Most mass air flow sensors used in automotive engine control applications employ this principle.
It is important for engine control computers to measure mass air flow and not just volumetric air
flow because it is important to maintain proper air/fuel ratio even if the air density changes due
to changes in altitude. In other words, the computer needs to know how many air molecules are
entering the engine per second in order to properly meter the correct amount of fuel into the engine
for complete and efficient combustion. The “hot wire” mass air flow sensor is simple and inexpensive
to produce in quantity, which is why it finds common use in automotive applications.
Industrial thermal mass flowmeters usually consist of a specially designed “flowtube” with two
temperature sensors inside: one that is heated and one that is unheated. The heated sensor acts
as the mass flow sensor (cooling down as flow rate increases) while the unheated sensor serves to
compensate for the “ambient” temperature of the process fluid