Ali Fikree(Offline)

A friend of mine emailed me the following email. I think it came of some rebreather list. Not sure.

Now before anything starts up, I have only copied it to this forum. It is not to create a flame war etc. Just some good/debatable reading.



I realise it's not the done thing to cross post from another list but some of you may be interested in a recent post by Richard Pyle to the Deco list.

The discussion was regarding deep stops and how different people calculate ( or more accurately guesstimate) the stops.
Richard Pyle's name has become synonomous with Deep Stops ( Pyle stops) and his recent post elaborates on his technique, it highlights the fact that decompression is not an exact science.
Original Question:
> I'd be interested to know how people calculate their "real"
> decompression time when using deep stops, and particularly a lot of
> deep stop time.
Richard Pyle's response:
Oddly enough, I generally don't do deep stops -- at least not by any particular method. Instead, what I do is fight the desperate urge to "get the hell out of deep water" upon beginning an ascent from a deep dive. In other words, I keep my initial ascent very slow (usually <10m/min). More often, though, this transit from bottom to first "required" decompression stop is not continuous, but punctuated by various stops along the way. The stop depths are not determined by any method or formula; they are determined by where I happen to see something interesting on my way back up. Or, sometimes they are determined by the need to take a moment to vent gas from the swim bladders of live fish we've collected during the dive.
The reason I'm so loose about deep stop methodology is that I measure by eyeballing, then cut with an axe: i.e., I apply a level of precision to the decompression profile commensurate with the precision of our understanding of how decompression physiology works, and what chaotic cascade of parameters and causes and effects lead to the manifestation of unpleasant symptoms following a deep dive.
A more direct answer to the question, however, is that I follow a decompression computer for the "real" part of the decompression, so whatever time I spend making my way from the bottom to the deco ceiling indicated by the computer is taken into account by the computer, and thus the remaining decompression profile is calculated accordingly.
But the numbers on the computer are just a guide. I'm very fortunate in having a computer (three, actually) based on a really good algorithm (DCAP), which provides for much deeper initial stops than most non-bubble-based decompression models (see:
www.marineexploration.org/images/DeepStopCompare.gif). I usually only catch up to computer's ceiling when I get to about 10m or so.
The only reason I proposed a "method" for deep stops in the first place is to give divers something to think about on the way up. If you just say "slow down your initial ascent", they never do. But if you tell them to stop at certain depths, then it's more likely they will, and thereby achieve more or less the same axe-cut. For those without dive computers, I wonder whether if they halved the rate of ascent from the bottom to the first "required" stop, they would still feel as much of a need to extend their shallow decompression schedules as they would if they kept their original ascent rate and inserted a couple of deep stops along the way. I suspect not. People seem less likely to trust their deco schedules that were calculated without deep stops, when additional deep stops are inserted (which betrays the fact that they "trusted" their deco schedules in the first place!)
Nevertheless, there are a couple of methods I recommend, depending on the
circumstances:
1) If you've got a dive computer, then the shallower stops are
(theoretically) adjusted anyway.
2) If your deco calculation software allows multi-level profiles, then plug the deep stops in as though they were part of a multi-level dive.
3) If you have fixed tables or single-level deco software, then you can
either:
a) add the total deep-stop time to the bottom time;
b) add half the total deep-stop time to the bottom time;
c) assume that the deep stops are doing more good than harm, and follow the original unaltered shallow stops.
I figure all of these are equally correct, since they all fall within the same "eyeballing" principle that the deco models are based on, and also fall within the same region where the axe hits the wood.
I apologize if I seem overly cynical about decompression models. It's not so much that I think they are useless (indeed, I use the information calculated by my computer to guide my decompression profile on all of my deep dives). Rather, I increasingly believe that there are other factors at play that are not taken into account by any of the deco models (including the more modern bubble-based models). And (again, increasingly), I believe some of these factors matter much more than, say, +/- 15m of depth or +/- 15 minutes of bottom time when trying to figure out how to get back to the surface without any unpleasantness (at least in the context of 90m+ trimix diving). Perhaps the most underappreciated factor of all (in my opinion) is exertion at the bottom (further confirmed during my most recent dive expedition -- a story I'll recount in full another time).

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