What does a Coriolis flowmeter measure? – an Expert writes.

Coriolis Principle

The principle of this flow meter depends on the Coriolis effect. A Coriolis mass flow meter measures:

  • Mass flow rate
  • Density
  • Volume flow rate, by calculation, and
  • Temperature

In simple terms, the answer is mass flow: the amount of mass per unit of time flowing through the tubes of the meter.

However, there is more information from the vibration of the tubes than just mass flow. By analysing the frequency of the vibrating tubes, the density can be calculated.  Therefore, as a by-product the volume flow rate can also be determined. For compensation of measurement due to thermal effects temperature is also measured and is usually available as a further output.

What’s the benefit?

While volumetric flow is adequate, mass flow can be desirable. Many items are sold by weight rather than volume, and it is beneficial to quantify mass flow in these situations. 

In the chemical industry, mass flow calculation is frequently needed because chemical reactions are commonly based on mass rather than volume.

The most accurate flow meters are Coriolis flow meters. For example, whilst many electromagnetic flowmeters have accuracies in the 0.5 percent range, many Coriolis flow meters have 0.1 percent accuracy. One of the main reasons for the extremely rapid growth of Coriolis flow meters is their high accuracy.

The theory

Coriolis flow meters operate on the idea that the inertia produced by fluid flowing through an oscillating tube causes the tube to twist in proportion to mass flowrate. 

Two tubes are common in Coriolis meters and a magnetic coil is used to make them vibrate in opposition to each other. The coils generate a sine wave voltage as they pass through the magnetic field generated by the magnet. The sine waves enable determination of mass flow.

The inlet and outlet sine waves are in phase when there is no flow. The tubes twist in proportion to the mass flowrate as fluid moves through them. The inlet and outlet sensors detect the amount of twist by detecting a phase shift (time difference) in the sine waves.

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