What gas is usually compressed in enriched air nitrox mixtures to increase oxygen levels?

Enriched air nitrox mixtures are designed to increase the proportion of oxygen in a breathing gas while reducing the amount of nitrogen compared to regular air, which is primarily composed of about 21% oxygen and 79% nitrogen. The process of creating these mixtures involves adding oxygen to a base gas, typically air or pure nitrogen, to achieve a desired oxygen concentration.

Therefore, when considering which gas is compressed in enriched air nitrox mixtures to increase the oxygen levels, oxygen itself is the gas that is added to the mixture. This addition raises the overall oxygen content, making nitrox mixtures beneficial for divers as they allow for longer bottom times at certain depths and reduce the risk of nitrogen-related issues, such as decompression sickness.

Other gases, such as helium and argon, do not contribute to increasing oxygen levels in nitrox mixtures. Helium is sometimes used in specific diving scenarios, such as in trimix (a combination of oxygen, nitrogen, and helium), to reduce narcosis but is not used to create enriched air nitrox. Argon, while an inert gas, is not relevant in the context of increasing oxygen levels in nitrox.