1) Direct mass flow measurement is more accurate and cost effective.
2) Direct Mass flow does not need any pressure or temperature compensation equipment or flow computers.
3) Provides virtual immunity to process changes in P & T; i.e., very repeatable measurements.
4) In Gases, density is highly variable. This makes volumetric gas flow very sensitive to changes in pressure and temperature.
5) Over 80% of all industrial processes—such as those involving chemical reactions, combustion, respiration, and HVAC—are based on mass flow, not volumetric flow.

Summary:
In most processes it is mass, not volume, which is the critical variable. Volumetric flow measurements are less reliable than mass flow measurements because changes in gas temperature and pressure will change the density of a fixed volume of gas. In the control process, additional errors can be propagated by back pressure changes in the process. Unlike rotameters, turbine meters and other volumetric flow devices, thermal mass flow meters and controllers are relatively immune to changes in inlet temperature and pressure. Because these instruments directly measure molecular flow, they provide the most reliable, repeatable and accurate method for delivering material from a supply volume to a process at a desired rate.