The operating principle of a Coriolis flow meter is basic but very effective. This phenomenon is all around us in the physical world; for example the earth’s rotation and its effect on the weather.

A Coriolis flow meter contains a tube which is energized by a fixed vibration. When a fluid (gas or liquid) passes through this tube the mass flow momentum will cause a change in the tube vibration, the tube will twist resulting in a phase shift. This phase shift can be measured and a linear output derived proportional to flow.

Schematic of a Coriolis flow sensor

As this principle measures mass flow independent of what is within the tube, it can be directly applied to any fluid flowing through it - LIQUID or GAS - whereas thermal mass flow meters are dependent of the physical properties of the fluid. Furthermore, in parallel with the phase shift in frequency between inlet and outlet, it is also possible to measure the actual change in natural frequency. This change in frequency is in direct proportion to the density of the fluid – and a further signal output can be derived. Having measured both the mass flow rate and the density it is possible to derive the volume flow rate.

Volume flow versus mass flow

Coriolis flow meters measure real mass flow, whereas thermal mass flow meters are dependent of the physical properties of the fluid. True mass flow measurement is an important development across industry as it eliminates inaccuracies caused by the physical properties of the fluid, not least being the difference between mass and volumetric flow. Mass is not affected by changing temperature and pressure. This alone makes it an important method of fluid flow measurement.

Volumetric flow remains valid, in terms of accuracy, provided that the process conditions and calibration reference conditions are adhered to. Volumetric measuring devices, such as variable area meters and turbine flow meters, are unable to distinguish temperature or pressure changes.

Coriolis versus Thermal Mass Flow for Gases and Liquids

• Fluid independent flow measurement and control - no need for recalibration

• Gas and liquid can be measured with the same sensor

• Ability to measure undefined or variable mixtures

• Suitable for supercritical fluids, e.g. carbon dioxide (CO₂) or ethylene (C₂H₄)

• Same compact footprint

• High accuracy ± 0.2% of rate ± zero stability for liquids ± 0.5% of rate ± zero stability for gases

• Large turndown range of up to 1:2000

• Fast sensor response time: down to 50...100 msec