Scuba gas planning

Scuba gas planning is the aspect of dive planning which deals with the calculation or estimation of the amounts and mixtures of gases to be used for a planned dive profile. It usually assumes that the dive profile, including decompression, is known, but the process may be iterative, involving changes to the dive profile as a consequence of the gas requirement calculation, or changes to the gas mixtures chosen. Use of calculated reserves based on planned dive profile and estimated gas consumption rates rather than an arbitrary pressure is sometimes referred to as rock bottom gas management. The purpose of gas planning is to ensure that for all reasonably foreseeable contingencies, the divers of a team have sufficient breathing gas to safely return to a place where more breathing gas is available. In almost all cases this will be the surface.

Gas planning includes the following aspects:

Gas planning is one of the stages of scuba gas management. The other stages include:

The choice of breathing gas for scuba diving is from four main groups.

Air is the default gas for most shallow recreational diving, and in some parts of the world it may be the only gas easily available. It is freely available, consistent in quality and easily compressed. If there were no problems associated with the use of air for deeper and longer dives, there would be no reason to use anything else.

The limitations on the use of air are:

These limitations may be mitigated by the use of gases blended specifically for breathing under pressure.

In an effort to reduce the decompression problems resulting from the high partial pressures of nitrogen the diver is exposed to when breathing air at depth, oxygen may be added as a substitute for some of the nitrogen. The resulting mixture of nitrogen and oxygen is known as nitrox. The traces of argon and other atmospheric gases are considered to be unimportant.

Nitrox is a mixture of nitrogen and oxygen. Technically this can include hypoxic nitrox mixtures, where the gas fraction of oxygen is less than in air (21%), but these are not generally used. Nitrox is generally understood as air enriched by additional oxygen. Gas fraction of oxygen may range from 22% to 99%, but is more usually in the range of 25% to 40% for bottom gas (breathed during the main part of the dive), and 32 to 80% for decompression mixtures.

Helium is an inert gas which is used in breathing mixtures for diving to reduce or eliminate the narcotic effects of other gases at depth. It is a relatively expensive gas and has some undesirable side effects, and as a result is used where it significantly improves safety. Another desirable feature of helium is low density and low viscosity compared to nitrogen. These properties reduce work of breathing, which can become a limiting factor to the diver at extreme depths.

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