"Accidental" Deco

[gsk3]

Fair enough.
So it sounds like you must play around with V-Planner or HLPlanner before every dive to generate contingency ascent schedules in case of going into accidental deco (and not having a working computer?). That's great. The vast majority of recreational divers out there don't do that.

That I never said. The V-planner profile was merely to illustrate that there were reasons to prefer the RD profile to the one you offered, and that disparaging it as untested was not productive.

Because ratio deco is untested, relatively speaking. The strength of any deco algorithm is that others have used it to complete dives safely without a DCS incident.

You're missing the point. Your suggestion is also untested. The Navy table you posted implies that if you've got a deco obligation (planned or unplanned doesn't matter to your tissues), you should be doing stops <30ft in some cases.

Ratio deco appears to be a derivative method that generates ascent schedules that are similar to what other established deco algorithms might offer.
The strength of VPM-B is that people have been logging their dive profiles in a fairly sizeable database.
Are ratio deco users doing the same thing? Hopefully, they are. That would be meeting the empirical test of a "safe" deco algorithm.
I'm not really sure how you are defining "best" when referring to the various algorithms out there.

If you're doing it in your head, chances are you're not using a verifiable model. If you want a tested model, dive your computer's profile, which I suggested is best if available. Neither my random offering of a profile, nor yours, nor the pure RD profile, is tested in the same way that VPM/RGBM are. But if they produce numbers that are close, who cares? This is emergency deco; if the diver with the problem makes it to the surface only slightly bent, that's less than ideal but better than nothing.

I'm not guiding anyone up from depth. I merely made a suggestion on how to conduct an ascent if a recreational diver unintentionally crossed into deco and, for whatever reason, could not follow his computer's instructions on managing the ascent.

But you switched to talking about what you would instruct another person to do. If it's for myself, then complexity of algorithm isn't a problem--which is a fundamental criticism underlying most of your commentary. If it's for someone else, then my above if-then-else scenarios are one way to go about it.

FWIW, your world seems to be far more ordered (wet notes, "conversations" underwater) than what I've witnessed in the average recreational diver.
I'm more than a little surprised that you seem to be advocating to put the diver on higher FO2 mixes to accelerate deco at 20 fsw. How many recreational divers are trained to do that?

Yeah, I'm definitely not advocating higher FO2 mixes for untrained divers, especially panicked ones who've demonstrated poor skills already. Sorry if it looked like I was--it was just a side thought that would work in a very limited context. The average Caribbean boat wouldn't have >40% mixes anyhow.

And yes, my world is more ordered than it used to be. I've worked hard to make it so, and I try to be good about building a community of others who can acquire similar comfort. It's made diving a whole lot more fun.

I think we are in agreement that the diver should follow his computer (if available) and make the best possible use of his remaining gas supply prior to surfacing. In the couple of times that I've seen recreational divers inexplicably surface without fulfilling a deco obligation, the divers had plenty of remaining gas to do an extended shallow stop. Both times the divers admitted not being familiar with the deco mode of their computers. In both cases, the divers surfaced prematurely because other divers were exiting the water -- monkey see, monkey do.

Yeah, that's a shame, and I think we basically agree on this point. I think the point of disagreement comes down to this: If the diver appeared to not be on the verge of panic and if there were sufficient gas supplies, I'd likely slow down the ascent considerably in the top half. This would serve hopefully to calm them down, to help them maintain a controlled ascent (if they can't stop at 40ft why should they be able to stop at 20ft where buoyancy control is harder?), as well as to prevent bubbles from growing.

 

[Bubbletrubble]

You're missing the point. Your suggestion is also untested. The Navy table you posted implies that if you've got a deco obligation (planned or unplanned doesn't matter to your tissues), you should be doing stops <30ft in some cases.

Just two comments here:

• For a little bit of unintentional deco along the lines of the scenario we are discussing, what kind of a profile would RGBM recommend?
• Please download and read the US Navy Diving Manual. Kindly refer to the Air Decompression Table (Table 9-9). (I think you made a typo in your previous post and meant to specify stops deeper than 30 fsw.)

If you're doing it in your head, chances are you're not using a verifiable model. If you want a tested model, dive your computer's profile, which I suggested is best if available. Neither my random offering of a profile, nor yours, nor the pure RD profile, is tested in the same way that VPM/RGBM are. But if they produce numbers that are close, who cares? This is emergency deco; if the diver with the problem makes it to the surface only slightly bent, that's less than ideal but better than nothing.

Once again, I ask what kind of ascent profile RGBM would specify. Probably something along the lines of 30 fpm ascent rate with an extended stop in the 10-20 fsw range. With more egregious instances of exceeding NDLs, the deco ceiling is lowered progressively to a deeper depth.

Yeah, I'm definitely not advocating higher FO2 mixes for untrained divers, especially panicked ones who've demonstrated poor skills already. Sorry if it looked like I was--it was just a side thought that would work in a very limited context. The average Caribbean boat wouldn't have >40% mixes anyhow.

OK.

And yes, my world is more ordered than it used to be. I've worked hard to make it so, and I try to be good about building a community of others who can acquire similar comfort. It's made diving a whole lot more fun.

Unless you can fill all the seats on a dive boat with like-minded (DIR-adherent) divers or limit shore dive sites exclusively to DIR divers, you're going to be diving in the vicinity of other divers who haven't put as much thought into diving as you have. That complicates things.

Yeah, that's a shame, and I think we basically agree on this point. I think the point of disagreement comes down to this: If the diver appeared to not be on the verge of panic and if there were sufficient gas supplies, I'd likely slow down the ascent considerably in the top half. This would serve hopefully to calm them down, to help them maintain a controlled ascent (if they can't stop at 40ft why should they be able to stop at 20ft where buoyancy control is harder?), as well as to prevent bubbles from growing.

Honestly, I think that we agree more than we disagree on this topic. I think your suggestion to introduce "deeper" stops (at least deeper than the shallow stop I described) is one definitely worth considering.

 

[TSandM]

Well, there are several questions floating around here now, and I'm getting confused.

I know what I will do if I go into deco on a dive. I have done it, unplanned, when we have found something sufficiently interesting to make violating limits worth while. But I have a strong gestalt of the dive; I have tools for deciding if I have enough gas to go into deco, and what is likely to happen to my deco obligation during the rest of the dive. The average recreational diver does not have the training I have, or the equipment I have (double tanks) or the buddies I have.

The critical thing is gas. As I have heard said many times before, you can often survive bent, but not drowned. If you are the sort of diver who ends up with a surprise deco obligation, you probably haven't planned your gas to spend an extra 20 minutes underwater. (I use that number because Peter's Suunto gave him a 20 minute obligation on a dive where NO ONE else on the dive had any deco obligation at all . . . he didn't expect it, but had to honor the gauge, because he wanted to use it on the next dive.) In Peter's case, he would have been fine with surfacing before the computer was happy, if we hadn't had another dive planned. He didn't really think he had a dangerous decompression obligation, he just wanted to be able to use his gauge. And he had tons of gas -- we all did.

I do think it's important that divers know that going into deco is not a death sentence or a reason for panic. I think they should also know what the problems are with doing it, and why it's not a good idea. And I don't think anyone on an Al80 should ever go into deco -- there just isn't enough gas in those tanks for any long hangs.

 

[Mr Carcharodon]

I do not think anyone is recommending going into deco accidentally. Rather I think what is being suggested is that it would be a good idea to have a contingency plan on what to do if you end up there for whatever reason. Hopefully this is a rare event for everyone and if they do stray over the line the likely obligation is going to be a handful of minutes. At that level the usual 15 or 20 cubic feet of reserve gas is plenty to take care of things. It is not too hard to have a plan that is greater in scope that looking at your computer and doing what it says. And having that planning framework provides the information needed to do the gas planning. It is hard to see why this is so scary. The conventional recreational approach is to leave people ignorant and terrified such that they make even worse decisions when they approach or stray over a no decompression limit.

 

[Bubbletrubble]

I do think it's important that divers know that going into deco is not a death sentence or a reason for panic. I think they should also know what the problems are with doing it, and why it's not a good idea. And I don't think anyone on an Al80 should ever go into deco -- there just isn't enough gas in those tanks for any long hangs.

In case some people would be more convinced by numbers...
My back of the envelope calculations indicate that, assuming a 0.5 cuft/min RMV, a 20 min. hang at 15 fsw would require a little less than 600 psi of an AL80 tank.
Assuming a 0.4 cuft/min RMV, that same 20 min. hang at 15 fsw would take a little less than 500 psi of an AL80 tank.

Example calc. for 0.5 cuft/min RMV hang:
((15fsw / 33fsw)+1)*(0.5 cuft/min)*(3000psi)/(77.4 cuft)*(20 min)= 567 psi

 

[Rainer]

Lol, if divers can maintain a 0.4 SCR while draining their cylinder to obviate the unexpected deco they've found themselves owing, good on them.

Seriously, instead of worrying about contingencies and what/ifs, it really would be better to spend the time and energy drilling into new divers the need to pay better attention to their depth and remaining NDLs. While I *do* know how to backgas deco out any moderate obligation I might find myself owing, I've never ONCE had to do so. The dives I do that have mandatory deco obligations are all pre-planned.

 

[Mr Carcharodon]

Bubble,

In most cases ratio deco profiles are enveloped by VPM. It is hard to understand how an approach that is more conservative than an approach you endorse could that much of concern. Of course it could be argued that VPM has not been formally validated either. But then after a decade plus of use both VPM and RD have many more dives that the formally validated approaches. Some people will do what works, even it is not the "official" approach.

 

[Bubbletrubble]

Bubble,

In most cases ratio deco profiles are enveloped by VPM. It is hard to understand how an approach that is more conservative than an approach you endorse could that much of concern. Of course it could be argued that VPM has not been formally validated either. But then after a decade plus of use both VPM and RD have many more dives that the formally validated approaches. Some people will do what works, even it is not the "official" approach.

I realize this is the "Basic" forum, but I just wanted to point out that a diver who hasn't been formally trained in RD (or using VPM, for that matter) won't know when there isn't overlap between the two, i.e., where there is no existing empirical evidence that an RD-determined ascent schedule is "safe."

What gives me comfort in using a deco algorithm is knowing that lots of people have used that same algorithm to conduct lots of dives similar to mine without being afflicted with DCS.

My main beef with advocating the use of RD in the case of unplanned, unintentional deco is that it's almost certainly too complicated for the diver who finds himself in that situation.

On a side note, I'm not here to persuade UTD/GUE/DIR divers to stop using the RD method. What you guys do is your business. I just ask that you consider recording whatever RD profiles you do along with the outcome (DCS/no DCS/possible subclinical DCS) and share it with the rest of us.

 

[bowlofpetunias]

I do not think anyone is recommending going into deco accidentally. Rather I think what is being suggested is that it would be a good idea to have a contingency plan on what to do if you end up there for whatever reason. Hopefully this is a rare event for everyone and if they do stray over the line the likely obligation is going to be a handful of minutes. At that level the usual 15 or 20 cubic feet of reserve gas is plenty to take care of things. It is not too hard to have a plan that is greater in scope that looking at your computer and doing what it says. And having that planning framework provides the information needed to do the gas planning. It is hard to see why this is so scary. The conventional recreational approach is to leave people ignorant and terrified such that they make even worse decisions when they approach or stray over a no decompression limit.

I think you hit the nail on the head here. IMHO it is better to give people some basic information to help them make good decisions for the level they are at than try to control their actions by creating fear and paranoia! Turning Deco into the Boogyman creates an environment for a panicked response rather than a reasoned one! I certainly agree that divers need to be aware that entering deco is not something they should be doing without adequate training and planning. They also need to be aware that stepping over the Deco line is not guaranteed DCI or Death! Too many new divers do not even understand the difference between a SAFETY stop and a DECO stop!

 

[scubadobadoo]

You can tell the average new rec diver all the info you want about deco obligations in the OW course but just keep in mind that the majority of new divers can't even remember how to read dive tables a month after their course. IMHO giving them a little info isn't going to help and may even give them a false sense of security or confuse the real culprit here which is a lack of dive planning and gas management. They will either want to know it or they won't. Not every diver is this same in terms of their desire to know more or bexome a better diver and most new ones just want to go on vacation and dive. So basically this whole discussion is kinda moot unfortunately for the basic forum anyway other than for the five new divers here out of one thousand who want to know more. In the end it's better to tell them not to do it and if you want to then take a course and if you accidentally do then you forgot what you learned or disregarded it so do what your computer says to do and read the manual. But again, most new divers will just say, " uh...okay dude. Can we dive now?"