Coriolis Flow Meter – Operating Principle

The physics of the Coriolis effect

The Coriolis effect is commonly used to explain why hurricanes, tornadoes, and typhoons spin clockwise in the Southern Hemisphere and counterclockwise in the Northern Hemisphere. This phenomenon occurs due to the fictitious Coriolis 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.

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.

Other mass flow measurement principles such as thermal or laminar DP 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.

Using the Coriolis effect to calculate mass flow

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.

Coriolis flow meters can vary greatly in size, with tube diameters as small as a few millimeters and as large as 30 millimeters.

Alicat’s CODA-Series of mass flow meters and controllers are low-flow (8 g/hr to 300 kg/hr) Coriolis instruments that use a very thin single tube setup described below. The measurement process is demonstrated in this video:

How do laminar DP flow meters work?

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