Two methods, Constant Temperature Difference and Constant Power, are used for mass flow measurement and both methods obtain the same results.
Constant Temperature Difference:
Maintaining a constant temperature difference between the heated (velocity) sensor and a reference (temperature) sensor is the objective of this method. A predefined temperature difference, ΔT, is set during initial calibration. The amount of power to the heater is controlled to maintain the temperature difference. A non-linear relationship between the mass flow rate and power signal is observed. At higher flow velocities, more power is required to maintain the temperature difference. Whereas at low flow velocities, less power is to maintain the temperature difference. This provides excellent low flow sensitivity as well as outstanding turn down.
Constant Power:
This technology works on maintaining a constant power to the heating sensor. The instantaneous fluid temperature varies depending on rate of flow and is sensed by the reference temperature sensor. Temperature difference between the heating sensor and reference sensor is constantly monitored. It decreases when the mass flow rate increases. At the lower flow rates, the change in temperature is larger which offers great low flow sensitivity. As the flow rate increases the temperature difference reduces.