How does Coriolis flow meter work?

Have you ever wondered why the wind direction is mainly east-west under low-pressure systems in the Northern Hemisphere?

The answer lies in the Earth's rotation-the Earth rotates primarily east-west, and its rotation speed is faster near the equator than at the poles. This rotation drives the wind direction, causing it to blow mainly from west to east in the Northern Hemisphere. French scientist Gaspard-Gustave de Coriolis discovered this effect of lateral forces on flowing air and named it the Coriolis effect after him. First and foremost, it's important to remember that the Coriolis effect explains the deflection of flowing air in a rotating system. In fact, the Coriolis effect is a mass inertia effect.

The Coriolis effect is also applied to a more practical application: mass flow measurement of gases and liquids. For this, the fluid to be measured is forced to flow through a vibrating tube. In a Coriolis flow meter, an actuator causes a thin tube to vibrate continuously around its natural frequency. Two sensors arranged along the pipe measure the deflection of the vibrating tube over time. When no fluid flows through the tube, both sensors measure the same deflection at the same moment.

However, when a gas or liquid flows through the tube, its mass causes additional twisting due to the fluid's inertia.

The difference between these two measurements is called "phase shift," which directly measures the mass flow rate through the tube.

Phase shift is directly proportional to mass flow rate: the greater the phase shift, the greater the mass flow rate.

Mass flow meters based on the Coriolis effect can do much more than this-they can also measure fluid density!

Phase shift is a measure of mass flow rate, while the (natural) vibration frequency is a measure of fluid density. The fluid density affects the tube's vibration frequency: denser fluids vibrate at lower frequencies than less dense fluids. Therefore, the vibration frequency directly reflects the density of the liquid or gas.

Mass flow rate and density can be measured independently using the same device, demonstrating the versatility of Coriolis flow meters.

Coriolis flow meters directly measure mass flow rate. Direct measurement eliminates errors caused by the fluid's physical properties. Thermal flow meters, on the other hand, can only measure mass flow rate indirectly. These fundamental differences in measurement methods affect their typical applications.

Thermal mass flow meters utilize the fluid's heat capacity to measure mass flow rate. Their core components include a heater and one or two temperature sensors. The power required for heating (using one sensor) or the temperature difference between the two sensors is proportional to the fluid's mass flow rate. Thermal mass flow meters are primarily used for gases.

Because the Coriolis principle directly measures mass flow rate, Coriolis flow meters can be used for both gases and liquids.

Coriolis flow meters vibrate the sensor tube via an actuator. External vibrations of approximately the same frequency can interfere with the measurement.

For example, nearby trains, air conditioners in buildings, or other machinery can generate vibrations. Identifying these external vibration sources is the first step in minimizing their impact. For example, vibration can be reduced by moving the Coriolis flow meter to a less affected location, rotating the device, using a (larger) mass block, or using dampers and/or flexible tubing for decoupling.

Coriolis-based mass flow meters are particularly suitable when measuring the mass flow rate of varying or unknown gas or liquid mixtures, or when measuring supercritical gases. In addition to directly measuring mass flow rate (thus eliminating errors caused by fluid physics), these devices offer extremely high accuracy and repeatability. Coriolis flow meters are flexible, reliable, and extremely accurate flow meters.

Coriolis flow meters have been successfully applied to improve the accuracy of automated extraction of natural compounds from raw materials such as plants, thereby improving extraction quality. Here, selective and precise extraction can be achieved by measuring and controlling the flow rate and temperature of supercritical carbon dioxide, ensuring repeatable yields.