Scuba gas planning is the aspect of dive planning and of gas management which deals with the calculation or estimation of the amounts and mixtures of gases to be used for a planned dive. It may assume 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:
Choice of breathing gases Choice of scuba configuration Estimation of gas required for the planned dive, including bottom gas, travel gas, and decompression gases, as appropriate to the profile. Estimation of gas quantities for reasonably foreseeable contingencies. Under stress it is likely that a diver will increase breathing rate and decrease swimming speed. Both of these lead to a higher gas consumption during an emergency exit or ascent. Choice of cylinders to carry the required gases. Each cylinder volume and working pressure must be sufficient to contain the required quantity of gas. Calculation of the pressures for each of the gases in each of the cylinders to provide the required quantities. Specifying the critical pressures of relevant gas mixtures for appropriate stages (waypoints) of the planned dive profile (gas matching). Gas planning is one of the stages of scuba gas management. The other stages include:
Knowledge of personal and team members' gas consumption rates under varying conditions basic consumption at the surface for variations in workload variation in consumption due to depth variation variation in consumption due to dive conditions and personal physical and mental condition Monitoring the contents of the cylinders during a dive Awareness of the critical pressures and using them to manage the dive Efficient use of the available gas during the planned dive and during an emergency Limiting the risk of equipment malfunctions that could cause a loss of breathing gas The term "rock bottom gas planning" is used for the method of gas planning based on a planned dive profile where a reasonably accurate estimate of the depths, times, and level of activity is available, so the calculations for gas mixtures and the appropriate quantities of each mixture are known well enough to make fairly rigorous calculations useful. Simpler, easier, and fairly arbitrary rules of thumb are commonly used for dives which do not require long decompression stops. These methods are often adequate for low risk dives, but relying on them for more complex dive plans can put divers at significantly greater risk if they are unaware of the limitations of each method and apply them inappropriately.
Choice of breathing gas The choice of breathing gas for scuba diving is from four main groups.
Air 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:
the effects of nitrogen narcosis at depths greater than about 30 m, but depending on the individual diver. limitations on no-decompression stop diving and decompression duration due to solution of nitrogen in the body tissues. These limitations may be mitigated by the use of gases blended specifically for breathing under pressure.
Nitrox
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 air and 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, as that is the usual method for producing it. 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 based 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. Undesirable properties of helium as a breathing gas component include highly effective heat transfer, which can chill a diver rapidly, and a tendency to leak more easily and rapidly than other gases. Helium based mixtures should not be used for dry-suit inflation. Helium is less soluble than nitrogen in body tissues, but as a consequence of its very small molecular weight of 4, compared with 28 for nitrogen, it diffuses faster as is described by Graham's law. Consequently, the tissues saturate faster with helium, but also desaturate faster, provided bubble formation can be avoided. Decompression of saturated tissues will be faster for helium, but unsaturated tissues may take longer or shorter than with nitrogen depending on the dive profile. Helium is usually mixed with oxygen and air to produce a range of effectively three component gas blends known as Trimixes. Oxygen is limited by toxicity constraints, and nitrogen is limited by acceptable narcotic effects. Helium is used to make up the rest of the mixture, and may also be used to reduce the density to reduce work of breathing.
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