Putting on my Nomex underware, Im going to challenge many of the assumptions made in calculations here.
While total lung capacity can be in the neighborhood of 6 liters, I know very few people who can reach a vital capacity of 4.8 liters and according to my measurements over the past few months with friends, fellow divers, and students my vital capacity of 4.5 liters is higher than anyone else Ive tested with my Voldyne inspiration exerciser. Most people are turning funny colors before they are able to inspire more than 3 to 3.5 liters. Typical predictive ranges run from a 58 tall 20 year old male at 2 liters to a 78 tall 20 year old male at 4 liters with it dropping considerably for females or older persons. Normal respiration is only about .5 liters per breath and Ive calculated that a upping my respiration while diving to about 1 liter per breath with 11 to 12 breaths per minute yields a SAC of about .389 CFM, which is very close to the .42 to .45 CFM I typically see diving. Most people are shocked the first time they find out how big a breath 1 to 1.5 liters really is.
If we assume that typically the Spare Air only gets filled to 2750 psi and at 99 we cant use the last 100 psi, that leaves us with only 2.65 CF (75 liters) usable at depth and when adjusted to sea level equivalent that would be .66 CF (18.75 liters). That would translate into about 18 normal diving breaths or 30 normal surface type breaths or about 1 minute 20 seconds at depth, which converts back very close to the 5 minutes 30 seconds I got off a Spare Air at the surface last time I checked.
When a diver runs out of air at depth 2 things happen to compound the problem and increase their anxiety while looking for their buddy and comprehending whats happened. The first is they are typically left with well under a normal lung fill of air, and if theyre a liter short of air in their lungs theyre caught 2.2 pounds heavier than normal with no way to get positively buoyant. Reality is that most vacation divers vary from 5 to 15 away from their buddies on dives, and with a normal vacation diver swim rate it will take 10 to 15 seconds for them to find and catch up with their buddy 15 away after about 5 seconds to comprehend the problem. Deeply exhale and than dont inhale for 20 seconds and youll see how quickly you start to feel CO2 buildup and how high your anxiety level goes. If a Spare Air only gives a diver 1 breath its enough to keep them neutrally buoyant and cut the amount of time they have to be out of air before they reach their buddy from 20 seconds or more to less than 10 seconds which will greatly reduce the tendency to panic that can be the real killer in cases like this. It also reduces the stress of having to make a split second decision as to whether it is best to get to a buddy 20 seconds away or to make an OOA emergency ascent.
One complaint from many vacation divers is the weight of all the equipment they have to carry to the boat, walk to the water with, and climb back on the boat with and a Spare Air is typically 8-10 pounds lighter than even a 13CF pony and doesnt require additional techniques for entry or exit from the water. The big complaint about Spare Air units in this thread is that theyre inadequate to make a solo ascent, but in the half dozen times Ive seen them used over the years not one person made an ascent with it, but they did easily and calmly get to their buddy to ascend with them and without the Spare Air they might have been forced to ascend without air on their own.
I wont argue that a Spare Air is any substitute for an adequately sized pony bottle for the types of diving many people here do, but were doing a disservice to the larger diving community by claiming there is no useful purpose for them and that people are better off without one than using one for what meets their requirements not the requirements of the minority of hardcore divers on this board. If our purpose here is to be elitist snobs that dont want casual recreational vacation divers to participate in the sport than by all means we shouldnt allow anyone to board a dive boat without a 40CF pony or doubles on a BP/W with an HID light. Otherwise we need to realize that there are plenty of competent divers that would find the sport losing much of its appeal if they had to lug a pony bottle around to feel comfortable (or accepted) and for the application many use a Spare Air for it does add a significant safety margin for them.
http://en.wikipedia.org/wiki/Lung_volumes
