Scuba gas planning

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,, 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 of the planned dive profile.

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

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, 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, and 32 to 80% for decompression mixtures.

Helium based mixtures

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.

Oxygen

Pure oxygen completely eliminates the decompression problem, but is toxic at high partial pressures, which limits its use in diving to shallow depths and as a decompression gas.
100% oxygen is also used to replenish oxygen used by the diver in closed circuit rebreathers, to maintain the set point — the partial pressure of oxygen in the loop that the electronics or diver maintains during the dive. In this case the actual breathing mixture varies with the depth, and is made up of a diluent blend mixed with oxygen. The diluent is usually a gas blend that can be used for bailout if necessary. Relatively small amounts of diluent are used in a rebreather, as the inert components are neither metabolised nor exhausted to the environment while the diver remains at depth, but are rebreathed repetitively, only being lost during ascent, when the gas expands in inverse proportion to the pressure, and must be vented to maintain the correct volume in the loop.

Choosing a suitable breathing gas mixture

The composition of a breathing gas mixture will depend on its intended use. The mix must be chosen to provide a safe partial pressure of oxygen at the working depth. Most dives will use the same mixture for the whole dive, so the composition will be selected to be breathable at all planned depths. There may be decompression considerations. The amount of inert gas that will dissolve in the tissues depends on the partial pressure of the gas its solubility and the time it is breathed at pressure, so the gas may be enriched with oxygen to reduce decompression requirements. The gas must also have a breathable density at the maximum depth intended for its use. A recommended value for maximum density is 6 grams per litre, as higher densities reduce the maximum ventilation rate sufficiently to induce hypercapnia.

Calculating the composition

states:
On short duration dives the P_O₂ can be raised to 1.2 to 1.6 bar. This reduces the P_N₂ and/or P_He, and will shorten the required decompression for a given profile.
Breathing air deeper than has a significant narcotic effect on the diver. As helium has no narcotic effect, this can be avoided by adding helium to the mixture so that the partial pressure of narcotic gases remains below a debilitating level. This varies depending on the diver, and there is significant cost in helium mixtures, but the increased safety and efficiency of work resulting from helium use can be worth the cost.
The other disadvantage of helium based mixtures is the increased cooling of the diver. Dry suits should not be inflated with helium-rich mixtures.
Apart from helium, and probably neon, all gases that can be breathed have a narcotic effect which increases with raised partial pressure, with oxygen suspected to have a narcotic effect comparable to that of nitrogen, though the evidence is inconclusive.
Example: Choose a gas mixture suitable for a bounce dive to 50 metres, where P_O₂ must be limited to 1.4 bar and equivalent narcotic depth to 30 metres:
These are optimum values for minimizing decompression and helium cost. A lower fraction of oxygen would be acceptable, but would be a disadvantage for decompression, and a higher fraction of helium would be acceptable but cost more.
The gas can be checked for density at maximum depth as this can have a significant effect on the work of breathing. An excessive work of breathing will reduce the diver's reserve capacity to deal with a possible emergency if physical exertion is required. A preferred maximum gas density of 5.2 g/L and a maximum gas density of 6.2 g/L are recommended by Anthony and Mitchell.
The calculation is similar to calculation of mass of gas in the cylinders.