ullage
noun
Definitions
In a wine bottle, the empty space between the cork and the top of the wine.
In a cask or barrel, the empty space, occupied by air, that is created by not completely filling the cask or barrel, or through spillage.
The topping-up of such a barrel with fresh wine.
In an industrial setting, the empty space in a tank, such as for fuel.
verb
Definitions
To gauge the amount of empty space between the top of a cask and the level of liquid inside it.