Liquid filled transformers are once whose coil and core assemblies are totally immersed in an insulating liquid, all of which is usually enclosed in a sealed tank. The liquid circulates through the windings and through cooling tubes or fins projecting from the tank, which in turn radiates internal heat to the external atmosphere. Liquid circulation by internal convention provides self-cooling. Liquid filled transformers must include an air or gas expansion space within the tank itself or within an overhead conservator or expansion tank. The tank of a liquid filled transformer is often sealed. Prior to sealing, internal air is removed by purging with an inert gas, usually dry nitrogen, which provides an inert gas blanket over the liquid. These transformers may be designed for indoor or outdoor application. Most outdoor transformers are of liquid-filled and sealed construction. Liquid-filled transformers are almost exclusively medium voltage rated and above.