Thursday, October 8, 2015

The Coriolis Principle and It's Use in Flow Measurement

Image courtesy of
Wikipedia
The Coriolis effect acts on a medium that is accelerated through a rotating system, like a ball on a rotating disk its movement is straight, however, if the observer turns with the disk the ball is apparently deflected (see image).

The same effect occurs with a water hose that rotates around its own axis, like a skipping rope. As soon as water flows through the host also twists. The twisting is stronger or weaker, depending on the amount of water flowing through the hose.

Coriolis flow meters function according to the same principle (measuring the force resulting from acceleration caused by mass moving toward, or away from, a center of rotation).
Oscillation with flow
(courtesy of Wikipedia)
Oscillation without flow
(courtesy of Wikipedia)
 
The Coriolis effect also appears with an oscillating movement, and in a Coriolis flowmeter, two symmetric metal tubes are set vibrating by an internal driver coil. The tubes oscillate with a resonance frequency similarly to that of a tuning fork.

The oscillation is measured precisely by two pick-ups at the inlet and outlet sections. If liquids or gases flow through the tubes, a phase shift occurs the pickups measure the spatial and temporal displacement (twist). The amount of twist is proportional to the mass flow rate of fluid passing through the tubes. The greater the amount, the stronger the tubes oscillate outwards.

Finally, sensors and transmitters are used to measure the twist and create a linear flow signal as an output for monitoring and control.

This video, although marketing oriented, does a great job illustrating the Coriolis effect and how Coriolis flowmeters measure mass flow (the video references the Siemens SITRANS FC430 as the example).