Sideband:
Why would you go back down deeper? What is the point of taking on MORE nitrogen that you will have to get rid of? If my thinking is right, and you didn't have a deco obligation before the ascent, you would not have one after the ascent. The exception being that if by diving back down to half your max depth you pick one up. So far I see your getting MORE nitrogen into your system and possibly picking up a deco obligation that you didn't have before. Where is the benefit? If this was an NDL dive to start and you happened to go over a bit then (according to NAUI tables) all deco is done at 15 feet anyway and going deeper makes the deco obligation worse...
You are of course free to do as you want, but would you mind explaning it a bit for me? It seems to go against the training I have had, which is admittedly minimal. I do know that it was specifically said not to go back down if a stop is missed though.
Joe
Well, a good question. I will see if I can answer. The procedure comes from a set of tables which, inheritly, are "deco-tables", although they indicate NDLs for a given depth too. That's what I am trained with, and what I prefer to dive with myself, so I follow the procedures reccomended for these tables.
Notice, that I do not reccomend to anyone to follow these procedures. Always dive according to training.....not according to what some imp writes on an internet www-site!
However this is how it's been taught to me: the idea is, that if you enter a "rapid ascent", which could mean either that you somehow miss your stops or you just ascent faster than the reccomended max speed (15m/min up to the first stop, 6m/min between stops and between last stop and surface), then you've violated the "model" behind the tables -- and to get back in to the model, you need "emergency recompression" within a short amount of time (the tables say 3 min...) Since we do not commonly dive with a recompression chamber on the RIB, going back down is the best option for increasing the preasure exerted on you -- of course, in the case where you do not detect an onset of DCS.
E.g.. if your weightbelt break off and you suddenly find yourself at the surface like a rocket, but you otherwise feel fine, you grab a weightbelt, and go down for emergency recompression within 3 min (DCI symptoms may take a while to show, remember?). If you end up at the surface with a DCS hit, you don't -- the prescribed treatment is then the usual (O2, water, aspirin and a call to DAN to navigate to the nearest ready chamber....)
Going back down to half of max depth is (in the model behind the tables in question supposed to) decrease the size of any (sub-problematic) bubbles to allow them to be elliminated, rather than to give them time to get together and form bigger, problematic bubbles, as well as to force some N2 back into solution. I am sure that DrDeco could give a much better explanation of what benefits this might have, but the above is how it's been explained to me.
Then, once the 5 min stop at half depth is done, we calculate the entire dive, including the 5 min stop, and do deco accordingly, since we -- as you observe -- on-gas during the 5 min at half depth. However the 5 min at half-depth do not represent off-gassing, rather it represents a thought similar to "we've gotta get the bubbles under control"....
You write one thing, which disturbs me a little:
"If my thinking is right, and you didn't have a deco obligation before the ascent, you would not have one after the ascent."
That's, IMO, wrong. You may not have a deco obligation, but you do have an off-gas obligation. That is, in part, done through your slow ascent, which allows you to expell N2. Later, it's done on the surface in your surface interval (with most tables/models, at least). If you do not ascent slowly, then you may very well end up with a "deco obligation" -- in the sense that you may find problematic bubbles in your system due to rapid preasure decreases.
So in the situation which we discuss here: a rapid ascent to assist another diver on a "recreational" NDL dive. If I did this, then after the rapid ascent, there may be "almost-problematic" bubbles in my system, even if I feel OK immediately when surfacing (remember, DCI doesn't necessarilly show instantaneously, but it may take a few minutes to hours before symptoms become visible). Then to prevent those bubbles from "ganging up on me to form one gigant super-bubble", I'll pop down to half of my max depth, and hang out there for a while...ongassing some, yes, but reducing bubbles in size and having my lungs and the rest of my system do their job in filtering N2 bubbles. Then, I can ascent and do deco.
So the short answer to the question of "what's the idea of going back down" is the following: On-gassing during a stop isn't a problem, as long as I can calculate what to do to off-gas appropriately. Having big bubbles in my system is a problem, so I'll do whatever I can to get the bubbles minimized -- then deal with off-gassing.
But, as I stated, if I arrive on the surface with even the slightest sign of DCS, then I'd not go back down. It's O2, water, aspirin, DAN without hessitation.
Again, that's how it's been explained to me. I am no expert in decompression, bubble-mechanics, deco-diving etc., so don't take what I say as advice on how to do deco dives or emergency recompression.
well, especially since you have only 35 dives yourself.
