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Measurements of mass flow are preferred over measurements of volumetric flow in process applications where mass balance (monitoring the rates of mass entry and exit for a process) is important. Whereas volumetric flow measurements express the fluid flow rate in such terms as gallons per minute or cubic meters per second, mass flow measurements always express fluid flow rate in terms of actual mass units over time, such as pounds (mass) per second or kilograms per minute.
In the past, mass flow was often calculated from the outputs of a volumetric flow meter and a densitometer. Density was either directly measured or was calculated using the outputs of process temperature and pressure transmitters. These measurements were not very accurate, because the relationship between process pressure or temperature and density are not always precisely known. This is because in the measurement process each sensor adds its own separate error to the overall measurement error, and the speed of response of such calculations is usually not sufficient to detect step changes in flow.

The advent of modern mass flow meters has led to a remarkable increase in the accuracy associated with flow measurements. This flow measurement requires no compensation for a change in density, temperature or pressure. Examples of mass flow meters include:
(a) Coriolis Mass Flow meters
(b) Thermal Mass Flow meters

Coriolis Mass Flow meters
Coriolis flow meters are true mass meters that measure the mass rate of flow directly as opposed to volumetric flow. Because mass does not change, the meter is linear without having to be adjusted for variations in liquid properties. It also eliminates the need to compensate for changing temperature and pressure conditions. The meter is especially useful for measuring liquids whose viscosity varies with velocity at given temperatures and pressures.

Coriolis mass flow meters artificially introduce Coriolis acceleration into the flowing stream and measure mass flow by detecting the resulting angular momentum. When a fluid is flowing in a pipe and it is subjected to Coriolis acceleration through the mechanical introduction of apparent rotation into the pipe, the amount of deflecting force generated by the Coriolis inertial effect will be a function of the mass flow rate of the fluid. If a pipe is rotated around a point while liquid is flowing through it (toward or away from the center of rotation), that fluid will generate an inertial force (acting on the pipe) that will be at right angles to the direction of the flow. This inertial force is used to measure flow rate of the fluid.

Thermal Mass Flow meters
Thermal mass flow meters also measure the mass flow rate of gases and liquids directly. Volumetric measurements are affected by all ambient and process conditions that influence unit volume or indirectly affect pressure drop, while mass flow measurement is unaffected by changes in viscosity, density, temperature, or pressure.

Thermal mass flow meters are often used in monitoring or controlling mass-related processes such as chemical reactions that depend on the relative masses of unreacted ingredients. In detecting the mass flow of compressible vapors and gases, the measurement is unaffected by changes in pressure and/or temperature.
One of the capabilities of thermal mass flow meters is to accurately measure low gas flow rates or low gas velocities (under 25 ft. per minute)- much lower than can be detected with any other device.

Thermal mass flow meters are most often used for the regulation of low gas flows. They operate either by introducing a known amount of heat into the flowing stream and measuring an associated temperature change, or by maintaining a probe at a constant temperature and measuring the energy required to do so.