The Breathing gas reference article from the English Wikipedia on 24-Jul-2004
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Breathing gas

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Air is the commonest and only natural breathing gas. Other artificial gases, either purifications or blends of gases, are used in closed breathing environments such as SCUBA equipment, recompression chambers, submarines and space suits.

A safe breathing gas has three essential features:

Most breathing gases are a mixture of oxygen and one or more inert gases.

Table of contents
1 Common diving breathing gases
2 Individual component gases
3 Unwelcome components of breathing gases
4 Gas cylinder colour coding

Common diving breathing gases

Common diving breathing gases are:

Individual component gases

Oxygen

Oxygen must be present in every breathing gas. This is because it is essential to the human body's metabolic process, which sustains life. The human body cannot store oxygen for later use as it does with food. If the body is deprived of oxygen for more than a few minutes, unconsciousness results. The tissues and organs within the body (notably the heart and brain) are damaged if deprived of oxygen for much longer than four minutes.

The proportion of oxygen in a breathing gas determines the depth at which the mixture gas can safely be used:

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 due to hypoxia.

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. Above this partial pressure or longer than this exposure, the diver risks oxygen toxicity involving a seizure similar to an epileptic fit.

Filling a diving cylinder with pure oxygen costs around five times more than filling it with compressed air. As oxygen supports combustion and causes rust in diving cylinders, it should be handled with respect when gas blending.

Nitrogen

Nitrogen is an inert gas that causes nitrogen narcosis in the diver, so its use is limited to shallower dives. Nitrogen can cause decompression sickness. Air is the cheapest diving, breathing gas.

Equivalent Air Depth is a calculation often used to determine the maximum proportion of nitrogen in max for a particular depth of dive. Many divers find that the level of narcosis caused by a 30 metre / 100 feet dive, whilst breathing air, is a comfortable maximum. The partial pressure of nitrogen at this depth on air is 3.16 bar (Fraction of nitrogen x Absolute pressure = 0.79 x 4 bar). So, what fraction of nitrogen would cause the same narcosis at 60 metres? The answer is 45% nitrogen. (3.16 / 7 bar)

Helium

Helium is an inert gas that is less narcotic than nitrogen at diving pressures, so it is more suitable for deeper dives than nitrogen. Helium can still cause decompression sickness, though. It is not very suitable for dry suit inflation due to its poor thermal insulation properties — helium is a very good conductor of heat, as opposed to air which is a rather poor conductor of heat. Helium fills typically cost ten times more than an equivalent air fill.

Neon

Neon is an inert gas sometimes used in deep, commercial diving but is very expensive.

Hydrogen

Hydrogen has been used in deep diving gas mixes but is very expensive and very explosive when mixed with oxygen (such as the oxygen found in breathing gas).

Unwelcome components of breathing gases

Many gases are not suitable for use in diving breathing gases. Here is an incomplete list.

Argon

Argon is an inert gas that is more narcotic than nitrogen, so is not suitable as a diving, breathing gas. It is used for dry suit inflation because of its good thermal insulation properties. Argon is much more expensive than air.

Carbon dioxide

Carbon dioxide is produced by the metabolism of the human body and causes carbon dioxide poisoning.

Carbon monoxide

Carbon monoxide is produced by incomplete combustion. This may occur in the air being drawn into a diving compressor or in the lubricants of the compressor. It causes carbon monoxide poisoning.

Gas cylinder colour coding

In the European Union gas cylinders are colour coded. The "shoulder" is the top of the cylinder close to the pillar valve.