HWFA4 series heat flow switch adopts heat diffusion principle. There are two temperature sensors in the probe: one is used as a reference sensor to measure the temperature of the medium, and the other is used as a measurement sensor after being heated. The temperature difference between the two sensors is used as the basis for measuring the flow velocity. When the flow velocity of the medium increases, the temperature difference decreases, and vice versa. The temperature difference value is converted into standard electrical signal and displayed after processing. The HWFA4 series are all designed with all-metal shell, with 8-bit LED showing flow trend and switching status. One product is suitable for various pipe diameters. It can be set by keyboard or computer. No moving parts, maintenance-free, can be used in a variety of media.