Ullage
Ullage is the empty space between the surface of a liquid and the top of the tank that holds it. It is the counterpart of sounding (the depth of liquid measured from the bottom): tank contents can be worked out from either measurement using calibration tables.
On tankers and in bunkering, ullage is the standard way to gauge how much liquid a tank contains — measured by tape, sounding rod or automatic gauging system, then corrected for temperature and trim to give volume and weight. Accurate ullage measurement is essential to cargo quantity determination, bunker surveys and custody transfer, where small measurement errors on large tanks translate into significant quantity — and cost — differences.
On a tanker, the money is in the tanks, and ullage is how you measure it. A small error in reading the empty space above a large parcel of oil becomes tonnes of cargo and real cost at custody transfer. Precise ullage — and its temperature and trim corrections — is the basis of trustworthy liquid-cargo and bunker figures.
What is the difference between ullage and sounding?
Ullage is the empty space from the liquid surface to the top of the tank; sounding is the depth of liquid from the bottom. Either gives the volume via calibration tables.
Where is ullage used?
On tankers and in bunkering, to determine cargo and fuel quantities for custody transfer and bunker surveys.