The thermal mass flowmeter adopts the principle of thermal diffusion, which is a technology with excellent performance and high reliability under harsh conditions. Typical sensing elements include two thermal resistors (platinum RTD), a speed sensor and a temperature sensor that automatically compensates for changes in gas temperature. When the two RTDS are placed in a medium, where the velocity sensor is heated to a constant temperature above the ambient temperature, the other temperature sensor is used to sense the medium temperature. The mass flow of gas through the velocity sensor is calculated by the heat transfer of the sensing element. As the gas velocity increases, the heat carried away by the medium increases. The temperature of the sensor decreases accordingly. In order to maintain a constant temperature, the working current through the sensor must be increased, and this increased portion of the current is not proportional to the velocity of the medium.