Scuba Cylinder Sizes and Working Pressures

“12-liter” and “80-cubic-foot” sound like comparable labels, but they describe different things. A useful cylinder plan needs internal water volume, rated free-gas capacity, working pressure, actual fill, and reserve.

Two ways to name cylinder capacity

Metric cylinders are commonly identified by water volume. A 12 L cylinder at 200 bar contains approximately 12 × 200 = 2,400 surface liters. Imperial cylinders are commonly identified by rated free-gas capacity: an AL80 nominally supplies about 80 cu ft at its service pressure.

That is why “80 cu ft = 12 L” is incomplete. Rated free-gas capacity changes with working pressure; internal water volume does not. The unit converter asks for working pressure when translating these labels.

Common cylinder examples

Those steel rows explain why calling every 15 L cylinder “120 cu ft” is loose: at 232 bar the simple pressure-volume calculation is about 123 cu ft. Manufacturer specifications and stamped pressure control, not a generic label.

Working pressure and the actual fill

Working or service pressure is the rated operating pressure marked on the cylinder. The actual pre-dive gauge reading may be lower, may change as a warm fill cools, and must never be assumed from the model name. Inspection and hydrostatic-test markings determine whether a cylinder may be filled; the fill station interprets them under local rules.

For gas planning, use the actual starting pressure and retain the planned reserve. For imperial SAC-to-RMV conversion, use the capacity and working pressure belonging to that exact cylinder. The RMV converter keeps those fields together, while the gas planner models usable gas after reserve.

Buoyancy, trim, and choosing a cylinder

Gas capacity does not describe in-water behavior. A common AL80 swings from roughly 1.4 kg negative when full to 1.4 kg positive near empty3 lb negative when full to 3 lb positive near empty. Many steel cylinders remain negative throughout the dive, but their diameter, length, mass distribution, and exact buoyancy vary by model.

Choose a cylinder that supplies the plan, can be filled locally, fits the harness, supports horizontal trim, and does not create an unrecoverable buoyancy problem if the BCD fails. Then redo the end-of-dive check described in the weighting guide.