Which deco stop to skip?

[DevonDiver]

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...?

 

[Tortuga68]

I would follow the more conservative computer, if my dive plan was to follow what a wrist computer says.

That is the conventional & conservative answer, yes. But does it mean the more aggressive profile is wrong, or that you will bend anything other than your second computer if you follow it instead? No.

Besides which "more conservative" isn't always clear... What if both give you the same total TTS but with different gradients etc.

Personally I would try to shave the curve overall but tend more to omitting deep(er) stops rather than shallow - although not for the reasons the OP mentioned

 

[boulderjohn]

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...?

What evidence do you have that the Bühlmann ascent profile used by thousands of deco divers around the world every day is unsafe?

Several years ago I was engaged in a debate with people within my agency about certain aspects of the Ratio Deco ascent methodology. I needed some research, and I enlisted the personal assistance of Gene Hobbes of the Rubicon Foundation. We had a number of email exchanges regarding this research. As a consequence, I know that unless something has been done since then, there is no research indicating that any of the current models is any safer than any of the others.

 

[Ste Wart]

Just edging back to the fact that the guy is taking advanced nitrox.

Why are they having a discussion about skipping stops? You should have enough back gas on advanced nitrox dive to cover you lost gas. Unless something radically different is being taught on this course.

What evidence do you have that the Bühlmann ascent profile used by thousands of deco divers around the world every day is unsafe?

Several years ago I was engaged in a debate with people within my agency about certain aspects of the Ratio Deco ascent methodology. I needed some research, and I enlisted the personal assistance of Gene Hobbes of the Rubicon Foundation. We had a number of email exchanges regarding this research. As a consequence, I know that unless something has been done since then, there is no research indicating that any of the current models is any safer than any of the others.

I actually agree with you John. :shocked2:

 

[DevonDiver]

What evidence do you have that the Bühlmann ascent profile used by thousands of deco divers around the world every day is unsafe?

Zero... and that wasn't what I was suggesting.

It just seemed odd that divers would get into the water using one model/approach, then abandon (rather than amend) that approach when changes needed to be made.

It'd be a lot easier to put forward a specific strategy if we had a hypothetical scenario to discuss....

 

[boulderjohn]

It just seemed odd that divers would get into the water using one model/approach, then abandon (rather than amend) that approach when changes needed to be made.

What is the difference between abandoning and amending?

 

[ajduplessis]

What is the difference between abandoning and amending?

Since you asked; the difference is actually very big!

Amending: to make minor changes
Abandoning: give up completely

What I do agree with is changing VPM by skipping the deep stuff completely; it becomes Haldain/Bhulmann approach. Its going to depend on how big those changes are

 

[Nirvana]

Firstly, thank you all for your answers.

Now, to address some of the issues raised. As some of you have pointed out, there is no need to say "you should never skip any deco". I think it is pretty obvious to anyone with a minimum of scuba diving knowledge that missing decompression stops puts you at serious risks. It follows pretty clearly that contingency plans must not include omitting decompression. The original question was thought of in a scenario of some imperative need to ascend more quickly than would be desirable, such as in the case of a developing medical issue (not DCI).

On another note, I understand that establishing the best course of action for broadly varying dive profiles (different depth/bottom time/ bottom mix/ deco mixes etc) is difficult at best and this was not what I had in mind.

I was curious to know if anyone thought the "deco stop depth / compartment" correspondence theory held any water. Apparently, after reading the responses already posted, no one thinks it does.

I recommend reading Mark Powell's 'Deco for Divers' to gain a better understanding on the finer points of decompression theory and modelling.

I have read this book, and part of the reason I posted the original question was that I did not remember reading anything in there that went in line with the theory I presented.

Which one? IANTD, TDI or A.N other?

IANTD

First off, let's try and be grown ups. Anyone who answers the question: "if you have to skip deco, where should you cut from"? With the answer: "never skip deco," is being a little childish (not to mention patronising) about the question. Yes, I think we all understand the very basic fact that is is never a good idea to skip deco. But equally we all recognise that not all dives go to plan.

Thank you for your defense, and for the considerations that followed.


Finally, some of you have asked what kind of deco obligation I might be talking about, as I indicated I am taking advanced nitrox. I was not referring to actual specific dive plans I might shortly execute, it was intended more as a general discussion. I also thought it would be worth asking as I will start the normoxic trimix course after I complete the advanced nitrox.

Best regards.

 

[boulderjohn]

Since you asked; the difference is actually very big!

Amending: to make minor changes
Abandoning: give up completely

What I do agree with is changing VPM by skipping the deep stuff completely; it becomes Haldain/Bhulmann approach. Its going to depend on how big those changes are

let's keep things in context, shall we?

I asked what is the difference between abandoning a deco plan to deal with an emergency and amending it.

According to the definition you provided, abandoning it would be saying "I can't do the deco profile as originally planned, so I might as well just go to the surface." I want to go on record as saying that I do not approve of that and do not suggest it.

But that is not what Andy (DevonDiver) seemed to be saying. He seemed to me to be saying that what I suggested (shaving time off the deep stops and adding time to the shallow stops to make it more in keeping with a Bühlmann algorithm) is abandoning the plan, and he things that is wrong. Instead he said we should amend the plan. Personally, I would apply the word "amend" to what I suggested. I just wanted to know how he was using the terms.

 

[rjack321]

This probably sounds too detailed...

But what to skip depends A LOT on why you need to skip anything at all. Are you hypothermic? Did you lose backgas? Lose deco gas? Being attacked by the Tiger Shark? Dramatically overstay so your gas reserves are no longer enough to complete the called for deco? Not have enough lead on and struggling to stay down?

About half of these situations can be adjusted for in the schedule. I.e. still doing enough deco just doing it on a different gas or at a different depth than scheduled for. The other portion (e.g. Tiger sharks chomping at you, flooded drysuits, maybe not enough lead) are more critical decision points.

Doing the deeper portions of a schedule but skipping the shallow stuff is one strategy, and it may even be 'optimal' for some situations. But it can also be a huge mistake in other situations too. For example: you have a 30min obligation. You have backgas (I guess air in your case), but you only have enough EAN 50% to do 20mins at 20ft or 10mins at 70ft.

In this situation I would not burn up all my 50% at 70ft and try to do air deco for the remainder. I would not just blow through the deeper portion of the profile either. I would switch to 50% at 70ft, skip some but not all of the 60,50,40ft time. Drain the 50% at 20ft. Then remain on backgas to complete as much of the obligation as possible. Yes pushing the gradient pretty hard, but also saving gas and thus doing more time (albit on the "wrong" gas).

The soundbite that deep stops = CNS and shallow stops = cartilidge is just a soundbite. Its roabbly superficially true to some extent. But its not valid enough to base a deco schedule on.