Due to the lack of expansion source in hermetic transformers, visual oil is used to control the oil level. Its installation place is on the oil charging pipe, and its main body is made of brass, and an oil-resistant float is placed in its chamber. The high level of the ball inside it indicates the proper oil level in the transformer. If the pipe is full of oil, the float will be in the upper part of the chamber, and due to the drop of oil in the pipe, the float will be at the lowest point of the oil chamber.