At that point you consider the dive, "thumbed" and act according to plan. In my case that would be to immediately begin an ascent to my first stop.
No, that is not strange, when something goes wrong you don't wait to see if your self-rescues procedures are going to work, you start to use them and, at the same time, you get your buddy involved. One thing going wrong rarely kills anyone, but two things ... often can. There have been more than a few fatalities where a diver was last seen, dropping out of sight down the wall, continuing to unsuccessfully do arm sweeps to find their lost second stage.
Nope, the second that I identify a failure or start to go for any of my redundancies, the dive is thumbed, and I want my buddy right next to me prepared to help in any way possible.
There is no confusion, there is no panic, there is only well oiled, well practiced, standard procedures that you go to immediately. It is unusual, though not unheard of, to incur a deco obligation and not be planning for full gas supply failure at the least opportune moment.
That's a great reason to avoid diving with your average vacation diver, especially any deeper than you feel that you could surrender all your gear and still make it the the surface managing both yourself (with no gas) and the average vacation diver ... I'd guess that to be on the order of about 30 feet.
I guess I don't see what the big deal about SMBs are either, I've actually used one perhaps a dozen times in my life, but it is (I feel) a rather simple process as long as you no longer have to manage your buddy.
When I dive I plan with tables (or the scrolling display from my computer). Let's say that I've got an ANDL at 140 feet of 10 minutes and I'm planning a 10 min dive. I'd make damn sure that I have a plan for a 140 for 10 dive, and a 150 for 15 dive. If anything goes wrong, I make sure that I am up and out and I decompress according to the 150 for 15 schedule (which I have calculated gas requirements for me and my buddy for). If I am outside of 150 for 15, all I can do is follow the 150 for 15 schedule and stop at 10 on my way up where I would empty my tank (or use my computer if it is working).
That opens up the entire question of how well trained should divers be and how much recent (and total) experience should they have if they are to dive without a real baby-sitter (as opposed to a "dive guide" who may or may not be useful in an emergency). If you use the window of experience that I use, 12 dives to 30 before 60, 12 dives to 60 before 100, 12 dives to 100 before 130, etc. for 150 and 190; and 12 dives within the previous 12 months; and one dive within the depth bracket that you are about to dive to withing the previous three months, then I'd say it's not biggie. If it is a biggie, either because of inadequate training, lack of experience, or rusty skills, I suggest being careful and not undertaking a potentially challenging dive, that's no biggie either.
If I have to make such a decision, then I am above 30 feet, even under ideal conditions (see above); otherwise, WE (as in the team I am diving with), all need to play our proper roles, not make decisions. These are roles that we have learned and practiced , and that is what makes it, "no biggie," even low vis, cold, current, what-ever.
That's a SAC rate of about .75, which is not out of line, let's say that you're planning a dive to 190 for 5 minutes. The plan is 3 minutes down, two on the bottom and then 6 minutes up to a 10 foot safety stop. If all goes right you've used 7.6 cubic feet going down, 10.2 on the bottom and 15.2 coming up. That's less than half a tank, even if you plan on another three cubic feet for a deep stop and three more for a safety stop. Your "exit" amount of gas is 15.2+3+3, round up to 22, so Bingo Air is twice that or 44 cubic feet, and you've still got 18 cubic feet in secondary reserve. So here's what the plan looks like: leave surface, arrive bottom (3 minutes, 2700 PSI), leave bottom (5 minutes, 2250 PSI), etc. If you overshoot and overstay, your bottom air consumption rises to 38 cubic feet and your deco obligation chews up another 5.2 cubic feet. To make that "problem" diving work you'd need 7.6 down, 38 on the bottom and then twice your ascent gas (22.8*2=45.6), for a total of about 92. Sounds like this dive, with contigencies, needs a bigger gas supply.
What's to "touch on" there that's unusual? First there's no EAN, too deep. Then, at 0.75 I'm sure not building up much CO2 and if I'm working hard this dive doesn't work because I'd be thumbing it on air consumption long before my 2 minutes of bottom time are up.
That is precisely the point, with crapola guidelines that appear absurd, even at first analysis, it is quite impossible to defend them or to provide intelligent guidelines.
PS: Please excuse any math errors, I ran these numbers in my head solely as an example and would not dive them without recalculating carefully.
