Thermal flow meters use the thermal properties of the fluid to measure the flow of a fluid flowing in a pipe or duct. In a typical thermal flow meter, a measured amount of heat is applied to the heater of the sensor. Some of this heat is lost to the flowing fluid. As flow increases, more heat is lost. The amount of heat lost is sensed using temperature measurement(s) in the sensor. The transmitter uses the heat input and temperature measurements to determine fluid flow. Most thermal flow meters are used to measure gas flows.
Thermal flowmeters is capable of providing direct measurement of mass flow, as opposed to most other methods that measure volumetric flow and require separate measurements for temperature and pressure in order to calculate density and, ultimately, the mass flow. These thermal flow instruments actually measure and control the flow on a molecular level and so are able to provide an extremely accurate, repeatable, and reliable delivery of gas into the process.
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.