The primary sensor of a thermal flowmeter is a heated wire or film, typically a platinum or tungsten Resistance Temperature Detector (RTD), that is exposed in the flow. A second sensor, possibly another RTD, is used to measure the flow temperature.
As the flow passes over the hot wire, it carries away heat. The heat loss depends on the mass flow rate, the heat capacity of the fluid, and the temperature difference between the wire and the fluid.
Since the heat capacity of the fluid is known and the temperatures are monitored in real-time, the mass flow rate can be determined from the heat loss (related to the electric resistance of the wire via the Ohm's law) and the thermal expansion coefficient of the wire, as discussed in the hot wire theory section.