The Coriolis flow meter principle of operation

What is the Coriolis effect?

Already know all about the Coriolis effect? Skip directly to “What does this have to do with mass flow?” to learn about the Coriolis flow meter principle of operation.

You may have heard of the Coriolis effect, a term commonly used to explain why hurricanes, tornadoes, and typhoons spin counterclockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere. Behind this phenomenon is the Coriolis force, a fictitious force responsible for the apparent deflection of an object moving within a rotating frame of reference. In the weather example, air traveling through the atmosphere is directed to either the right or left (depending on the hemisphere) and determines the spin direction of weather bodies.

While Newtonian laws of motion sufficiently describe the motion of objects within a stationary frame, they require an additional correction factor provided by the fictitious Coriolis force to describe motion within a rotating frame of reference. This is necessary because the object isn’t physically tethered to the frame of reference, or coordinate system, which is used to characterize its motion. So, an object will appear to be deflected from the initial path as the frame of reference rotates beneath it.

The difference between the intended and actual path is measurable as a deflection resulting from the Coriolis effect.

Figure 1. The Coriolis effect affects trajectory

A real-world example to help explain: Imagine that a person calculates a ball trajectory using only Newtonian laws. The person then stands at a spot near the North Pole and launches the ball directly south towards a target on the equator. If the Earth were a perfectly still, stationary frame of reference, the ball would land on the target. But since the Earth is spinning, the ball actually lands somewhere to the west of the target. The magnitude of this westward deflection increases the longer the ball is in the air and the closer it is to the equator.

Coriolis and mass flow: a dynamic duo

What does this have to do with mass flow?

Standard mass flow devices such as thermal or differential-pressure based mass flow meters calculate mass flow rates using measured temperature change or volumetric flow values in conjunction with known fluid properties. Flow meters and controllers based on the Coriolis operating principle, however, are unique in their ability to measure mass flow directly with no dependence on such properties.

This is made possible by a clever utilization of the Coriolis effect. A tube (or a set of tubes) is electromagnetically actuated and functions as a moving frame of reference. All fluid entering the device travels through the moving tube and experiences a very tiny deflection from its intended path. Sensors measure the magnitude of the deflection as a vibrational phase shift between different points in the tube. This deflection is only dependent on the mass of the fluid, allowing Coriolis instruments to provide precise mass flow measurements regardless of the fluid’s properties, composition, and temperature. A single temperature sensor is typically included to measure the temperature of the tube as its physical properties can vary slightly as a function of temperature.

How does Alicat’s CODA-Series work?

Figure 2. Coriolis flow instrument internal diagram

The CODA-series of mass flow meters and controllers utilizes the Coriolis operating principle and uses the single tube setup described above. The measurement process is as follows:

1. The tube is electromagnetically actuated to create a fixed vibration at the tube’s natural resonance frequency.
2. In a no-flow condition, the vibrational frequencies of corners C1 and C2 (see figure above) are in phase.
3. Flow through the tube induces a change to the vibration (i.e. a “twist” of the tube).
4. The twisting motion produces a phase shift in the vibrational frequency of C1 compared to C2.
5. A series of sensors measures the magnitude of the phase shift, which is directly proportional to the mass flow rate.
6. The PCB converts the sensor data to a measurement of flow rate as mass per unit time.

Figure 3. Coriolis instrument flow path and tube oscillation