One of the first alternative methods of preserving oil was the tank with a preservative. In conservative transformers, the main reservoir is completely filled with oil, and a smaller reservoir with a small amount of oil, the conservative tank, is mounted above the main tank and connected by a tube called a gooseneck.
The conservator is ventilated by air, and during its normal operation, its oil level will be halved to allow expansion and contraction of the oil at the time of a critical operation.
The goal of the gooseneck is to prevent the contact of hot oil, generated by the core and the transformer coils with the conserved oil cooled. This result is partial, since there is always some contact between the two oils.
The oil in the main tank of the transformer is under a slight positive pressure, equal to that of the conservator. In the conservator, a dehumidifier can be installed to keep the oil free of moisture, which works for time-fitted transformers.
The dehumidifier has to be replaced at regular intervals and the change from pink to blue will indicate when replacement needs to be made.