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E-Zine August 2006Coriolis mass flowmeters use the properties of mass to measure mass. In an analogy, when a fixed mass is rotating on a turntable, centrifugal force pushes the mass outward, but there are no forces pushing the mass in a plane tangent to the rotation. However, if the mass is moving inwards or outwards from the center of the turntable, the radius of rotation changes, and a force (the Coriolis force) is produced in the tangential plane at right angles to the centrifugal force. This effect can be readily experienced when riding a merry-go-round. In a Coriolis mass flowmeter, the "rotation" is typically generated by vibrating tube(s) in which the fluid flows. In a U-tube Coriolis mass flowmeter design, the fluid (mass) in the tubes flows away from and towards the axis of vibration, and a Coriolis force in the tangential plane is produced. Fluid flows in opposite directions relative to the axis of vibration cause the Coriolis forces on the inlet and outlet halves of the U-tube to be in opposite directions. These opposite Coriolis forces cause the U-tube to twist. The amount of twist is proportional to the mass flow rate of fluid passing through the U-tube. Other designs utilize different geometries to develop Coriolis forces. Sensors and a Coriolis mass flowmeter transmitter are used to measure the twist and generate a signal proportional to the mass flow of the fluid. The mechanical characteristics of vibrating tubes are affected by temperature, so the twist of the tube generated by a given Coriolis force will vary with temperature. To maintain accurate mass flow measurement, Coriolis mass flowmeters generally require compensation for tube temperature or operation within a narrow temperature range. Temperature compensation is typically implemented electronically in the Coriolis mass flow transmitter using a measurement from a temperature sensor located on the surface of the flow tube or in the flowmeter housing. In many designs, suppliers offer an additional analog output that can represent temperature. It should be noted that this output is the temperature used for compensation. It is not the fluid temperature, but rather the temperature of the outer surface of the tube or the flowmeter housing. Nonetheless, this temperature is often sufficiently close to the fluid temperature and may provide useful process information. The frequency of the vibrating tube(s) is related to the density of the fluid in the tube. The Coriolis mass flow transmitter typically analyzes the sensor signals to determine density of the fluid in the tube. In many designs, suppliers offer an additional analog output that can represent the density of the process fluid. Excerpted from The Consumer Guide to Coriolis Mass Flowmeters ISSN 1538-5280 |
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