Bottling operations require management of multiple variables to ensure proper wine development. Calculating proper ullage levels are necessary to maintain proper levels of internal bottle pressure over a range of storage temperatures. The two key variables are bottling temperature and headspace management.
Thermal Expansion is a critical factor in calculating fill heights during bottling. The most common miscalculation occurs when wineries do not compensate for bottling at cooler temperatures than the eventual storage conditions. This can lead to underestimating the headspace. For instance, wine bottled at 50°F will see approximately 2ml of headspace disappear at normal storage temperature of 68°F. Failing to take this expansion into consideration can lead to serious increases in internal pressure if the wine is exposed, for even several hours, to higher than normal storage temperatures.
Headspace management is important to avoid potential oxygen ingress. Most bottling lines use some combination of gas sparging and/or vacuum application. Lower internal pressure levels are preferred, and the CQC recommends a maximum of 2psi internal pressure after bottling at 68°F.
Here are two headspace calculators that offer a quick way to see the relationship of temperature, internal pressure, and ullage. One is designed for determining the minimum headspace volume at bottling. The second is designed for wines that have already been bottled, and illustrates the pressure and ullage levels that can be expected at different storage temperatures.