Alternate Air Source (Octopus)
Alternate air source actually means any air source you can breath underwater other than your primary second stage (read Regulator page for information about the second stage of the regulator). But we generally point the octopus when we say “alternate air source” in general. An octopus does what a regulator second stage does, the difference is it is a back-up.
Mounted to the regulator first stage (read Regulator page for information about the first stage of the regulator), octopus looks the very same with the regulator second stage. It is not for you, but for your buddy when needed. If your buddy runs out of air somehow, you can share your air with him/her.
Because of its importance, an octopus has a different (generally eye-catching) color. Mostly it is phosphoric yellow. It is important to attach the octopus to your BC. There are some accessories for this. Different color and the attachment is for finding it easily when needed.
In addition to these, octopus is longer than your primary second stage. This will ease your buddy’s movement while using it.
An octopus may not show high performance like the regulator second stage, but this doesn’t mean that it should be of poor quality. Some divers even use identical second stages, one acting like an octopus.
Another type of octopus is the BC integrated alternate air sources. A mouthpiece is attached near the inflator hose of the BC. Diver gives his/her primary second stage to the diver who is experiencing an out-of-air situation. Then takes this alternate air source attached to the BC. This kind of alternate air source requires a longer primary second stage.
I don’t have any unique advise for choosing an octopus. It should be quality, long enough and have a different color. Because it is only used in emergencies, you’ll carry it as a back up.
But an octopus (alternate air source) is a must in today’s dive gear.
Every diver must have an octopus.