Perhaps an anecdote can help to underscore this. I was showing a visitor to the area around our local wreck the HMCS Yukon. a 366-foot long destroyer escort lying on its port side in about ~95 feet of water (+/- depending on tide and where you are along the bottom). The diver was from the Pacific Northwest, was accustomed to "real" cold water diving (his words) and came armed with all the gear you'd expect to see a reasonably experienced cold water diver in (dry suit, hood, gloves, etc).
We descended along the mooring line nearest the forward guns and got out in front of the bow with terrific visibility - I wanted to show him a front on look at the wreck. We are at ~90 feet, about 8 minutes into the dive (approaching the halfway/turn point as we'd planned 20 minutes to the bow and back to the forward guns). I turn to signal to him to look at the wreck and I see him pull off his reg 3 times and not exhale. He'd managed to suck his tank dry in 8 minutes. Not low on air - out of air. I deployed the long hose on my bail out (I am closed circuit, so he cannot receive directly the gas I am breathing) an AL40 with EAN32 with a standard stage kit. He didn't fight me in receiving the regulator and I pushed him quite briskly to the nearest upline (on the bow) and up to the surface. I maintained our ascent speed but opted out of the safety stop as he was plowing through gas and I wanted out of the water.
We arrived at the surface and I had to orally inflate his wing - his tank was THAT empty. The swim to the upline and the ascent had taken 5 minutes. He informed me that he was unaware that he'd emptied his tank, admitted to having been "freaked out" under water in the moments leading up to my giving him my bailout tank.
So in the 8 minutes we'd been in the water, the ~1-2 minutes it took us to get the donation sorted and the 5 minutes to ascend (15 minutes total dive time to a max of 95 feet, call it an average of 60 feet) he'd sucked his AL80 and my AL40 dry. Keep in mind that on the ascent I was still on my loop and not sharing his gas, so he had access to all 40ft^3.
120ft^3 in 15 minutes from an "experienced" cold water diver who had a panicky episode.
Still think 500psi is enough to get you home safe?
Well I'm sure that if I took an LP108 to 20ft and paid absolutely no attention to my air and stayed down way too long, I may not have enough air to surface.
You guys are missing the point. No matter what you do, you can't plan for stupid. My post was in relation to a statement that if you increase your tank size so your bottom time matches your buddy, it may not be a good thing because your buddy has a smaller tank.
And I believe that in an emergency out of air ascent, you wouldn't stick to the 30fpm rule. And again, if you are at 100ft with 500 lbs of air, that's just stupid. Even if one person goes out of air, the other should have plenty to make a direct ascent for both divers.
As far as the anecdote, shouldn't your buddy have been prepared as if a solo diver considering that you would not be much help in an OOA situation? Where was his pony bottle?
We are talking about the average situation here, not the extreme worst. First, you should be paying close attention to your air. Second, you shouldn't be at depth with only 500 lbs of air. And third, in such a situation, 500 lbs is more then enough. You don't have to get all the way to the surface on it, just close enough. If I were at 100ft and had no air, trust me, my weights would be off and I would get back to the surface. I would much rather spend time in a chamber then a coffin. The closer you get to the surface on that 500 lbs of air before ditching your weights may just make a little difference.
---------- Post added July 3rd, 2014 at 04:03 PM ----------
You said, "It won't get you a ton of extra bottom time though."
I think I just clarified that after your explanation of how to save air and get more bottom time that way, your statement that "it won't get you a ton" = "it won't get you barely any at all." My comments went on further to point out that if it is getting you a lot more bottom time, then you should look at other reasons for the problem, and it suggests those reasons.
Is there something wrong with what I said?
Nothing other then I never said that you would get significant time. But a person that is constantly inflating their BC might get an extra minute or two if they manually inflated. And the poster said that they had buoyancy issues.
And I never said it was a great way to conserve, but it is a way.
If anyone has better suggestions on how to conserve air that don't include things that take time and practice such as be more efficient or control your buoyancy better, help the man out. I had already suggested diving a little shallower then his buddy.