How it looks like
Yes, you guess it right, it is made of rubber ( Rubber plus fabric i.e. nitrile and neoprene coated Nylon).
The shape of the air cell in the image above is cylindrical however, it can also be elliptical, octagonal or rectangular-shape depending upon the shape of the conservator tank.
Operation of the air cell:
When oil inside conservator becomes more due to increased loading on transformer, Air cell releases air outside from it and takes the air inside it when volume of transformer oil becomes less inside conservator or it inflates or deflates on rising/declining transformer oil volume.
The benefits of having air cell and why Air cell is required?
Basically it is for the safety of transformer oil located in the conservator. it protects transformer oil from the moisture and oxygen or we can say, preserves the transformer oil
Air-cell acts as a barrier to separate undesirable elements in the atmosphere, such as water vapor, nitrogen, ozone and oxygen, from transformer oil within a conservator. This barrier also reduces condensation and oxidation inside the transformer and suppresses gas bubble formation in the transformer oil.
Air cell weight and pressure prevents the oil from becoming supersaturated and formation of bubbles inside the oil.
Other names of the Air cell: Air-cell is also known as flexible separator, transformer conservator membrane and by Air bag.