Commercial diving/Emergency gas supply (scuba)

An emergency scuba gas supply is a supply of breathing gas which a scuba diver can rely on to be available in case of failure of the main breathing gas supply, and which is both suitable for the depth and sufficient to get the diver to the surface or another place where sufficient suitable breathing gas is available. A buddy diver who is not immediately and consistently available throughout the dive to supply breathing gas in an emergency, is not an emergency gas supply. The diver must be competent to operate the emergency gas supply under stressful circumstances.

Indications for carrying an emergency gas supply
Provision of an emergency gas supply should be informed by the authorised Code of Practice,(for example, the Code of Practice for Scientific Diving), the company Operations Manual and the HIRA for the operation, and must take into account the actual water conditions expected during the dive.
 * If the dive is shallow, the risk of entrapment is negligible, and the risk incurred by allowing a controlled emergency swimming ascent (CESA) is acceptable, an emergency gas supply may not be necessary.
 * If the visibility and other environmental conditions are good enough that an in-water standby diver can be reliably available at all times during the dive, this option may be used. The standby diver should be able to supply sufficient gas for a safe ascent for both divers at all times during the planned dive, with an allowance for reasonably foreseeable contingencies. If the working diver cannot be reliably available to provide emergency breathing gas to the stand-by diver throughout the dive, the standby diver must carry an independent emergency gas supply. The delivery system for gas sharing should be by a second demand valve wherever reasonably practicable. Sharing a single DV is acceptable for training purposes, but should generally not be relied on for working dives.
 * (list any other circumstances where an EGS is not mandated)
 * In other circumstances each diver must carry their own emergency gas supply, which must be provided with a reliable delivery system, and an indication of available gas quantity.

Independent emergency gas supply configurations
The choice of configuration will be dependent on assessed risk and company policy, and to some extent on equipment available and operational constraints. It is preferable that the diver is trained in the use of all likely options, and must be competent to use the chosen option. It is not necessary for the training school to train with all possible configurations, but the diver should practice with each configuration in benign conditions and be assessed as competent by an appointed company assessor in the use of any specific configuration before it is used on a working dive.
 * A pony cylinder, independent twins or manifolded twins with an isolation valve which can be reliably reached and operated by the diver are generally acceptable options.
 * A manifolded twin with a single outlet and independently isolatable cylinders may be acceptable for some applications, taking into account that there is a single point for critical failure in the single regulator first stage.
 * A "Spare Air" (low volume cylinder with integral DV) unit may be acceptable for applications where the gas supply is sufficient for any reasonably foreseeable contingency.
 * The delivery system must be compatible with the primary breathing gas supply.

Pony cylinder
A pony cylinder, also known as a bailout cylinder is versatile and can be the most economical option. The same cylinder with the same fill can be used for several dives, as it is not used except in an emergency. It must be tested before use, which uses a small amount of gas, but then can be closed during the dive to prevent free-flow and other gas loss. The second stage must be easy to reach and deploy, and the cylinder valve must be easily reachable to open. Mounting depends on the size of the cylinder and preference of the diver. Sling mount at the left is convenient for most purposes, but is slightly more cumbersome than parallel strapping to the back gas cylinder. Sling mount puts the valve in reach of both hands, and the cylinder can be detached and handed off to another diver in seconds. This makes the arrangement particularly suitable for in-water standby diver use. Parallel mounted cylinders can be mounted with the valve down if the hoses are suitable. Sling mount pony cylinders tend to range in size from about 2 litres to 7 litres water capacity, though it is possible to carry larger cylinders if necessary. Parallel mount cylinders strapped to the primary are in the same size range, and narrower cylinders are more convenient. 3 to 5 litres is enough for most commercial scuba applications. 2 litres is marginal at deeper depths and more than 5.5 litres is not necessary. However 7 litre steel cylinders are no bigger than 5.5 litre aluminium, weigh almost the same, may be easier to find, and can be used when not completely full (7 litres at 150 bar provides the same volume of gas as 5.5 litres at 190 bar).

Independent twins
Carrying both cylinders on a twin-cylinder backplate can also be convenient, as it puts them out of the way during work. It is not as easy to operate the valve as for sling or side-mount cylinders, and some divers cannot reliably reach the cylinder valve if it is mounted with the vale to the top. The valve can be left open during the dive, but this makes the system vulnerable to loss of gas through free-flows, and the DV should be stored where the diver will notice gas loss, and it is easily reached by the diver and the buddy. Depending on the backplate arrangement, it may be possible to carry cylinders of different sizes in this configuration. This arrangement can also be used to supply primary gas from one cylinder (usually the right side) and the left cylinder is only used in an emergency. Larger cylinders can be carried this way, depending on the strength of the diver, and the bailout cylinder may also be used for several dives with this setup. The bailout cylinder is securely attached to the harness and can not be handed off or passed up to the boat separately, but may be easier to manage than a sling mount during shore entries and exits.

Manifolded twins
Two configurations are useful for providing emergency gas:

Isolation manifold
This arrangement uses a DV on each cylinder valve outlet and allows the diver to breathe off both cylinders from either DV, or to isolate the cylinders, when each DV will be supplied from one cylinder. It is a very versatile arrangement, and has been optimised for high risk technical diving, but it is bulky and heavy and cylinders cannot be switched on the manifold while there is pressure in them. The isolation valve is behind the back of the neck and some divers have considerable difficulty reaching it and the cylinder valves, particularly if wearing a bulky diving suit.

Centre outlet manifold
The original style of manifold connects to the outlet of two cylinders, which can be opened independently or together, to supply a single first stage regulator. This arrangement is vulnerable to regulator failure of any kind, but it does provide a form of reserve which does not rely on vision, as one cylinder can be left closed at the start of the dive, and only opened when the pressure in the opened cylinder becomes too low to comfortably breathe, at which stage there is still 50% of the original gas available. This can be a useful arrangement in very low visibility and darkness where the SPG is not legible. The system can be extended a further step, by closing the valve after allowing the cylinders to balance, thus keeping 25% in reserve for the ascent and using the 3rd 25% to finish the work.

Spare air
These are very small cylinders with integral regulator which are generally carried in a pouch on the front of the harness. They are better than nothing, and very compact, but should not be considered for depths much more than appropriate for a free ascent, as they only supply a few breaths at depth.