The Theory of Coriolis: How the Technology Measures Mass Flow and Density

by | Jan 4, 2016 | Flow, Measurement Instrumentation | 0 comments

Emerson’s Micro Motion is widely recognized as the leader in innovative measurement practices, where the Coriolis flow meter is the primary example of technology that has revolutionized accuracy across many different applications and industries.

Micro Motion Coriolis Flow Meter Technology

So how does the Coriolis flow meter measure mass flow and density? Our patented technology relies on a special design of dual parallel flow tubes at the core of the flow meter.

The process fluid splits when entering the sensor, and half enters each tube. During operation, a drive coil stimulates the tubes to oscillate in opposition to each other at the natural resonant frequency of the tubes. Magnet and coil assemblies, called “pickoffs”, are mounted on the flow tubes, and as the tubes oscillate, the voltage generated from each of these pickoffs creates a sine wave. These waves indicate the movement of one tube relative to the other.

When there is no flow, the tubes are ‘in phase’, moving in a synchronized motion. But when fluid is moving through sensors’ tubes, Coriolis forces are induced in both tubes, causing the tubes to twist in opposition to each other. As a result of the twist in the flow tubes, sine waves are now shifted in phase with respect to each other, and are now asynchronous.

The time delay between the two sine waves is measured in microseconds, and is called Delta T, which is directly proportional to the mass flow rate. The greater the Delta T created by the Coriolis force, the greater the mass flow rate.

While the sine wave phase shift indicates mass flow, wave frequency indicates density. When the liquid density changes, so does the vibrating frequency of the tubes.

Mass and Frequency

A similar example would be a system of springs with different weights on the end. The object with a larger mass has a lower frequency of oscillation, while the smaller mass item has a higher frequency. The movements vary according to the size of the weight.

In the Coriolis sensor, the tubes correlate to the spring, the mass and fluid they contain correspond to the weight at the end of the spring. The stiffness of the flow tubes remains essentially constant, so the only variable affecting the frequency is the mass and density of the liquid contained in the flow tubes.

Liquid volume flow is derived from the mass flow and density measurements already acquired. Fluids can have the same volume, but different densities and masses, and it is the mass and density which allows the calculation of the volume flow.

These techniques allow the Micro Motion flow meters to deliver highly accurate direct mass flow and density measurements in a broad range of applications.

Visit the Emerson Micro Motion site for more information and to learn more about the Coriolis flow meter process.

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