True mass flowmeters measure the mass rate of flow directly as opposed to the volumetric flow rate. As a result, entrained air does not affect the accuracy of their measurement. Many so-called mass flowmeters, however, infer the mass flow rate via the equation:

In this equation, QM is the mass flow rate, QV is the volume flow rate, and r is fluid density. Such mass flowmeter instruments essentially combine two devices, one to measure fluid velocity and the other to measure density. These inputs are typically combined in a microprocessor, along with additional data, to provide an output indicative of the mass flow rate. In contrast, the following meters measure mass flow directly without the intermediate calculation from volume and density.

Thermal Meters

Thermal meters are commonly applied to gas streams only; in fact, to gas streams where the transfer of heat to and from the stream is a usual element of the metering process. Measuring this heat transfer supplies data from which a mass flow rate may be calculated. As mass meters, thermal meters operate independent of density, pressure, and viscosity.

Coriolis Meters

The Coriolis meter uses an obstructionless U-shaped tube as a sensor and applies Newton’s Second Law of Motion to determine flow rate. Inside the sensor housing, the sensor tube vibrates at its natural frequency (Figure 11). The sensor tube is driven by an electromagnetic drive coil located at the center of the bend in the tube and vibrates similar to that of a tuning fork.

The fluid flows into the sensor tube and is forced to take on the vertical momentum of the vibrating tube. When the tube is moving upward during half of its vibration cycle (Figure 12), the fluid flowing into the sensor resists being forced upward by pushing down on the tube.

The fluid flowing out of the sensor has an upward momentum from the motion of the tube. As it travels around the tube bend, the fluid resists changes in its vertical motion by pushing up on the tube (Figure 12). The difference in forces causes the sensor tube to twist (Figure 13). When the tube is moving downward during the second half of its vibration cycle, it twists in the opposite direction. This twisting characteristic is called the Coriolis effect.

Due to Newton’s Second Law of Motion, the amount of sensor tube twist is directly proportional to the mass flow rate of the fluid flowing through the tube. Electromagnetic velocity detectors located on each side of the flow tube measure the velocity of the vibrating tube. Mass flow is determined by measuring the time difference exhibited by the velocity detector signals. During zero flow conditions, no tube twist occurs, resulting in no time difference between the two velocity signals. With flow, a twist occurs with a resulting time difference between the two velocity signals. This time difference is directly proportional to mass flow.

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