The ideal-gas law (PV = nRT) assumes gas amount scales linearly with pressure. At cylinder pressures that's off by a few percent: a real gas is described by PV = ZnRT, where the compressibility factor Z depends on the gas and pressure.
Near 200 bar and room temperature, Z ≈ 0.96 for O₂, ≈ 1.04 for N₂, and ≈ 1.05 for He — so an “AL80” really holds ~77 cf, not the 81 the ideal estimate gives. Helium-rich mixes hold even less per bar than ideal.
This tool uses a first-order linear approximation of Z (reference only). The global “real-gas” toggle applies it across every tool.