Ullage is the empty space remaining at the top of a tank above the liquid cargo surface — measured downward from a fixed reference point at the top of the tank to the liquid surface.
Ullage vs Sounding
- Ullage — measured from the top of the tank down to the liquid surface
- Sounding — measured from the bottom of the tank up to the liquid surface
Both methods measure the same liquid level, just referenced from opposite ends — which one is used often depends on the specific tank gauging equipment and company/vessel convention, but both feed into the same cargo quantity calculations.
Why Ullage Space Is Left Intentionally
Tanks are never filled completely to the top when loading liquid cargo — a deliberate ullage space is left specifically to accommodate thermal expansion. Liquid cargo expands as temperature rises during a voyage (moving through different climate zones, for instance), and a completely full tank with no expansion space would risk dangerous overpressure or cargo loss through vents. Loading calculations specifically account for expected temperature range to determine safe ullage at loading.
Using Ullage for Cargo Quantity Calculation
Ullage measurements, combined with the tank’s known dimensions (via calibration tables specific to that tank) and cargo density/temperature, allow calculation of the actual volume and weight of cargo in the tank — a standard, essential part of tanker cargo operations and quantity verification, similar in purpose to draft surveys on dry cargo vessels.
Automatic Tank Gauging (ATG)
Modern tankers commonly use automatic tank gauging systems providing continuous, real-time ullage readings without requiring manual sounding tape measurements — though manual verification remains standard practice for quantity disputes or as backup confirmation.
Ullage and Safety
Insufficient ullage space (overfilling) is a genuine safety hazard — risking overflow during loading, dangerous overpressure from thermal expansion during the voyage, or cargo loss through pressure/vacuum relief valves. Ullage monitoring throughout loading is a real, active safety responsibility, not just a quantity-calculation exercise.
Exam Relevance
Ullage measurement, its role in cargo quantity calculation, and the safety reasoning behind intentional ullage space are core Cargo Work topics in deck officer competency exams, particularly for tanker operations.