THERE was a time when you were expected to share a regulator mouthpiece with your buddy if one of you ran out of air.
Thankfully that era is dead and buried, as are some of the poor souls who found themselves in such a predicament. Today every diver carries an alternate air source, which gives a buddy easy access to the air supply he carries on his back.
The most common set-up is to use an octopus rig - an extra second stage connected to a medium-pressure port of the regulator, and made easily accessible in case of emergency. This should be as easy to breathe from as the primary second stage. For ease of use, many alternate second stages have extra-long hoses, from 1m (about 20cm longer than the primary second stage) up to 2m or even 3m, as favoured by some technical diving training agencies.
Some divers like to attach an octopus rig to a small 3-litre pony cylinder, and carry it alongside their main TPR(s). Others prefer an independent hand-held 200ml unit like the Oceanic Spare Air, which will give just a few breaths of air - enough, one hopes, to get them out of trouble.
While today's "technical" divers are tending to use more and more equipment, other divers are trying to simplify their rigs. For them an alternate air source that is integrated with the BC direct-feed control is most appealing.
This set-up means carrying fewer hoses and a more streamlined configuration but, in the event of an emergency, the primary second stage must be surrendered from the mouth of the donor diver. The feed hose of the Octoplus, Air II or Auto Air is limited by the length of hose of the BC.