In explaining Coriolis principles of operation, Marc notes how mass is measured directly because the vibrating tubs are sensitive to bulk inertial forces of the fluid’s mass. As a mass moves toward or away from the center of rotation inside of a rotating tube, the particles generate inertial forces on the tube. The measurement of this vibration is not affected by changes in fluid properties and velocity profile. Vibration pickup coils measure the sinusoidal waves of the two vibrating tubes. In a no flow condition the two sine waves are in sync with one another.
Flow causes a twisting effect in the tubes, which puts the sine waves out of phase with one another. By measuring this phase shift, the amount of mass flow is calculated. See 4:30 into the video for Marc’s description of how this measurement is performed.
For natural gas metering applications, a Micro Motion Coriolis meter with Tall Tube Geometry is recommended, since the mass of gas is so much less than liquid and longer tubes are required to get the Coriolis effect and the level of accuracy required for custody transfer applications. These applications require highly accurate measurements since custody transfer involves the change in ownership of the natural gas.
He referenced the 2nd edition of the AGA Report No. 11 which tightened measurement performance requirements from +/- 1.0% to +/- 0.7%. Diagnostic verifications were required to guide the users on when flow tests were needed to recalibrate and this calibration could be performed in the field.
To convert mass flow measurements to standard volume measurements, one doesn’t need to know pressure and temperature, like traditional volume flow measurements, but the density of the natural gas.
Learn more about Coriolis flow measurement in the Micro Motion area of Emerson.com and connect and interact with other flow and oil & gas experts in the Flow and Oil & Gas groups in the Emerson Exchange 365 community.