In the case of the thermal flowmeter, the instrument works on the principle of convective heat
transfer : heat energy extracted from a hot object as cooler molecules pass by. The ability for fluid
molecules to transport heat is a function of the specific heat of each molecule and the number of
molecules moving past the warmer object. So long as the chemical composition of the fluid remains
unchanged, the convective transfer of heat is a function of how many fluid molecules pass by in a
given time. The heat transfer rate inside a thermal flowmeter will thus double if the volumetric
flowrate of a given fluid doubles and all else remains constant; the heat transfer rate will likewise
double if the density of a given fluid doubles and all else remains constant (i.e. twice the number of
molecules passing by with each time interval). Either way, the convective heat transfer rate becomes
a representation of how many molecules of fluid are moving through the flowmeter, which for any
given fluid type is proportional to the fluid’s mass flow rate. This makes the thermal flowmeter a
true mass flow instrument for any (calibrated) fluid composition