Thermal
flowmeters are most commonly used to measure the mass flow of clean
gases, such as air, nitrogen, hydrogen, helium, ammonia, argon, and
other industrial gases. Mixtures, such as flue stack flow and biogas
flow, can be measured when their composition is known. An advantage of
this technology is its dependence upon thermal properties that are
almost independent of gas density. Be careful when using thermal
flowmeters to measure the flow of gases with unknown and/or varying
composition, such as hydrogen-bearing off-gases and other mixtures that
can disproportionately affect the thermal flowmeter measurement.

Thermal
mass flow meters are used almost entirely for gas flow applications. As
the name implies, thermal mass flow meters use heat to measure flow.
Thermal mass flow meters introduce heat into the flow stream and measure
how much heat dissipates using one or more temperature sensors. The
amount of heat lost from the sensor is dependent upon the sensor design
and the thermal properties of the fluid. The thermal properties of the
fluid can (and do) vary with pressure and temperature, however these
variations are typically small in most applications. In these
applications where the thermal properties of the fluid are known and
relatively constant during actual operation, thermal flow meters can be
used to measure the mass flow of the fluid because the thermal flow
measurement is not dependent upon the pressure or temperature of the
fluid.
dasar

mass gas flow meter