the theory of sensor is below:

The sensor unit of a thermal mass flowmeter consists of two sheath pipe-shaped sensors acting as flow speed sensor and temperature sensor, respectively. The flow speed sensor is controlled (heated) to a specified temperature difference from ambient temperature. The heat taken away from the flowrate sensor due to the fluid flow is correlated to the mass flow. Therefore, the mass flowrate can be obtained from the amount of power applied to the flowrate sensor to compensate for the removed heat. A thermal mass flowmeter features wide flow range and enables direct measurement of mass flowrate without compensating temperature and pressure.