ullage

Ullage is the empty space inside a container that is not filled with liquid. It is the headspace between the liquid surface and the container’s top and can contain air or vapors released by the liquid. Ullage equals the vessel’s total capacity minus the liquid volume, and its size is influenced by temperature, evaporation, and container integrity.

In winemaking and storage, ullage is especially significant. Wine and other liquids in oak barrels or fermentation tanks lose volume through evaporation, creating ullage often referred to as the angel’s share. The ullage level affects exposure to oxygen, microbial activity, and chemical reactions that influence maturation, aroma, and color. Producers monitor ullage and top up the container to maintain a desired headspace and minimize undesired oxidation or spoilage.

Beyond wine, ullage is a general term used in various industries to describe the space in storage tanks, ships’ cargo tanks, and road or rail tank cars. Measuring ullage helps determine remaining liquid volume and assess vapor pressures and safety risks, such as flammability or tank integrity. In petroleum and chemical storage, ullage measurements may be taken with dipsticks, rods, or electronic sensors.

In aerospace, ullage denotes the space above the liquid propellant within a rocket tank. Ullage motors or pressure pulses are employed to settle propellants to the tank bottom before ignition, reducing gas bubbles and ensuring reliable engine start and stable combustion.