Do you know for fact that it gets difficult to breathe when running out of air at 100'? At ambient pressure with a firefighter SCBA you get at least 4 or 5 higher resistance breaths. In the pool at 12ft it seems more like only 1 or 2 more breaths after you sense something is wrong. I am guessing that due to the increased consumption at 100' that you would run out part way through a breath with no warning and no time to develop a plan. For clarification, when I said it sounded unlikely in my previous post I was referring to the fact that I think it is unlikely that it would not be a big deal. I have always felt like I could survive the ascent with a trip to the chamber being the best case scenario. My backup plan(besides my pony bottle) had always been to breathe the air from the BC with the understanding that I would likely have a nasty lung infection. I wonder how may more breaths you could get from an empty tank during a 100' ascent?
Well I have run a tank down to where I could sense I was low on air many, many times.. hundreds of times, sometimes in shallow, sometimes in deeper water.
Some generalities: the lower performing your regulator is, the more likely it is to respond to a low tank pressure by showing some resistance in breathing effort. A very high performance balanced reg will allow you to breath down a tank to a LOW pressure without you sensing it (in general).
If you are working hard, breathing hard, SUCKING on the reg, you will get a little hint of resistance at the very end of an inhalation cycle. This is NOT good practice, but sometimes i don't watch my air that carefully on a rec dive and I know I am low on air, instead of looking at the gage, I can sometimes take a super hard, fast, full inhalation and if I feel a little slow down in air delivery at the END of the inhalation, I am outta there.. might not even look at the gage. Generally will put a puff of air in the BC, and just start my ascent.
Now if you compare this type of diving to a diver who is warm and is doing macro photography, resting, doing no activity and trying to breath very slowly and gently in order to avoid scaring his subject... well this diver really can suck his tank to almost zero without knowing it, because he is just sipping it.
I do not dive hardly ever without a pony bottle, so running low on air is a non event for me, for a solo, no deco stop.....I hardly ever use the pony to ascend.
Another comment: If you are ascending on a very low tank and you have no redundancy, the most important thing to do is relax. Don't use your thigh muscles, it is better to be SLIGHTLY bouyant and just gently float up in a very controlled manner, venting air from the BC as needed to maintain complete control. of course you want to breath very slowly and conserve your air. When it gets hard to draw air on ascent, it can be scary and FEEL bad and when you get in this mind set, your metabolism and adrenaline will make you burn more oxygen, so it is REALLY important to try to keep calm.
One thing which can help you, is to press the purge button rather than simply suck on the reg. Pressing the purge,, will deliver a slow and steady stream of air which is pschologically much easier to "accept" than to be sucking super hard and getting just a slow stream of air from your reg.
If you don't beleive me try this... try to breath through two straws while sitting in a chair.. for the first experiment, try to inhale really hard and fast... it makes you feel bad.... then rest a little and do the same thing but concentrate on inhaling super slowly and gently... you will be able to breath through one or two straws for a lot longer in this manner.
