While there are many technologies to measure the flow rate of air and gas, most of these methods measure the flow rate at the actual operating pressure and temperature, and require pressure and temperature correction to obtain the mass flow.
Traditionally, the most common benefit of thermal dispersion flow measurement is the inherent ability to directly measure the mass flow without need for pressure and temperature correction, as required with volumetric gas flow measurement. This not only provides a more useful flow measurement, but also makes thermal very cost-effective. Magnetrol thermal dispersion mass flow meters also allow users to conduct calibration verification in the field, an option of increasing importance for process industries.
Principle of Operation
Thermal dispersion technologies are based on the operational principle that states the rate of heat transfer by a flow stream is proportional to its mass flow. The flow measurement is accomplished by precisely measuring the cooling effect as the mass (molecular) flow passes the heated sensor. The sensor consists of two elements:
- The reference, which measures the temperature of the gas.
- A second element, which is heated at a variable power to maintain the desired temperature difference between the two sensors.
The illustration below shows the amount of power required to maintain a constant temperature difference between the two sensors. Under low mass flow conditions, there is minimal cooling and little power is required. As the mass flow increases, more power is required. This provides excellent low flow sensitivity and high turndown capabilities.
The calibration of a thermal mass flow meter determines the relationship between heat transfer and mass flow. Increasingly, it has become an important option for users to conduct calibration verification on site. In the case of Magnetrol thermal mass flow meters, factory calibration is permanent and recalibration is not required. While true calibration can only be performed with a flow bench, some users may want to periodically check calibration.
Magnetrol has developed a procedure to check the heat transfer characteristics of the sensor under two sets of controlled conditions: zero flow and a simulated high flow. Upon conducting verification measures, if the heat transfer characteristics are the same as originally received (indicated on the calibration certificate and stored in the unit), the unit remains in calibration and there is no need to send it back to the factory for recalibration. For further information, a demonstration video on the Thermal Mass Flow Technology Portal provides detailed information on calibration verification of Magnetrol THERMATEL® TA2 mass flow meter units. View the video by logging in to the Thermal Mass Flow Technology Portal.