Based on the thermal principle, the sealed probe contains two resistors, one of which is 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 resistors is used as the basis for judging the flow rate. No moving parts, maintenance-free, easy installation, one model is suitable for a variety of pipe diameter requirements, the switch-ing quantity can be adjusted continuously, very low pressure loss, compact structure, OLED display flow trend and switching state, integrated temperature measurement function, support temperature alarm and remote measurement.