Posted by seahunt on March 09, 2004 at 22:12:42:
In Reply to: Re: What are you smoking? posted by seahunt on March 09, 2004 at 19:14:50:
Put it this way....
You and Kendall both gave considerations for looking at what pony bottle might be needed.
You put in a very nice description of gas consumption calculations.
Kendal put in that those calculations would be meaningless in the real world based on that normal stress reactions in an emergency would cause gas consumption to increase dramatically.
Calculations in both cases are complicated by the changing pressure as the diver ascends. The quantity of gas is constant, yet usage changes as the diver ascends, without even considering changes in the diver's breathing rate.
As a model, if the diver ascended from 60 feet at 30 feet per minute, then you could calculate their consumption as 2 minutes at 2 atmospheres of pressure or 4 minutes on the surface. A tank should last half as long on this ascent as it would on the surface.
The reality of it for me is that if I am at 60 feet and I SUDDENLY lose my primary air, I expect to be up at least 10 feet before I take my first breath off the pony bottle. Until I am at 30 feet, I'm going to be wheeling for the surface whether I am coming up from 90 feet or 50. At 30, I'm going to want to slow dramatically. Maybe I can even make the equivelant of a Pyle Stop, but it is going to have to be based on judgement, not calculation. Then I will move much slower up to 15 feet, then 10 feet and wait there as long as I can until I think I am nearly out of air. By then I should be relaxed enough that 2 breaths of air should allow me more than 20 seconds to make the last 10 feet to the surface. That does not easily get described by air consumption calculations.
That is an ideal scenario, but not unrealistic. And if I have to come up a bit faster, oh well, I'll get there. From what I hear, that still constitutes less of a deco obligation than I have heard is common on some technical dives. Also that ascent is far slower than what I used to regularly do 15 years ago before stops were widely recognized as a good thing.
Realize, I've sucked tanks until my eyes hurt.
When I bought the tank, I calculated how long I would need a tank to last at the surface, to make it up from a dive in an emergency. It was not even based on my potential maximum dive depth, but it was a pretty good model and based on an empirical test. Personally I am great at calculations, but prefer empirical data. So as I said, I really should test it in a real ascent situation for the best answer available by my standards.
As for Kendall's point, it is almost always valid, but is very unlikely to be correct for me. Quite the contrary, my gas consumption is almost certainly going to decrease in an emergency. That sounds unlikely you say. Well, think of this. I almost always swim fairly fast. I am certainly going to want to surface slower than I swim normally. These days, unless I am shallow (and even then quite often) I usually swim at least neutrally bouyant. by the time I have ascended 20 feet, I have to be worring about dumping air, not swimming. That combined with my freediving practice and hypnoticly trained breath control means that (if I don't panic or stress) I'm almost certainly going to actually relax and my air consumption is going to go down, not up. As I and others have said in many posts, panic is the greatest danger there is to a diver. As I also said, I have repeatedly demonstrated that I am very unlikely to panic.
So pardon me if I like my air consumption calculations, empirically tested and judged with a great deal of experience.
Notice in all my posts, I said nothing about calculations, I just said how I performed my empirical test and evaluated it.
Do you see anything else I missed?
Enjoy, seahunt