Scuba gas management
Scuba gas management is the aspect of scuba diving which includes the gas planning, blending, filling, analysing, marking, storage, and transportation of gas cylinders for a dive, the monitoring and switching of breathing gases during a dive, efficient and correct use of the gas, and the provision of emergency gas to another member of the dive team. The primary aim is to ensure that everyone has enough to breathe of a gas suitable for the current depth at all times, and is aware of the gas mixture in use and its effect on decompression obligations, nitrogen narcosis, and oxygen toxicity risk. Some of these functions may be delegated to others, such as the filling of cylinders, or transportation to the dive site, but others are the direct responsibility of the diver using the gas.
Management of breathing gas during the dive is a critical skill to avoid potentially fatal consequences. For the basic case of no-decompression open-water diving, which allows a free emergency ascent, this requires ensuring sufficient gas remains for a safe ascent and for the possibility of an assisted ascent, where the diver shares gas with another diver. Gas management becomes more complex when solo diving, decompression diving, penetration diving, or diving with more than one gas mixture. Other necessary knowledge includes awareness of personal and other team members' gas consumption rates under varying conditions, such as at the surface, at varying depths, for different dive task loadings and personal physical effort and mental states.
Divers need to be aware of the remaining gas available, so a submersible pressure gauge is fitted to each diving cylinder to indicate the remaining gas pressure, and the cylinder is clearly labelled to indicate the gas mixture. The amount of available gas remaining can be calculated from the cylinder pressure, the cylinder internal volume, and the planned reserve allowance. The time that a diver can dive on the available gas depends on the depth, work load, the fitness of the diver and that the gas is safe to breathe at that depth. Breathing rates can vary considerably, and estimates are largely derived from experience. Conservative estimates are generally used for planning purposes. The divers must turn the dive and start the exit and ascent while there is enough gas to surface safely. This may require the calculation of minimum acceptable pressures for various stages of a dive, known as critical pressures.
To limit the risk of equipment malfunctions that could cause a loss of breathing gas, divers maintain their breathing apparatus in good order, assemble it with care and test it before use. This does not entirely eliminate the possibility of a malfunction that could cause a loss of gas, so the requisite skills for dealing with the reasonably foreseeable malfunctions should be learned and maintained, and redundant supplies carried to allow for circumstances of unrecoverable malfunction.
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 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 most cases this will be the surface.Gas planning includes the following tasks:
- Choice of breathing gases to suit the dive,
- Choice of scuba configuration for primary breathing gas,
- Choice of scuba configuration for emergency breathing gas,
- Estimation of gas quantities required for the planned dive, including,, and decompression gases, as appropriate to the planned 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's volume must be sufficient to contain the required quantity of gas at or below its working pressure.
- Calculation of the required pressures for each of the gases in each of the cylinders to provide the required quantities.
- Specifying the of relevant gas mixtures for appropriate sectors of the planned dive profile, taking into account the estimated breathing rates of the divers who may have to use the gas in a contingency.
Rule of thumb gas planning
The formal and relatively complete procedure for scuba gas planning assumes that a dive plan is available that is sufficiently detailed that most of the variables are known, but many recreational dives are conducted on a more ad hoc basis.The majority of recreational divers do not do penetration dives or dives exceeding the no decompression limit, and can safely ascend directly to the surface at any point of a dive. Such ascents do not use a large volume of gas, and these divers are commonly taught to start the ascent at a given remaining pressure in the cylinder, regardless of the depth, size of cylinder, or breathing rate expected, just because it is easy to remember and makes the dive leader's work simpler on group dives. It may occasionally be insufficiently conservative, but is more often unnecessarily conservative, particularly on shallow dives with a large cylinder. Divers may be told to notify the dive leader at 80 or 100 bar and to return to the boat with not less than 50 bar or 700 psi or something similar remaining, but one of the reasons for having the 50 bar in reserve is to make the return to the boat safer, by allowing the diver to swim on the surface in choppy water while breathing off the regulator. This residual gas may also be well used for an extended or additional safety stop when the dive approached the no decompression limit, but it is good practice not to entirely use up the gas, as an empty cylinder is easier to contaminate during handling, and the filling operator may be required to internally inspect any cylinder which does not register a residual pressure when presented for filling, or reject it for filling until a competent person has made an internal inspection.
For deeper dives, dives with some planned decompression, or solo dives, a bailout cylinder can be carried, with sufficient gas suitable to surface safely from any point on the planned dive profile. If the bailout cylinder is reserved for use only in emergencies, it can last for many dives, as very little gas need to be used when performing the pre-dive checks on the cylinder and regulator.
The rule of thirds is another such rule of thumb. This rule generally only applies to diving in overhead environments, such as caves and wrecks, where a direct ascent to the surface is impossible and the divers must return the way they came, and no decompression stops are intended.
For divers following this rule, one third of the gas supply is used for the outward journey, one third for the return journey and one third is held in reserve in case of an emergency. The dive is turned when the first diver reaches one third of the starting pressure. However, when diving with a buddy with a higher breathing rate or a different volume of gas, it may be necessary to set one third of the buddy's gas supply as the remaining 'third'. This means that the turn point to exit is earlier, or that the diver with the lower breathing rate carries a larger volume of gas than would be required if both had the same breathing rate. The rule of thirds does not allow for higher consumption rates under stress.
Reserves are needed at the end of dives in case the diver has gone deeper or longer than planned and must remain underwater to do decompression stops before being able to ascend safely to the surface. A diver without gas cannot do the stops and risks decompression sickness. In an overhead environment, where it is not possible to ascend directly to the surface, the reserve third allows the diver to donate gas to an out-of-gas buddy, providing enough gas to let both divers exit the enclosure and ascend to the surface.
Rock bottom gas planning
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, do the calculations for gas mixtures and the appropriate quantities of each mixture are known well enough to make fairly rigorous calculations useful.Gas blending
Gas blending for scuba diving is the filling of diving cylinders with non-air breathing gas mixtures such as nitrox, trimix and heliox. Use of these gases is generally intended to improve overall safety of the planned dive, by reducing the risk of decompression sickness and/or nitrogen narcosis, and may improve ease of breathing.Filling cylinders with a mixture of gases has dangers for both the filler and the diver. During filling there is a risk of fire due to use of oxygen and a risk of explosion due to the use of high-pressure gases. The composition of the mix must be safe for the depth and duration of the planned dive. If the concentration of oxygen is too lean the diver may lose consciousness due to hypoxia and if it is too rich the diver may suffer oxygen toxicity. The concentration of inert gases, such as nitrogen and helium, are planned and checked to avoid nitrogen narcosis and decompression sickness.
Methods used include batch mixing by partial pressure or by mass fraction, and continuous blending processes. Completed blends are analysed for composition for the safety of the user. Gas blenders may be required by legislation to prove competence if filling for other persons.