Thermal flow meters are most commonly used to measure the mass flow
of clean gases, such as air, nitrogen, hydrogen, helium, ammonia,
argon, and other industrial gases.
How Thermal Flowmeters Work
Thermal flowmeters use the thermal properties of the fluid to
measure the flow of a fluid flowing in a pipe or duct. In a typical
thermal flowmeter, 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 flowmeters are used to measure
gas flows.
The amount of heat lost from the sensor is dependent upon the
sensor design and the thermal properties of the fluid. The thermal
properties of the fluid can and do vary with pressure and
temperature, however these variations are typically small in most
applications. In these applications where the thermal properties of
the fluid are known and relatively constant during actual
operation, thermal flowmeters can be used to measure the mass flow
of the fluid because the thermal flow measurement is not dependent
upon the pressure or temperature of the fluid.
