"Accidental" deco with 1-day group, what to learn?

[rjack321]

Do your homework, educate yourself regarding available algorithms, choose your risk tolerance and choose your algorithm. Then dive it without faking it out, manipulating it, or violating it. How difficult is that?

RGBM is a total black box, nobody knows all its weird repetitive dive penalties and ascent rate penalties or even what L2 means 3 days into a weeklong trip.

And its impossible to determine what the DCS risk for any PDC actually is, none of them run models based on DCS probability. Those models exist, just not for PDCs. Everything you said "sounds good" but the reality is that you are making up the data for all of that as much as @fmerkel is by using a higher EAN than actual.

 

[rjack321]

Randomly changing one parameter (like diving one gas while telling the computer you are diving another) doesn't have that same support behind it. It's not known how that will affect the underlying algorithm and math. And while a diver may be fine doing that within a certain limited range of parameters (like following another divers computer), it's just not good practice and shouldn't be encouraged.

Actually using a different EANx is really easy to model

 

[doctormike]

Actually using a different EANx is really easy to model

What do you mean? You mean that there is a body of data with large numbers of dives done with people diving computers set to the wrong mix?

I don't think so.

 

[DevonDiver]

Do you have data to support that a relatively liberal decompression algorithm, like DSAT, has a higher rate of DCS than a relatively conservative algorithm, like Suunto, Mares, or Cressi?

I wouldn't know where to find such information. If we're talking statistics of DCS in the general diving population, then we don't have any accurate and fair means of testing algorithms comparatively... because divers don't generally follow an algorithm to it's max parameters/NDL, at exact ascent speeds, at the same conservatism setting, same gasses etc etc.. We'd also need to know the total amount of dives conducted globally per algorithm, to ascertain what % of DCS occurred. Otherwise, we'd just find out that Suunto caused the most bends... but the vast majority of divers use Suunto....so...

If there were such statistics... then surely we'd all be using the same algorithm... regardless of computer brand... as it'd be proven safer.

It's my personal opinion that different algorithms best suit different 'styles' of diving. You've got weekend divers, who might dive deeper/longer profiles. You've got holiday divers who might dive short-burst aggressive multi-day, repetitive schedules. You've got recreational dive pros who routinely dive moderate profiles, but rarely get a day to desaturate totally. And so on...

Can you match algorithm principles against those diving types? Yes... I think so, assuming we have faith in those principles.

Suunto RGBM does your thinking for you. That's prudent for the vast majority of divers....those who either cannot, or do not wish to...apply informed consideration to their surfacing habits.

Having worked extensively in the dive industry, I'd suggest that divers who consider their diving habits, along with DCS pre-disposing factors.. and voluntarily apply conservatism (via a computer setting and/or through ascent stops or nitrox use) are the rare exception rather than the norm. RGBM removes the need for voluntary application of conservatism - as the algorithm is adaptive to certain trigger behaviors; repetitive dives, short surface intervals, fast ascents etc etc.

It's great for novice divers and those who do 90% of their total dives within 1-2 (vacation) weeks each year.

A less-adaptive algorithm might be better suited for divers who do understand the mechanisms of DCS and how they apply to diving patterns and pre-disposing factors. Such algorithms are less categorically restrictive, but it's the diver who (should) be taking the necessary steps to insulate themselves from undue risk.

It's good for experienced divers who educate themselves beyond the course manuals, dive regularly but not intensively. Those who make a habit of voluntarily applying practical DCS safety measures (beyond algorithm conservatism settings) in line with a refined understanding of relative risk factors.

Finally, we have the 'set the parameters yourself' algorithms. I'm thinking Buhlmann ZHL-16 w/GF. These can be awesomely effective in the right hands.... or a license to bend yourself if you're an idiot. They're open to abuse... as they'd let you do the diving you want to do...giving you the numbers you want to see. Hence, it's the algorithm of choice for most technical divers...but rarely features on dedicated recreational diving instruments.

Where does that leave us? Well.... a prudent diver doesn't get out of the water until they have cleanly and efficiently off-gassed to an extent where DCS risk is virtually (statistically) nil. Algorithm conservatism is just one component in that - a greater or lesser ingredient in a mix of many options and approaches that can get us back on the boat fit and healthy.

"Tricking" dive computers... or willingly straying into mandatory deco without effective risk mitigation... is merely a sign of having your cake and wanting to eat it. It's an imprudent approach to surfacing safely and assuredly.

This 'clever' divemaster and his 36% shenanigans is a perfect example of 'inconvenient obligation'.

He/she wants (needs...) to do the longest bottom times... the repetitive dives... the multi-day schedules.... but doesn't want to apply any other factors to off-set the inherent DCS risk associated with that. Their computer is trying to keep them safe, but they over-rule it by lying about their breathing gas.

The alternative to shorter bottom times would be to use nitrox... and do longer stops....and take more dry days...and do longer surface intervals... but no, they don't want to do that....

Calculated off-gassing becomes an 'inconvenient obligation'.... the response is to re-define DCS as some sort of hypothetical-only risk. Computer conservatism becomes the enemy; an obstruction to what they want to achieve. So they cheat the algorithm... avoid the obligation... clever people.

 

[doctormike]

the reality is that you are making up the data for all of that as much as @fmerkel is by using a higher EAN than actual.

I don't agree with this statement. Yes, all models are predictive based on research, theories or observation data. And you can criticize any of them, since they are all just models. It is not possible (yet) to actually measure decompression stress in a diver in real time. But that doesn't mean that the ascent profiles that they generate are just random guesses, no better than someone putting in erroneous inputs.

GF, VPM and RGBM all have been implemented on commercially available dive computers, with millions of dives accumulated over the years. There is no evidence that any of them, properly followed within recreational diving parameters, is associated with a significant DCS rate. The fact that RBGM is proprietary and is a "black box" to you doesn't really change that, it just means that you can't pick apart the algorithm and make your own edits.

I consider tricking a dive computer to make it less conservative to be a bad practice. My opinion.

 

[leadduck]

some details about the RGBM used in dive computers are given here: http://www.archeonet.org/sub/biblio... with Coupled Phase and Material Dynamics.pdf

The OP may have run into two issues mentioned on p.22: RGBM adds hefty penalties for short surface intervals and reverse profiles (i.e. second dive deeper than first one). The OP did both, and if he was the only one using a Suunto, he may have accumulated a lot more deco time than the other divers and the guide.
There's another penalty for multi-day diving, reducing your NDL based on the number of consecutive days of diving.
Besides these penalties, RGBM is not particularly conservative.

The reasoning behind these penalties is doubtful, see for example DAN Europe - Reverse profiles on reverse profiles.

The main effect I see from these penalties is, that divers who want to stay inside NDL are forced to do fewer and shorter dives on a multi-day dive trip. It increases safety by keeping you from diving. Which may be a good thing for some who really don't know any better, but a diver should not need a computer to tell that more frequent diving means higher overall DCS risk.

The unnecessary penalties that the OP suffered from on his dive probably did not increase his safety. But I don't like the idea of cheating on the Suunto by increasing EAN or other tricks, this may backfire in other situations. The few Eon Steel users I know set their personal level to (-2) despite all the warnings in the manual. I prefer a computer that tells me a rather non-conservative limit, because I extend shallow stops and surface intervals anyway.

 

[DevonDiver]

The unnecessary penalties that the OP suffered from on his dive probably did not increase his safety..

Weinke and a few other noteable decompression scientists strongly objected to the results of the DAN Reverse Profiles symposium.

"The discussion turned to the participants to arrive at findings and conclusions, and the discussion got heated. Several people who work with bubble models had serious reservations about a “complete retraction” of warnings against reverse dive profiles since the bubble models suggest that you might get into trouble on an improperly planned or executed reverse dive profile. Many were concerned that divers, especially inexperienced sport divers, would get the wrong message about reverse profiles and think that it was okay to do them without any special consideration.

The bubble modelers obtained a couple of key concessions. Practical diving experience showed few problems with reverse profiles, but bubble models showed there could be. Thus, they adjusted some wording to make it clear that it was only in the diving experience that there had been few problems, not that there’s a lack of evidence that reverse profiles are or could have a higher DCS risk. The sentiment prevailed also that there should be a pressure differential limit, noting that most of the safely executed reverse profiles were in 40 fsw or less between the repetitive dives. Another point of agreement was that the sport diving limit of 130 fsw should apply to any relaxation of current prohibitions on reverse profile diving,"

Undercurrent Magazine, May 2000​

The point being that some of these issues are highly contested in the upper echelons of decompression science. It's unwise, as a layman, to make sweeping and categorical statements that a given algorithm gives "unnecessary penalties" and/or did not "increase safety".

In fact, it's almost a certainty that the extended shallow decompression time conducted by the OP did increase his safety. What's debatable is whether he needed to increase his safety by that margin. That's purely an issue of where you want to draw the line on acceptable safety... as it's a grey area.

It's also pretty short-sighted to make sweeping statements when we don't know the precise parameters of the OP's dive... and prior dives. It's wrong to jump to conclusions that immediately assume a ludicrously conservative algorithm. As I mentioned before, don't make an algorithm into "the enemy"... it isn't.

There's another penalty for multi-day diving, reducing your NDL based on the number of consecutive days of diving.....The main effect I see from these penalties is, that divers who want to stay inside NDL are forced to do fewer and shorter dives on a multi-day dive trip. It increases safety by keeping you from diving.

It's important to understand the differences between dual-phase (bubble) and dissolved gas models.

Traditional dissolved gas models factor tissue compartments on and off-gassing based on half-times. Slow tissue half-times aren't that relevant to recreational no-stop divers - we just don't do the bottom time to significantly saturate the slower tissues. Hence, many dissolved gas models feature less slow tissue compartments.

Once we progress beyond considerations of gas-in, gas-out based on tissue compartment half-times slower compartments become much more relevant. Slow tissue compartments, whilst not approaching dissolved gas m-values, still play a role in the gas differential calculated to influence bubble growth, persistence and collapse. Hence, dual-phase algorithms feature more slow tissue compartments.

An algorithm like RGBM places more emphasis on multi-day diving because it attributes a role to slower tissue compartments in the critical dynamics of bubble control.

I do feel that some divers over-simplify this issue of 'conservatism'. Almost as if they don't understand the principle differences between dual-phase and dissolved gas modelling.

 

[leadduck]

Weinke and a few other noteable decompression scientists strongly objected to the results of the DAN Reverse Profiles symposium.
(...)
The point being that some of these issues are highly contested in the upper echelons of decompression science.

No.

There was a lot of discussion at the symposium, but the findings were agreed upon by the participants including Wienke and others.

These findings were:

1. Historically neither the U.S. Navy nor the commercial sector have prohibited reverse dive profiles.
2. Reverse dive profiles are being performed in recreational, scientific, commercial, and military diving.
3. The prohibition of reverse dive profiles by recreational training organizations cannot be traced to any definite diving experience that indicates an increased risk of DCS.
4. No convincing evidence was presented that reverse dive profiles within the no-decompression limits lead to a measurable increase in the risk of DCS.
CONCLUSIONS: We find no reason for the diving communities to prohibit reverse dive profiles for no-decompression dives less than 40 msw (130 fsw) and depth differentials less than 12 msw (40 fsw).

Please see the proceedings including discussion protocol here:
Proceedings of Reverse Dive Profiles Workshop.

In fact, it's almost a certainty that the extended shallow decompression time conducted by the OP did increase his safety. What's debatable is whether he needed to increase his safety by that margin. That's purely an issue of where you want to draw the line on acceptable safety... as it's a grey area.

Extending shallow decompression stops will reduce DCS risk, but not indefinitely. See for example
Effect of varying deep stop times and shallow stop times on precordial bubbles after dives to 25 msw (82 fsw).
where extending the 6m stop time from 2min to 5min didn't show any effect on the bubble score after a 2.5min stop at 15m. There seems to be a bottom line that the recreational diver can reach with a few minutes extra time, but there's no evidence that going beyond that has any positive effect.

I don't say extending shallow stops hurt, but it's far from a certainty that the extended shallow deco time conducted by the OP did anything for his DCS risk.

Slow tissue half-times aren't that relevant to recreational no-stop divers - we just don't do the bottom time to significantly saturate the slower tissues.

No.

Slow compartments may be irrelevant for single recreational dives, but repetitive multi-day recreational diving will load the slower tissues. That's the reasoning behind the multiday repetitive diving penalty in Suunto RGBM.

Hence, many dissolved gas models feature less slow tissue compartments.
(...)
Once we progress beyond considerations of gas-in, gas-out based on tissue compartment half-times slower compartments become much more relevant. Slow tissue compartments, whilst not approaching dissolved gas m-values, still play a role in the gas differential calculated to influence bubble growth, persistence and collapse. Hence, dual-phase algorithms feature more slow tissue compartments.

No.

At first, the number of compartments and their halftimes has nothing to do with the number of gas phases. The halftimes and number of compartments in ZHL16, VPM, and RGBM, are about the same. Other dual phase algorithms such as used by the Navy feature only three compartments.
Secondly, VPM and RGBM have a tendency to assign less, not more, relevance to the slow compartments; that's the reason why they create deep stops.

An algorithm like RGBM places more emphasis on multi-day diving because it attributes a role to slower tissue compartments in the critical dynamics of bubble control.

No.

The Suunto RGBM places more emphasis on multi-day diving because there's an additional penalty function that reduces NDL time based on the number of consecutive days of diving.

It's unwise, as a layman, to make sweeping and categorical statements that a given algorithm gives "unnecessary penalties" and/or did not "increase safety".
It's also pretty short-sighted to make sweeping statements when we don't know the precise parameters of the OP's dive... and prior dives. It's wrong to jump to conclusions that immediately assume a ludicrously conservative algorithm. As I mentioned before, don't make an algorithm into "the enemy"... it isn't.

A more precise description would be: these penalty functions are not justified neither by the decompression model nor by experimental findings.
Just think about the reverse dive profile: The model is not particularly conservative if you do just one 35m NDL dive. If you did a shallow one shortly before the deep dive, then there will be a little additional gas load remaining from that shallow dive, reducing the deep dive's NDL time. Every model does that. The special effect of RGBM's additional penalty function is, that it will reduce the second dive's NDL time a lot more than it should according to the decompression model alone.
From the reverse profile symposium you see that there's no evidence that such behavior has any positive effect on safety.

I wouldn't say that Suunto RGBM is conservative in general, and that's the problem I have with it. Instead of being conservative in general for every dive, its conservatism varies between dives for no good reason.

I do feel that some divers over-simplify this issue of 'conservatism'. Almost as if they don't understand the principle differences between dual-phase and dissolved gas modelling.

Please note that these issues of the RGBM in dive computers have nothing to do with dual-phase models. It's still a dissolved gas model, plus some penalty functions on top.

 

[DevonDiver]

You do understand that Wienke developed RGBM? Why would he concur with the repetitive dive issues, then write an algorithm to the contrary? LOL

You keep saying "no good reason", so you obviously know better than the leading dual phase model developers.

Where can I find your scientific papers disputing their theories?

Sarcasm aside... I suggest you do some real reading on the subject. The reduced gradient factors you're assuming to be unrelated to bubble theory.... somehow plucked from thin air by Wienke... are very much rooted in predicted physiological bubble mechanisms.

Repetitive dives, multi-day/slow tissues, ascent rate violations... are all bubble factors.

This is where only reading a brief synopsis of RGBM in 'Deco for Divers' let's you down..... go deeper into the RGBM source articles for more than a skin-deep understanding.

 

[Diver0001]

Although I agree in general, I think you're missing a few points here. There's a reason I put "Accidental" in scare quotes.

First, I was diving with a group who were all more experienced than me -- quite a bit more, in fact. Second, I was diving with an Instructor, acting as a dive guide, who when I pointed out my computer (having not ignored it, and having communicated it to him frequently and in a timely manner) *told me* to stay put.

I made several mistakes here -- one being that I didn't then tell him "no, we're terminating the dive", but it's intimidating to do that to an instructor and a group of people who are all more qualified than you. That's what I'm trying to work past -- and what I'm asking for advice on.

Hi Kalleth,

You're right. I might have been too harsh in my first post.

As you pointed out, it's hard to go against the instructor if you believe there is a power dynamic at play. I appreciate that you're asking what to do about that.

Personally I wouldn't do anything that I believe is unsafe. There is a very strong unwritten rule in diving: "anyone can call any dive at any time for any reason. No questions asked".

In this case, you were engaged in a guided dive. It is not a course and in that context if there was no pressing reason to not ascend then I would inform the guide that I was ascending. If, for some reason it were unsafe to go to the surface, then I would have ascended to a shallower depth and continued the dive at a depth at which I had enough NDL showing to do so. In the worst case scenario you can always ascend to 5m and extend your safety stop in the case that there is some reason not to ascend to the surface. There is practically no limit to how long you can extend the 5m stop without accumulating any more deco time.

The point here is that even when you're following a guide, you are the one who needs to be in control of your dive. It's a bad policy to relinquish responsibility for your safety to the guide and an even worse idea to follow instructions that you believe are unsafe. I personally don't think it's fair of divers to put the guide in the position of seeing to their safety and and think it's dumb for a guide to accept that responsibility in the context of a guided dive.

That leaves the question of the role of the guide. In my opinion, and this is what I do when I guide dives for certified divers, the guide briefs the dive and the site, sets the general parameters of the dive (max depth, max time), addresses possible safety concerns (entry/exit, lost divers etc) and if they actually go in the water with divers who wish to have a guide in the water, then I make a dive that is fully within those parameters and dive *with* the other divers, not *for* them.

R..