Liquid Oxygen – Oxygen is stored as a very cold liquid in a vessel very similar to a thermos. When released, the liquid converts to a gas and you breathe it in just like the compressed gas. This storage method takes up less space than the compressed gas cylinder, and you can transfer the liquid to a small, portable vessel at home. Liquid oxygen is more expensive than the compressed gas, and the vessel vents when not in use. An oxygen conserving device may be built into the vessel to conserve the oxygen allowing you a longer period of time away from the main unit.