Thermal Technology
Thermal mass flowmeters are based on the relationship between convection heat transfer and mass flow. There are two types of thermal mass flowmeters: Rate of Heat Loss Flowmeter and Temperature Rise Flowmeter.
Rate of Loss Flowmeter
Rate of Heat Loss Flowmeters measure the rate of heat loss to the flow stream from a heated element such as a resistance wire, thermistor, thermocouple, or thin film sensor.
Governing Equation
In this equation, 
, Vavg, qt, and 
T are the unknowns, because they change with time while the other variables are known. However, qt and T can be obtained through measuring devices, leaving in the product of and Vavg and cross section area of the pipe.
Temperature Rise Flowmeter
Temperature Rise Flowmeters measure the temperature of the flow changes as it passes through a heat source.
Governing Equation
W and T are unknowns in this equation. T can be measured. W can therefore be calculated.
Note that thermal mass flowmeters are used for gas measurements only.
