A thermal mass flowmeter is a direct‑type mass flowmeter used to measure mass flow rate. When the fluid is stationary, the temperature distribution upstream and downstream of the heating element in the pipeline is symmetrical, and the indicated temperatures of the element are equal. When the medium flows, the downstream temperature is higher than the upstream temperature. As the flow velocity increases, the temperature difference rises. Once the flow velocity reaches a certain threshold value, the temperature difference between the two elements gradually decreases with further increase in flow velocity. To ensure the single‑valued characteristic of the instrument, the characteristic of increasing temperature difference is generally adopted for low‑flow measurement, while the characteristic of decreasing temperature difference is used for high‑flow measurement.

The thermal conductivity measurement method realizes flow measurement based on the relationship between heat dissipated by the heating element and flow velocity. In the conventional method, a fine metal wire placed in the fluid is heated by an electric current. Fluid flow cools the hot wire, and the degree of cooling varies with flow velocity. Therefore, the flow velocity and flow rate can be determined by measuring changes in the resistance value of the hot wire.