Earth’s atmosphere consists of 21% oxygen in human atmospheric conditions. On the other hand, compressed or cylinder oxygen is 99.5% (±0.5) pure oxygen. And, through cryogenic distillation, oxygen is separated from other gases and compressed or filled in oxygen cylinders. And oxygen cylinders are filled to 2000 pressure per square foot. So the oxygen cylinder pressure is always 2,000 (±100) PSI. It’s pressure it’s not liters.