The liquid level detector is a principle that uses the low energy photon source and the measured material to interact with the measured material, and the ray intensity varies with the material's material surface position, so as to control the capacity of the liquid material. Because of its non-contact measurement method, the problem that the conventional weighing method can not measure the capacity of liquid material in the production line is fundamentally solved. Therefore, it is widely used in on-line automatic control of food, medicine, chemical and other liquid filling materials.