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.
Thermal Mass Flow Measurement
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 mass flow meter/controller 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 mass flow meters and mass flow controllers 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.