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# Coriolis flow meters: how do they work?

How does the accurate measurement of low gas and liquid flows relate to the Coriolis effect? This phenomenon is well known in meteorology on a world scale. Learn how the largest and smallest flows rely on the same phenomenon.

The video explains the Coriolis flow meter working principle

## 1. What is the Coriolis effect?

Ever wondered why wind in the Northern hemisphere mainly blows in a west/east direction during a low-pressure system?

The answer lies in the Earth’s rotation on its axis – mainly in east/west direction and at a higher speed at the equator than at the poles. This rotation takes the wind along and forces it to blow mainly in a west to east direction in the Northern hemisphere. It was the French scientist Gaspard-Gustave de Coriolis who discovered the effect that flowing air experiences a lateral force, which was named after him – the Coriolis effect. The first take home message is: the Coriolis effect explains the deflection of flowing air moving in a rotating system. In fact, the Coriolis effect is a mass inertia effect.

## 2. How does a Coriolis flow meter work?

This Coriolis effect is also applied to a more ‘down to earth’ application: in mass flow measurement of gases and liquids. To this end, the fluid to be measured is forced to flow through a vibrating tube.

In the Coriolis flow device, an actuator allows a small tube to vibrate continuously around its natural frequency. Two sensors positioned along the tube measure the deflection of this vibrating tube in time. Without a fluid flowing through the tube, both sensors measure the same deflection at the same moment in time. However, when a gas or liquid flows through the tube, the mass of this fluid poses an additional twist to the tube due to the fluid’s inertia. The difference between the two, known as the ‘phase shift’, is a direct measure of the mass flow through the tube. This phase shift is proportional to the mass flow rate: a larger phase shift is the result of a higher mass flow rate.

Mass flow devices according to the Coriolis effect do even more – they measure fluid density! While phase shift is a measure for mass flow rate, the vibration (natural) frequency is a measure for fluid density. The density of a fluid affects the vibration frequency of the tube: denser fluids vibrate with a lower frequency than less dense ones. So this vibration frequency is a direct measure of the density of the liquid or gas. Mass flow rate and density are measured independently of each other using the same device, illustrating the versatility of Coriolis flow meters.

Coriolis flow meter working principle

## 3. Coriolis flow meter vs thermal flow meter

Coriolis flow meters allow the direct measurement of mass flows. Direct mass flow measurement eliminates inaccuracies caused by the physical properties of fluids. Thermal flow meters, on the other hand, allow indirect measurement of mass flows.
Due to the way of measuring, there are fundamental differences between the two kinds of devices, which influence their typical applications.

A thermal mass flow meter employs the fluid’s heat capacity to measure the mass flow rate. With a heater and one or two temperature sensors in the heart of the device, the applied power for heating (with one sensor) or the temperature difference between the two sensors is directly proportional to the fluid mass flow rate. Thermal mass flow meters are mainly used for gases.

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

## 4. Challenges to face with a Coriolis flow meter

With Coriolis instruments, the sensor tube is set into vibration by an actuator. In the case of external vibrations of approximately the same frequency, this could interfere with the measurement.

Take trains in the surroundings, air conditioning or other machines in the building itself. Identifying these external vibration sources is a first step to minimising their effects. This can be done, for example, by moving the Coriolis flow meter to a less vulnerable place, by rotating the device, by using a (bigger) mass block, decoupling using dampers and/or flexible tubes.

## 5. Applications where the Coriolis flow meter is the best choice

A Coriolis-based mass flow meter is particularly suitable when you want to measure the mass flow of varying or unknown gas or liquid mixtures or for measuring supercritical gases. Besides measuring direct mass flows which eliminates inaccuracies due to the physical properties of the fluid, these devices are highly accurate and have a high repeatability. The Coriolis flow meter is the ultimate flexible, reliable and extremely accurate flow meter.

## 6. Coriolis flow meters in our portfolio

Bronkhorst High-Tech B.V.

Nijverheidsstraat 1A
NL-7261 AK Ruurlo (NL)
Tel. +31 573 45 88 00
[email protected]