What is the function of the oil cup in a transformer breather?

The oil cup in a silica gel breather acts as an air seal and a moisture trap at the breather inlet. As the transformer breathes due to oil volume changes, incoming air bubbles through the oil cup, which helps prevent dust and moisture laden air from entering directly. The air then passes through the silica gel where remaining moisture is adsorbed. Operators check the oil level and cleanliness in the cup and the color of the silica gel, because low oil or contaminated oil reduces breather effectiveness and can accelerate moisture ingress and oxidation inside the transformer.