Which deco stop to skip?

[Lorenzoid]

As someone just learning about deco--Deco for Divers is all I've read so far--this thread caught my eye. So the answer to the OP's question is simply "no"--nobody here can think of any plausible reason why skipping a deeper stop over skipping a shallower stop might be the lesser of two evils?

You guys can turn anything into an argument.

 

[Diver0001]

No use. Mr Ph.D here feeds of his own assumptions and views. I know believe in "outcome based education"

I'm pretty sure John has a strong opinion about this and I assumed his playing the Ph.D card was probably a result of being a foul mood but one thing stands above all else for me...

He's one of the few people on Scubaboard whose posts I almost always read.

He's *that* good and I've learned *that* much from him.

R..

---------- Post added April 26th, 2013 at 03:36 PM ----------

As someone just learning about deco--Deco for Divers is all I've read so far--this thread caught my eye. So the answer to the OP's question is simply "no"--nobody here can think of any plausible reason why skipping a deeper stop over skipping a shallower stop might be the lesser of two evils?

I agree. The opposite is also true. I can't think of any reason why skipping a shallower stop as a result of spending more time deep is a good idea.

However, you've hit the nail on the head. There's been absolutely no testing done on any of this so we only have intuition/guesses and the word of those with the biggest opinions/mouths to go on.

I would advise anyone reading this thread to view what has been written here as pure speculation by all those who have been involved. It's tin-foil-hat philosophy and we can't make any more of it than that.

You guys can turn anything into an argument.

Actually, you may have missed the most important thing said, which is what Andy (DevonDiver) mentioned. He said that he really liked this kind of debate because (and I'm paraphrasing) when you're out on a limb your own thinking is under maximum scrutiny. Some of us really enjoy the mental gymnastics of the debate. It's not so much a question of who is right and who is wrong (we all know that we're guessing)... but the mere exercise of putting your thinking out there as a boxing bag for others has a value of its own.

The best debates are like this one, actually. People have been perfectly respectful and while the discussion has been vigorous, nobody has flown off the handle and some outstanding thoughts have been put up for consideration. I wish all debates were this good.

R..

 

[tracydr]

Are you sure this works with an dive computer? You can VIOLATE the computer, blow off the required deep stops and then all computers will happily go along with it and recalculate your adjusted deco plan after a violation? I find that hard to believe.

However, I DO believe the converse is probably true in most cases... You come up slower than the computer wants.... doing deep stops it did not predict and it will recalculate the correct deco. I have done that myself hundreds of times... as for the former... Not that I know of... Do you really have data to represent that violation of required deco stops won't cause "issues" with a dive computer?

Probably depended on the computer. Certainly the rec computers won't but I bet the XEO or Shearwater wouldn't stroke out in this sort of situation.
Olga, the nice lady," thinking my gps program" might start to get a little cranky, maybe even yell a little, and want you to turn around, though. But, I suspect they can recompute and manage nicely.

 

[Dive Bug Bit Me]

Rhone Man and Diver0001... so you'd basically ditch any 'bubble theory' and opt for an old-school Haldanian 'bend and mend' in the shallows?

If a more modern algorithm (dual-phase/gradient/bubble) got/gets you up quicker in general, shouldn't that remain the same (quicker) model you'd follow, but make more aggressive to get you up much faster than ideal?

I'm assuming you're using such a model to begin with...?

Back to post #21

I plan helium dives with VPM and do subscribe to bubble theory.

Lets pretend I am doing a 280ft dive to level 7 of the mine I use and that I am solo and cannot expect help. I get myself into a situation where I do not have enough gas for deco and need to choose what to do as I leave the bottom. (Plenty of soul searching about my dive planning if I survive, but that's for after I get myself out).

To get myself into this mess I have already abandoned my dive plan and also abandoned my contingency plan.

Yes, I abandon bubble theory for Haldane as well. I am beyond where my computer (30 / 80 Bulhman) or my pre cut VPM plans can help. My plan is probably to slowly (slower than 10m per minute) ascend to the shallowest depth that I dare. Hang as long as I can, then move to the next improvised stop, etc. Hopefully, I will get to the gas I normally stage (usually 120 ft, 70 ft and 20ft) then I hang a long time.

(If I lose more than two thirds of my back gas at depth, and then one of my stages is missing or has bled empty, I am probably dead).

My VR computer will flash and scream, but will recalculate a plan. I will probably accept that plan once I am back with the oodles of gas I staged.

Why abandon bubble Theory? Practically, I would rather be shallower sooner as gas lasts longer. I am closer to my staged gas if something else arises. Also I am in a position where I cannot do what my computer says, so I am calculating a deco plan on the fly. I find an aggressive Haldane profile more intuitive than an aggressive Bubble Theory based profile. So really, I'm going towards the model that I find easiest to improvise, even though I find bubble theory more appealing.

Then if I am lucky, I survive to post in the Near Misses Forum and put on a flame retardant vest.

 

[rjack321]

The opposite is also true. I can't think of any reason why skipping a shallower stop as a result of spending more time deep is a good idea.

In V-planner and other software you can "force" a profile to be mis-shapen. Not all decompression profiles can even follow a default profile since a cave might not allow you too. You might have to spend far "too long" at 80ft for instance since that's just how long that depth lasts on the exit. You can get a surprising amount of deco credit for these too deep/too long stops. Up to a point of course. All that credit allows for shorter shallow stops. Certainly much shorter than the quinessential parabolic ascent curve might suggest.

Is it a good idea? I don't know but it seems to work (in practice) and if you play around with "forced" problems in software like this you start to see that in many cases "where" you stop matters little. (As long as you are on a deco gas, I'm not talking about extending deep backgas stops). If you have a 45min obligation you can do 20mins on EAN50 and 25mins on O2 or 30mins on EAN50 and 15 on O2 or various other combinations all come out pretty close, as long as the total deco time ends up close to 45mins you aren't way outa line. Its doing 20mins total instead of 45mins where the real risk jumps up.

 

[DevonDiver]

Why abandon bubble Theory? Practically, I would rather be shallower sooner as gas lasts longer. I am closer to my staged gas if something else arises. Also I am in a position where I cannot do what my computer says, so I am calculating a deco plan on the fly. I find an aggressive Haldane profile more intuitive than an aggressive Bubble Theory based profile. So really, I'm going towards the model that I find easiest to improvise, even though I find bubble theory more appealing.

Thanks.

So, in this case, the change in solution reflects a K.I.S.S. approach to emergency management? Rather than a considered preference towards the efficiency of either model under the restrictions discussed?

I was interested to see some of the more experienced divers express a preference for a Haldanian-shaped approach. What I was wondering was whether there was some psychological inclination to revert to what might be perceived as 'tried and tested' under emergency conditions. In particular, whether there was an inherent 'trust issue' with bubble theory - that might come to light under extreme circumstances (which the parameters of this discussion certainly represent).

 

[Ste Wart]

So, in this case, the change in solution reflects a K.I.S.S. approach to emergency management? Rather than a considered preference towards the efficiency of either model under the restrictions discussed?

I was interested to see some of the more experienced divers express a preference for a Haldanian-shaped approach. What I was wondering was whether there was some psychological inclination to revert to what might be perceived as 'tried and tested' under emergency conditions. In particular, whether there was an inherent 'trust issue' with bubble theory - that might come to light under extreme circumstances (which the parameters of this discussion certainly represent).

I think Kevin Gurr put it best, when he said no matter dive he does he always carries an Air/EANX50 table. As it will always get you out of trouble...eventually.

 

[GrimSleeper]

Thanks.

So, in this case, the change in solution reflects a K.I.S.S. approach to emergency management? Rather than a considered preference towards the efficiency of either model under the restrictions discussed?

I was interested to see some of the more experienced divers express a preference for a Haldanian-shaped approach. What I was wondering was whether there was some psychological inclination to revert to what might be perceived as 'tried and tested' under emergency conditions. In particular, whether there was an inherent 'trust issue' with bubble theory - that might come to light under extreme circumstances (which the parameters of this discussion certainly represent).

All of this is entirely my own opinion, and in no way represents an invitation to get bent and then blame me...

I'm a believer in bubble-model decompression vs neo-Haldanean (especially since Buhlmann got me bent and VPM never has...), so assuming I had the gas and it was purely a question of getting out faster, I'd still want some deeper time in the ascent. There are a series of key points during an ascent, especially the off-gassing ceiling and gas switches. So I'd get above the off-gassing ceiling before I even stopped to think about the rest of the ascent (I always have it noted on my dive plans), then I'd look at 1-minute glide-and-pause stops up to somewhere close to the first switch, lengthen the stops a little as I approach, a longer stop at the switch to take advantage of the higher ppO2, then glide-and-pause again towards the next switch. Approaching the switch to O2 I'd be looking to lengthen the stops a little bit more, then it's onto the oxy and hang there for as long as I can, depending on the reason I'm bailing out. I'd still be aiming for an s-curve, just a bit more like one of those gothic 'f's.

If the change to the schedule is because of a lost gas, I've already got a plan for that, so there's no need for missed stops. If I've had a total loss of backgas, then my choices are to either share to the first switch, in which case we're just sticking to the original plan, or get to the switch as fast as possible and hope it doesn't hurt too much later. Since skipping the deeper stops intrinsically takes me out of bubble-model territory, I'd want to extend the O2 stop in the hope it compensated.

I can see why there would be a strong temptation to head towards neo-Haldanean territory in the event of a fecal matter/fan interface. It's simpler - get shallow and hang, rather than mentally shaping a curve and figuring out what you can shave where. It also has a long track record, in the form of US Navy tables, Buhlmann tables etc. On the other hand, I do believe that limiting bubble-growth earlier in the ascent makes off-gassing during the final part of the ascent more efficient, so that's (probably) what I'd be aiming to do.

Well, that or panic and rocket to the surface in tears, obviously.

 

[TSandM]

The fellow who taught my Cavern and Intro classes also teaches technical diving. He uses a pure dissolved gas decompression strategy, and one deco gas . . . 100%. He has done this for years, and taught it, and he's not a pretzeled cripple yet. So, although I suspect this is not the best decompression routine you can use, I hold it in the back of my head as a way to get as shallow as possible, as quickly as possible. As the best diving instructor I ever had put it, "You can fix bent; you can't fix drowned."

Reading the proceedings of the DAN Symposium on technical diving, I was struck by the fact that there is no evidence at all that very deep stops are useful, and I believe (unpublished communication) that there are some research studies being done that suggest that decompression strategies utilizing very deep stops are resulting in higher bubble grades. And although bubble grade is not an adequate proxy for DCS, if the deep stops are DESIGNED to control bubble formation and they do not do it, then we are wasting deco time and gas doing them.

The bottom line is that no one knows what the best shape for deco is, or even in some cases how MUCH deco is sufficient, or ideal. We know that at least several different approaches result in uninjured divers. In a last ditch, desperate attempt to get out of the water (which is how I see the OP's question) I would get shallow and spend as much time there as I could. Yes, that's "abandoning" the deco model I prefer -- but I would have had to abandon many of the other principles by which I plan such dives, to end up in such a bind.

 

[eponym]

The bottom line is that no one knows what the best shape for deco is, or even in some cases how MUCH deco is sufficient, or ideal. We know that at least several different approaches result in uninjured divers.

My avatar (the unbent goat) agrees, and accepts that every diver today is part of an ongoing statistical analysis of our theories.

In a last ditch, desperate attempt to get out of the water (which is how I see the OP's question) I would get shallow and spend as much time there as I could. Yes, that's "abandoning" the deco model I prefer -- but I would have had to abandon many of the other principles by which I plan such dives, to end up in such a bind.

Well said. Sometimes we hesitate to plan for (or even contemplate) the very worst case because we fear that would imply that our training and our discipline can fail so spectacularly as to need that ultimate "bent or dead" decision.

But circumstances change, gear fails in unexpected ways, and we do make mistakes. Kudos to the OP for beginning the discussion. The possibility of skipping planned stops was not part of my deco training, IIRC.