How Thermal Flow Meters Work
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How Thermal Flow Meters Work 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. 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 flow meters 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 flow meter 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 flow meters 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 flow meter 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 flow meter calibrated for a given gas mixture will be degraded if the actual flowing gas has a different composition. |
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