teknitroxdiver:
A spare air may help you a bit. But let's look at the math (always like proving stuff with numbers...).
You can get a plane-jane 3cf Spare Air for $180. (40 seconds of air) What could you do with $180?
Sling bottle: $70 for a used AL40, $100 for a used reg, $10 for hardware, and you got 40 cf of backup air. (9 minutes at 132 feet)
A spare air may be better than no air, but if you add up the cost you can get a lot more for the same.
I personally am not concerned with the comparable cost. I have BOTH a 3 cu ft Spare Air AND a 13 cu ft pony, along with an Atomic Z1 to put on the pony and a button gauge for it. But for shallow dives < 60 feet, I find it easier to use the Spare Air. It's alot less fussing around between tanks and you are carrying less weight when you are trying to get up the ladder -- a significant consideration for a middle-aged lady who has had a partial knee replacement, even though I do work out with weights and cardio. I can swim nearly the length of a 25 meter pool underwater on one breath, so I figure I could make an emergency ascent from 60 feet even without a spare air. Back in the 60's, my Dad had an out of air situation when he came to the end of his air, went to flip his J-valve and found it was already flipped. He made an emergency ascent breathing out the whole way up from 90 feet!
For sure, last time I did Punta Sur/ Devil's throat in Coz (where, for those of you who don't know, you go in the 30' coral tunnel at 80 fsw and come out at 130' fsw), I used my pony. There would have been no way I could have breathed from my buddy's alternate unless he had a 7' hose like the DIR people use, but it would have been easy to get to my pony bottle reg.
I am not a tech diver. I dive Nitrox and recreational limits, and watch my computer, staying on the conservative side of it -- so there is never a question of missing a deco obligation -- just of missing a safety stop, which is not essential -- just for extra safety.
The problem I am trying to work out is this: what depth limit should I set for myself for using the Spare Air as opposed to my pony? Of course, most of my diving trips have 2 morning dives each day (and maybe an afternoon or a night dive later -- both at the shallower depth)-- the first is usually 80-100 feet and the second 50-60 feet. It's much easier to have the same set-up for both dives, and yet my above argument would suggest I use my pony for dive 1 and my spare air for dive 2. That pony set-up is so darned inconvenient and heavy though -- I really hate it.
BTW, I am going to Little Caymen in October -- does anyone know what the usual depths are for dives there? I guess it's wall dving, so it's whatever you plan, right? Just trying to decide whether to bring my pony and extra reg. along.
) I did 3 dives a few weeks ago and my deepest depth was 22. I was toasty warm in my 7 mil 
