What is the use of a conservator in transformer?

When a transformer is over heated ,more hydrogen content is released from the transformer oil when burns when reaches air at the top of tank. so conservator is used to allow hydrogen flow upto it between which a BUCHolz relay get operated.

  The Conservator tank is a tank fitted above the level of top cover of Oil Filled transformer to allow for expansion and compression of the Oil in question. As goes its name, it CONSERVES the amount of oil filled in the transformer, allowing for expansion and contraction.
It also functions as a test reservoir for the Bucholz relay and also as a chamber though which transformer breathes air. 

Two main roles of conservator:
1) Due to temperature variation the transformer oil volume varied it was compensated by conservator and will allow pure air inside the TR.
2) To maintain the oil level inside the transformers.