When you are OOA, you're not really OOA. The reg needs at least IP (135-150psi) to deliver air. Depth will affect this too, increasing it a bit. So you can be "OOA" with a couple of hundred psi in your tank. (Which is another reason why we advise divers to arrive at the surface with 300-500psi.)
I snipped most of your very helpful post, but this part was confusing to me.
My experience is that the regulator will
actively deliver air as long as the tank pressure is above ambient. The IP of the regulator matters only in terms of achieving
full output of the second stages. When tank pressure is equal to or below ambient pressure, the force of the delivered air, and hence the volume rate, is essentially zero and the regulator is no longer able to actively provide air. That's when "sucking" the tank is required.
The mechanics are clear, if I understand them correctly, that as the tank pressure drops below the IP normally set by the spring adjustment, the spring is not compressed enough to close the high pressure orifice. The high pressure orifice simply remains open and the regulator delivers air to the 2nd stages at a pressure lower than the normal IP and equal to tank pressure. It could be said that there is no longer an Intermediate Pressure, since there is no step-down at that point.
As you ascend, the pressure drops. You will most likely be able to get another breath (albeit a hard one) or two out of the reg. If you have the reg in your mouth, you'll actually get that breath. Even if there's still no air, the worst you will get a closed reg. But you may get another breath of air. And even if you get no air, the act of inhaling against a closed reg may satisfy the air hunger enough to get you to the surface.
Yes, quite a few more breaths, not necessarily hard either, if "sucking the tank dry" started two or three atmospheres deeper.... :no
As for regs, requiring 130 psi to operate, that is not true. For your entertainment, pull the dust cap OFF of your regulator and try to suck air through it. Most will allow you to breathe with a lot of resistance. Theoretically, you can breathe your tank down until you reach ambient pressure. However, breathing becomes increasingly difficult under a 130 PSI, even with a balanced reg.
I've found my regs still breathe almost as easily as normal as long as tank pressure remains above ambient. One can get easy breaths right down close to 50 psi or so in 30' of water. The regulator won't put out air in high volume, but it will be plenty for normal, relaxed breaths. You really only need 30 psi at 33' to get a little active delivery of air, if I have the Absolute Pressure concept right.
As one ascends from 33 ft, you will find one additional tank full of air as you approach the surface. That would be 3 tank fulls of air from 100 fsw. I have emptied a tank in a pool just to see what happens. I was surprised at the results and how empty I could make that tank.
Yes, surprising, but wonderfully true! I see I'm not alone in wanting to stay in the water forever!
The physics of that always fascinated me. But we seem to get conflicting opinions. I wish I can PM Vance Harlow, the author of scuba maintenance and repair on it.
Some say, that few extra breaths comes from the tank. Some say, it is the expanding air coming from the hoses.
I wish there is a safe way to test it. Take a tank down to 100 ft, breath it till it dry. Go up to 20 feet, breath it again, and then count the breath. Or simply, go to the surface and count the breaths.
Why can't someone do this and let us know. But the report from folks who have experienced this say it does happen, and you do get a few more breaths on the way up.
I'm going to further risk my credibility by revealing the following....
I've "tested" my regs with an accurate spg at varying depths at tank pressures low enough to make the regulator begin to breathe harder than normal. This was done with a working redundant air supply
in hand, of course.
It is true what they say, that the regulator will become easier to breathe as ambient pressures decrease on ascent. "Sucked dry" at 100' will get easier to breathe on the way up.
(Please be advised these tests were done under controlled conditions by trained professionals. Do not try this at home!)
Dave C
