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 and the resistance value is changed. No moving parts, maintenance-free, easy to install, one type is suitable for a variety of pipe diameter requirements, switch volume continuously ad justable, very low pressure loss, compact structure, LED display flow trend and switch status.