiliary power. Electromagnetic devices use Faraday's law of induction and measure only conductive liquids with no pressure loss. Ultrasonic meters determine the transit-time difference of two acoustic signals and work non-invasively, some even as clamp-on units from outside the pipe.

How do the principles differ in practice?

The key selection factors are medium, accuracy, pressure loss and price. The overview below summarises the typical properties.

• Variable area: no auxiliary power, direct reading, but position-dependent (vertical installation, flow from bottom to top).
• Electromagnetic: no pressure loss and no moving parts, but needs a minimum conductivity of about 5 µS/cm.
• Ultrasonic clamp-on: retrofittable without interrupting the process, ideal for existing pipework.
• Coriolis (additional option): also measures mass and density, but is considerably more expensive.

What matters for medium and measuring range?

The medium decides the principle, the measuring range decides the size. Key factors are viscosity, conductivity, temperature, solids content and the expected flow rate in l/min or m³/h.

• Check conductivity: below about 5 µS/cm (e.g. demineralised water, oils) the EMF technique is ruled out.
• Solids and bubbles: for heavily contaminated media avoid transit-time ultrasonic and consider the Doppler principle instead.
• Verify the temperature and pressure rating of seals and measuring tube - especially for heating and process water.
• Size the range so the operating point sits at 40‑70 % of full scale, not at the edge.

What to consider during installation?

Installation often affects accuracy more than the device itself. A disturbed flow profile behind bends or valves distorts the reading, so defined inlet and outlet runs per EN ISO 5167 apply.

• Observe the flow direction (arrow on the housing) during installation.
• Avoid vibration and strong magnetic fields near the EMF meter.
• After installation, perform a zero adjustment with the medium at rest.
• Document calibration and inspection intervals, especially for billing-relevant measurement.

Frequently asked questions