Ullage. This is a measurement of space between the surface of liquid in a tank and the top of the tank inner surface.
Ship’s ullage tables are drawn up, based on the internal volume of a tank measured from some reference point, e.g., the lip of an “ullage” hole. The reverse of ullage is sounding. This is the depth of liquid in a tank measured from the liquid surface to some reference point at the bottom of the tank “Sounding tables” may complement or replace “ullage tables”.
This expression covers the free space left in the tanks after loading liquids in bulk. In oil tankers, ullage is left in order to leave room for expansion when the oil is heated to a higher temperature before discharge. Oil can also expand with atmospheric temperature changes so that oil tanks are customarily loaded to 98 per cent capacity.