At shallow depths, a diver using open-circuit breathing apparatus typically only uses about a quarter of the oxygen in the air that is breathed in, which is about 4 to 5% of the inspired volume. The remaining oxygen is exhaled along with nitrogen and carbon dioxide – about 95% of the volume. As the diver goes deeper, much the same mass of oxygen is used, which represents an increasingly smaller fraction of the inhaled gas. Since only a small part of the oxygen, and virtually none of the inert gas is consumed, every exhaled breath from an open-circuit scuba set represents at least 95% wasted potentially useful gas volume, which has to be replaced from the breathing gas supply.
A rebreather retains most of the exhaled gas for re-use and does not discharge it immediately to the surroundings. The inert gas and unused oxygen is kept for reuse, and the rebreather adds gas to replace the oxygen that was consumed, and removes the carbon dioxide. Thus, the gas recirculated in the rebreather remains breathable and supports life and the diver needs only to carry a fraction of the gas that would be needed for an open-circuit system. The saving is proportional to the ambient pressure, so is greater for deeper dives, and is particularly significant when expensive mixtures containing helium are used as the inert gas diluent. The rebreather also adds gas to compensate for compression when dive depth increases, and vents gas to prevent overexpansion when depth decreases.