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In many research and production processes the important variable is mass and not volume. Measurements of volumetric flow are not as reliable as mass flow measurement due to the effects that changes in temperature or pressure have on the density of a fixed volume of gas.

Unlike volumetric flow measurement devices such as purge meters (variable area meters) or turbine meters, thermal mass flow meters (MFM) and mass flow controllers (MFC) are relatively immune to fluctuations in temperature and pressure of the incoming flow. The MFM/MFC is capable of providing direct measurement of mass flow, as opposed to most other method that measure volumetric flow and require separate measurements for temperature and pressure in order to calculate density and, ultimately, the mass flow.
The MFM/MFC actually measures and controls the flow on a molecular level and so is able to provide an extremely accurate, repeatable, and reliable delivery of gas into the process.

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Originally developed for the semiconductor industry, MFMs/MFCs are now widely used for applications in research laboratories, pilot plants, and continuous processes. The thermal mass flow meter and mass flow controller advantages of low flow accuracy and repeatability, relative immunity to fluctuations of inlet flow temperature and pressure, and a complete PID control loop in a compact package have helped to improve productivity and reduce costs in a variety of analytical, industrial process, and OEM applications.

Principle of Thermal mass flow measurement

EL-FLOW Prestige mass flow meter/controller product tutorial