Abstract: The heat trigger of flowrate / liquid level switch is a sensor to monitor flow or level change, it is mainly composed of temperature measuring element in part of probe and a low power heater to heat the element. The response time can minimize to 2 seconds, suitable for various liquids and gases with different scope of medium pressure, temperature, and viscousity. The type of TFS-1 trigger is developed by Chinese Petroleum and Natural Gas Scientific Research Institute, and a comparison between the trigger and introduced one was made on test data and chart. From the point of calculation and test results, response time, heating power, signal value out of probe in different levels and different medium maximum power consumption, signal repeat, and automatic temperature compensation, all these parameters are basicly same, but the cost is much lower.