There are a number of reasons an out of air situation can happen; and, of course, the best way to deal with these situations is to take steps to prevent them. Still, every diver should know how to assist another who is out of air.
Buddy Breathing Dynamics
When buddy breathing, it is smart to assume that both divers are stressed and using more air than normal. In even the best cases, air is used twice as fast as usual, with a panicked diver requiring about 12 cubic feet of air per minute. If air supply is less than 300-500 psi, this could be more than even the best regulator can deliver.
Assume that the receiver needs to take at least two good breaths to regain his composure, and keep the purge button clear. Pay close attention to exhalation while ascending, but never force it. If needed, point to your lips as you exhale after getting your buddy’s attention, to remind him or her to exhale.
Trying to buddy breathe under emergency circumstances during an out of air situation is one of the most stressful things that can happen to any diver, and as such, it is extremely dangerous for the donor. Donors should always take an extra breath before passing the regulator, since getting it back can be difficult in cases where your buddy’s brain is oxygen starved. Expect to be kicked, hit, or fought for air; no matter how many times you have practiced buddy breathing, emergency circumstances are usually nothing like drill situations.
As a last resort, a diver who is out of air can usually breathe from his BCD, which is preferable to dumping air. The deeper you are, the more air will be available in the BC. Never breathe from a BCD if the CO2 cartridge has been discharged, and if the BC is fully inflated, release a little air prior to breathing from it since full pressure discharge into the lungs can cause injury.
If you think it is too dangerous to try to buddy breathe, see if you can start to ascend slowly and get your buddy to follow you; and, whether you’ve been buddy breathing or not, be very careful once you get to the surface. Always stop so both of you can gain your composure before moving forward. Signal for emergency assistance if help is not already on the way.
Pony bottles are an excellent alternate air source, and a safe second or an alternate air source is a second, second stage with a longer hose than usual and a left hand design that enables buddy breathing to occur without passing a single regulator back and forth. While you can learn to breathe off a second stage without a mouthpiece, this is a much better alternative; it’s something you might not think of until you need it.
Practice for all types of out of air situations, and learn to breathe from various air sources in a safe environment, such as a training pool. Get an instructor to help you learn proper techniques, and do your best to prevent problems from happening by ensuring you and your buddy keep a close eye on your tanks and gauges before and during dives. Remember, you never discover you are out of air until you have taken your last breath; being prepared and knowing how to deal with emergencies is your best chance of survival.