stunaep:
perhaps what i was trying to say, is how does one dive past 140 feet, since my computer and the dive table only shows/calculates decompression rates for diving those parameters... i've seen television shows where someone goes down a cave for 6 hours and then takes 8 hours to come back up... how do you calculate that?
Well, I think you asked a very good question.
Dives beyond rec limits are often called "extended range" dives.
So how do people plan dives like this in practice? In no particular order, there are skills, training, deco planning, and gas management to worry about. Probably more things to, but this list can keep one occupied for a good while.
Your question mainly focuses on decompression theory, e.g. the models and algorithms used to build the programs that your dive computer runs, or the programs the generate dive tables (below). Researchers have some pretty good
guesses about how your body absorbs gasses like nitrogen and helium. Researches have a pretty good (but not perfect) idea about how long it takes your body to fill up (staturate) and how long it takes your body to vent extra gas (also called off gassing). It gets complicated because blood absorbs/offgasses prety fast. Muscle and fat are slower. Bone is a lot slower. They all have to offgas to a safe level before you go shallower. If you go too shallow too soon, the pressure on your body becomes low enough that you litterally start to fizz inside (which is bad). *shrug* There is math out there if you're so inclined to follow it - but that is really why people use computers.
All of the algorithms (buhlman, microbubble, rgbm, blah blah blah) try to do the same thing - find a compromise for one to come back up to the surface quickly but not so quickly that they'll get bent. These threads have some basic deco questions, you'll find lots more on the respective forums:
http://scubaboard.com/showthread.php?t=150860
http://thedecostop.com/forums/showthread.php?t=19974
All of these things are huge areas, beyond what I can describe in one post. Here's a small gear example - if your tank craps out at depth, what will you do? You can't just kick up to the surface - you'll probably die (or worse, perhaps, paralyzed for life) from getting bent. One option is to carry backup breathing gas - that means an extra tank, and an extra regulator to breath from that tank.
So anyway, here's a little speculation about "deco planning".
For deco schedules, there are different schools of thought.
One is cutting your own tables (there are computer programs to do this: say you want to stay at 200ft for 1 hour, the program will tell you the stops (what depth and how long to wait at that depth) which will get you back to the sufrace with a
fairly low (e.g. non-zero) chance of getting bent. You sound more interested in the mecahnics of how those depths and times are computed - in general, the program will say something like this: "Ok, 60 min at 200ft will load BLOOD to 80% saturation, BONE to 20%. DIVER can slowly ascend to 100ft, wait for 3 minutes, then BLOOD will be down to 60% saturatio, BONE to 15%. Then DIVER can ascend to 80ft, wait for 5 minutes." Etc. Etc. all the way back to the surface (be advised that I'm just writing a made-up bu115h17 example here for the sake of illustration - the acutal algortihms are way more sophisticated, but this should give you a rough idea of what is going on).
Anyway, these are basically square profiles, and you'll use a timer (multi-profile dives are similar). Hmmm... a timer indeed, actually two or three timers if you're smart. I've read about people who have had their timer crap out at depth, and had to count "1 one thousand, 2 one thousand, 3 one thousand" all the way back to the surface, which would have a
high suck factor if your talking 90 or 180 minutes to make it back topside (and god help you if you're task loaded, e.g. buddy breathing or bringing up an unconscious buddy, for example). By timer here, I mean bottom timer (e.g. watch + depth gauge built in).
Oh yes, you'll need backup tables - one for your plan, one for emergency scenarios in case you need to go deeper (perhaps to rescue a buddy? An extra 20 feet can make a big difference, you're well beyond just "winging" it when doing extended range diving). Or stay longer (and extra 10 or 20 minutes also makes a big difference in extended range diving). So you'll have tables for plan-A, plan-B(extra depth), plan-C(extra time). You'll also have backup tables for all of the above, just in case you drop your main slate or something.
Another school of thought says it is ok to take one dive computer, and carry one set of worst-case backup tables (as per above).
Yet another school of thought says it is ok to take two dive computers.
There are lots of combinations. You sound dilligent and safe, and you will learn enough to make your own choices about your comfort levels risk-wise.
If you are interested in growing your skills to do extended range diving, I can't really tell you what you should do. I can tell you what I've been doing, but you'll have to make your own decisions. With that disclaimer, here are the choices I've made so far:
2002: 5 dives, my OW cert, late in the season so things got too cold.
2003: 40 dives, took AOW + Nitrox
2004: 50 dives, started practicing with doubles (2 tanks on your back).
2005: 40 dives adv-nitrox and deco procedures. rebreather mod-1.
2006: 30 dives so far, working on rebreather mod-2 & mod-3.
I went into rebreathers because I decided I didn't like hauling doubles around. What I'm learning about rebreathers is that they're far from ready for prime time - normal scuba is a lot more manageble, and often more reliable; you have to be very dilligant not to kill yourself on a rebreather.
Hmm... it takes lots of practice, study, and training to not kill oneself when doing extended range diving. I'm a long way from being ready to do extreme dives of any kind. I'm not sure I'll ever want to, now that I think about it. That is fine by me, as there is plenty of interesting stuff to look at at the levels I'm diving now. What I'm doing now helps me get 30 minutes of bottom time on that 130ft wreck (as opposed to 10 or 15 minutes).
I would encourage you to spend some time reading the accidents/incidents writeups on scubaboard.com, thedecostop.com, rebreatherworld.com, and any other dive sites you can find. These often have Really Good take home lessons in them (I would much rather benefit from "research" that others have already done instead of repeating it myself).
So. Extended-range diving is far from a silver bullet. The trade-offs can be severe. If you put 2 hours of bottom time in, you may be looking at 3 or 4 hours (maybe more) to get back to the surface. Will your wetsuit keep you warm enough? Will you have enough gas to breath? Will your drysuit keep you warm enough if part of it rips and you become waterlogged? There are all kinds of little problem scenearios to sort through, because there are all kinds of things that can be lethal.
There is a ton of stuff to learn.
Lots of things to read.
Lots of things to study.
Lots of questions to ask.
Lots of things to practice.
Hah! So if I haven't freaked you out by this point
, I would say to look into Nitrox soon. Nitrox is good because you learn more detailed gas theory; there are pros and cons to using nitrox, you'll like it. Work up to 100 or so dives. In that time, you'll see what other divers in your area are doing and talking about. There, that is my 2 cents worth of unsolicited advice.
fwiw, I think you're off to a great start. You sound enthusiastic, cautious, and pragmatic. Those attitudes can take you far - good luck and safe diving!
John G.
