The silica gel breather cup of a Transformer is partially filled with oil to avoid entering of moisture into the Transformer. Every Transformer breathes in and out because they are filled with oil and tightly sealed with gaskets. As the oil expands due to rise in temperature, the pressure builds up , consequently, the vapour and dry air accumulated in the conservator are breathed out. During the fall of temperature, oil in the Transformer contracts and creates partial vacuum in the tank which makes the Transformer to breathe in air from the outside. In the process of breathing in outside air, moisture and dust particles in the outside air are also sucked in. But, the oil in the breather cup obstructs dust particles and settles them at its bottom. The moisture entered is absorbed by the silica gel in the breather.
The oil in the breather cup is only partially filled up to the mark given on the cup. If oil in the cup is over filled, while brething in, the oil gets into contact and absorbed by the silica gel resulting in bottom portion of silica gel is permanently wasted as the oil absorbed silica gel cannot be reactivated again.