In recreational diving, the most common type of alternate air source is the octopus. An octopus alternate air source and is attached to the regulator first stage by a 3 or 4 foot hose. Usually, the octopus hose is routed from the right side of the regulator is attached to the diver's BC on the right side. A diver who is out of air simply grabs the alternate air source and breathes from it. The out-of-air diver and the diver donating air then turn to face each other, grasp arms, and make a slow and controlled ascent to the surface.
Octopus style alternate air sources are perhaps the simplest to use. However, they have several disadvantages. The divers sharing air must stay close together and face each other. Sometimes this positioning is not ideal. It is also possible that the alternate air source has broken underwater or has a flaw that the diver did not diagnose on the surface. It is difficult to ascertain whether an air source is working perfectly by breathing from it on the surface, and most divers do not test their octopus air sources underwater. Unless a diver tests his octopus underwater on each dive, the possibility exists that an out-of-air diver may attempt to breathe from an octopus only to discover that is does not function properly.
A long hose alternate air source is attached to a 5 or 7 foot long hose. This hose is routed from the right side of the diver's regulator first stage, passed over the diver's right shoulder, then looped across his body, up his left side, and around his neck. The extra length of hose is typically secured by tucking it into the diver's waist band or by looping it under a canister-style dive light at the diver's waist. The length of the hose allows a diver donating air to swim beside or behind an out-of-air diver. For this reason, long hose alternate air sources are the standard in most types of technical diving, such as wreck or cave diving. Divers in these advanced diving environments may need to exit the dive site in single file before surfacing. Many recreational divers prefer long hose alternate air sources to octopus style alternate sources because of the freedom and mobility that this type of air source gives divers who are sharing air.
Long hose alternate air sources require a different air sharing technique from octopus style air sources. In the long hose system, the diver breathes from the long hose regulator (the one he will donate in an emergency) during the dive. In the event that his buddy is out of air, the diver removes the regulator from his mouth and passes it to the out-of-air diver. He then switches to his second air source, which is carried on a bungee necklace around his neck. This avoids the possibility of handing off a non-functional air source to an out-of-air diver. In the rare case that the bungeed air source is non-functional, the donating diver, who is typically more calm and coherent than the out-of-air diver, can then begin buddy breathing from the long hose with his out-of-air buddy.
In many emergency air sharing situations, it has been found that a panicked, out-of-air diver will often attempt to grab an air source from his buddy's mouth, even if he has been trained to reach for an octopus air source. By planning to hand off the regulator in his mouth, a diver avoids an underwater fight for an air source in an emergency. In fact, when diving in reduced visibility, technical divers are trained to seek the air source that their buddy is breathing. In low visibility, it is easier to locate a regulator in a diver's mouth than find one which is dangling from his buoyancy compensator.
Using a long hose system requires additional training for divers who have learned to dive with an octopus style alternate air source, as the air sharing procedure is slightly more complicated. Of course, both a diver and his buddy must be familiar with long hose air sharing procedures (and practice them) to dive safely together.
Integrated alternate air sources are combined with the inflator mechanism on a diver's buoyancy compensator (BC) inflation hose. On typical BCs, the inflation hose has two small buttons which allow a diver to add and vent air from the BC in order to control his buoyancy during a dive. Integrated alternate air sources combine the inflation mechanism with an air source and mouthpiece from which a diver can breathe.
This system has the arguable advantage that it reduces the number of hoses on a diver's regulator, as he need only use a hose for his primary air source, a hose for the pressure gauge, and a hose attached to the inflator/air source mechanism.
In order to share air using an integrated alternate air source, a diver hands off the regulator he is breathing (ensuring a functional regulator is passed to the out-of-air diver) and begins breathing from the air source at the end of his inflator hose. The hose attached to the donated air source must be long enough to easily share air with another diver, meaning that it should be slightly longer than a typical primary air source hose.
The integrated alternate air source system can be tricky to use, and divers who select this system must practice air-sharing ascents to become proficient. The divers must stay very close together during ascent. For this reason, integrated alternate air sources are not used in technical diving situations.
Using an integrated alternate air source requires a BC that has a the air source attached to the inflator hose. These BCs may be more expensive than average BCs, and the air source requires servicing and maintenance just as any other air source would. Servicing both an integrated air source and a separate regulator may be more expensive and complicated than servicing a regulator with an octopus or long hose alternate air source.