Breathing gas
A safe SCUBA diving breathing gas has two essential features: it must contain sufficient oxygen to support the life, consciousness and work rate of the diver and it must not be toxic when being breathed at its design depth.
Types of breathing gas
Common diving breathing gases are:
- Air is a mixture of oxygen, 21%, and nitrogen, 79%, and is the most common diving gas being cheap and simple to use. As its nitrogen component causes nitrogen narcosis it has a safe depth limit of 40 metres/ 130 feet for miost divers.
- Nitrox is a mixture of oxygen, more than 21%, and nitrogen, less than 79%, and is mainly used instead of air, to reduce decompression requirements and to decrease the risk of Decompression sickness.
- Pure Oxygen is mainly used during the shallow phase at the end of a technical dive for allerated decompression stops.
- Trimix is a mixture of oxygen, nitrogen and helium and is used during the deep phase of a technical dive.
- Heliox is a mixture of oxygen and helium and is used during the deep phase of a technical dive.
- Heliair is a mixture of oxygen, nitrogen and helium and is used during the deep phase of a technical dive. It is easily blended from helium and air and so always has a 21:79 ratio of oxygen and nitrogen with the balance consisting of helium.
Oxygen
The proportion of oxygen in a breathing gas determines the depth at which that gas can be safely used:
- hypoxic mixes have lower proportion of oxygen than air, 21%, or more strictly less than 16%, oxygen and are design only to be only safely be breathed at depth. Trimix, Heliox and Heliair are typical of hypoxic mixes and are used in Technical diving as deep breathing gases.
- normoxic mixes have the same proportion of oxygen as air, 21%
- hyperoxic mixes have a higher proportion of oxygen than air 21% and are designed only to be used in shallow water. Nitrox is a typical hyperoxic breathing gas.
The minimum safe partial pressure of oxygen in a breathing gas is 0.16 bar. Below this partial pressure the diver risks unconsciousness and death.
The maximum safe partial pressure of oxygen in a breathing gas depends on exposure time, but for dives of less than 3 hours is 1.4 bar. Abaove this partial pressure or longer than this exposure, the diver risks oxygen toxicity involving a fit similar to an epileptic fit.
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