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.
However, in many applications, the thermal properties of the fluid can be dependent upon fluid composition. In these applications, varying composition of the fluid during actual operation can affect the thermal flow measurement. Therefore, it is important for the thermal flowmeter supplier to know the composition of the fluid so that the proper calibration factor can be used to determine the flow rate accurately. Due to this constraint, thermal flowmeters are commonly applied to measure the flow of pure gases. Suppliers can provide appropriate calibration information for other gas mixtures, however the accuracy of the thermal flowmeter is dependent on the actual gas mixture being the same as the gas mixture used for calibration purposes. In other words, the accuracy of a thermal flowmeter calibrated for a given gas mixture will be degraded if the actual flowing gas has a different composition.
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. Be careful when using thermal flowmeters to measure the flow of gases with unknown and/or varying composition, such as hydrogen-bearing off-gases and other mixtures that can disproportionately affect the thermal flowmeter measurement.
Thermal flowmeters can be applied to clean, sanitary, and corrosive gases where the thermal properties of the fluid are known. Thermal flowmeters are most commonly applied to measure pure gases, such as would be used for laboratory experiments, and in semi-conductor production. They can also used in chemical and petrochemical plants when the thermal properties of the gas are known. With proper attention to materials of construction, the flow of corrosive gases, such as hydrogen chloride and hydrogen sulfide can be measured.
Thermal flowmeters should not be applied to abrasive fluids because they can damage the sensor. Fluids that coat the sensor can alter the relationship between the thermal properties of the fluid and the measurement and adversely affect the flow measurement. Extensive coating can render the sensor inoperable unless the sensor is routinely cleaned. This can increase maintenance associated with these flowmeters.
Varying the percentage of certain components that have different thermal properties from calibrated values can cause thermal flowmeters to become highly inaccurate. In other words, thermal flowmeters are often not suitable for applications with fluids that have varying composition and unknown components. However, in some applications, thermal flowmeters could measure reasonably accurately when the flow stream contains components with similar thermal properties.
Aerosols and gases with droplets can cause thermal flowmeters to become erratic and/or measure full scale flow. This is because the large amount of thermal energy used to heat the liquid/droplet is interpreted as a high flow signal. Operating a thermal flowmeter above its maximum flow rate will generally not damage the flowmeter but can cause measurement error because its calibration curve can become unpredictable.