Mass flow meter
From Wikipedia, the free encyclopedia
A mass flow meter, also known as inertial flow meter and coriolis flow meter, is a device that measures how much liquid is flowing through a tube. It does not measure the volume of the liquid passing through the tube, it measures the amount of mass flowing through the device.
Volumetric flow metering is proportional to mass flow rate only when the density of the fluid is constant. If the fluid has varying density, or contains bubbles, then the volume flow rate multiplied by the density is not an accurate measure of the mass flow rate.
In a mass flow meter the fluid is contained in a smooth tube, with no moving parts that would need to be cleaned and maintained, and that would impede the flow.
[edit] Operating principle
There are two basic configurations: the curved tube flow meter and the straight tube flow meter. This article discusses the straight tube design.
The first image is a schematic representation of the vibration of the tube of a mass flow meter, in a no-flow situation. The surrounding machinery (not shown) drives the vibration of the tube. In this schematic, the representation of the amplitude of the vibration is very much exaggerated.
The second image shows what happens when there is mass flow through the vibrating tube. The vibration of the inlet part of the tube is dampened because fluid without kinetic energy in the sideways direction is entering the tube. The vibration of the outlet part of the tube is stronger than in the no-flow situation because fluid with a lot of kinetic energy is moving from the inlet part to the outlet part.
One way of understanding the dynamics involved is to concentrate on the time intervals when the vibrating tube is passing the equilibrium point of the oscillation. At the moment the tube and the fluid in the tube are passing the equilibrium point the velocity is at its maximum; at that moment the fluid has maximum kinetic energy (in the sideways direction). In the no-flow situation the fluid at the location of the largest amplitude has the most kinetic energy. In the flow situation that fluid is moving on, causing the outlet part of the tube to vibrate stronger.
The third image shows the difference in vibration shape between the no-flow situation and the flow situation. The vibration in the flow situation can be seen as a superposition of two vibrations: the original, single-bulge vibration, and a secondary two-bulge vibration that arises in response to the combination of the primary vibration and mass flow. the magnitude of the secondary vibration is proportional to the mass flow.
It may not be directly obvious why this operating principle is often called 'Coriolis flow metering'. However, the kinetic effect that is being exploited is the Coriolis effect. The Coriolis effect is usually described in the context of rotation, but there are analogies between oscillation and rotation. A pendulum can swing in one vertical plane, but it can also swing in two perpendicular planes, and then the resultant motion of the weight will be an ellipse (or a circle if the two oscillations are carefully set up to result in a circular motion of the weight of the pendulum.) It is simpler to have the measuring tube vibrate in one plane, but if the measuring tube were to vibrate in two perpendicular planes the resulting motion would be like the motion of a crankshaft, making it clearer how the Coriolis effect is involved in mass flow metering.
