Based on the thermal principle, the sealed probe contains two resistances, one of which is not heated as the detection resistance, and the other is not heated as the reference resistance. When the medium flows, the heat on the heating resistance is taken away, the resistance value is changed, and the difference between the two resistances is used as the basis for judging the flow rate. No moving parts, maintenance free, easy installation. One model is applicable to various pipe diameter requirements. The switching value is continuously adjustable, with extremely low pressure loss, compact structure, and LED display flow trend and switching status.