"Undeserved" DCS hits

[HowardE]

By your logic, Howie, I should be bent on every dive. I have done a number of aggressive dives, but have yet to encounter DCS.

I didn't say everyone who gets bent is overweight, nor did I say that overweight people get bent, nor did I say that the only ones who get bent are overweight people.

I said (and re-re-re-clarify), it's ironic that the overweight out of shape person who gets bent laments, "I don't know how this could have happened. I didn't do anything wrong"

 

[Rhone Man]

The other thing to bear in mind about these probability models is that they are based upon a narrow range of probable outcomes. Their accuracy diminishes rapidly when people do things that are unorthodox.

I recall reading about the Royal Navy doing some tests on a subject group of divers in either the 70s or 80s. Basically it involved three "bounce" dives to 180 feet, with an hour surface interval between each. According to all of the tables, they stayed within NDLs. But over 75% of the test group got bent.

Now, nobody actually dives like that. But the further you get from the predicated norm, the less accurate the probability model is likely to be.

 

[TSandM]

I do 10ft/min from 70' and up.

Well, here's an example of where "do more deco" is not a clear prescription. Marroni's studies showed, in the dives they did, that a 10 fpm ascent from 70 was one of the worst ascent profiles for bubbling. Simply doing a very slow ascent is not enough; it's pretty clear that one needs to move more rapidly deeper, and more slowly shallow (as the exponential decay curves would suggest), it's just that nobody really knows what "more rapidly", or "shallow" means.

Tortuga is entirely correct in saying that, for any individual diver, a .1% risk of DCS per dive does not translate into certainly of DCS in 1000 dives. But across a population, at some point, the likelihood SOMEONE will get DCS will start to approach certainty, given enough divers and enough dives, all done entirely within the parameters of the model.

There are certainly people who get bent from being stupid, or from making serious mistakes. Read Bernie Chowdury's book, if you want a graphic example. The Rouses died from making serious mistakes. But there are probably far MORE people who either minimally exceeded their limits, or didn't exceed them at all, using the best information they had, and ended up with symptoms. They "didn't do enough deco", but nobody could know it except in hindsight.

 

[Blackwood]

I take your point, but instead of asking what's worse, ask what's better: getting DCS or not getting DCS?

Clearly 'not getting DCS' is better, but I don't buy the premise that teaching divers to expect it will in anyway affect the likelihood of taking a clinical hit.

I've had the "inevitability of DCS" discussed in lectures yet I still strive to avoid it.

That said, I fully accept that it's a psychological point, and different people think differently.

 

[Duke Dive Medicine]

I think there's a common theme that everybody agrees on, and that's that divers have a responsibility to know the risks of DCS for the dive they're about to do, and mitigate them to the best of their ability.

If a diver fails to recognize and then mitigate the risks and ends up with DCS, we call that an expected hit. You can call it whatever you want.

The departure from logic (and overwhelming scientific evidence) comes with the belief that the risk of DCS can be ELIMINATED by controlling everything. That's impossible. DCS risk can be minimized, but not eliminated. Everybody starts somewhere, i.e. with a computer or a set of tables. John, you yourself mentioned your computer (and its unfortunate lack of a puke button). Most of those tables and computers have an adjustable conservatism factor. It's reasonable to assume that the higher the conservatism setting, the lower the risk of DCS, so adjusting the conservatism is a method of risk mitigation. Some divers go a step further and, based on their experience from past dives, alter their decompression on the fly to compensate for unexpected occurrences that may increase their risk of DCS on a particular dive: maybe heavy work, or a few minutes panic from a siltout inside a wreck. It's unscientific, but the proof is in the pudding in that they avoid DCS. The question of how to teach this method is one I'll let someone else take a crack at. At any rate, some divers make dozens or hundreds of incident-free dives like this and then commit a logical fallacy: they equate their absence of DCS with an absence of RISK of DCS. They believe that they have successfully eliminated all their DCS risks and so will never get bent. Maybe they'll even berate someone who, using the same methods they do, gets DCS because obviously the (insert insulting profanity here) shouldn't have made the dive in the first place.

Birth control is a good analogy. You can use oral contraceptives, which carry about a 1% statistical risk of failure. You can use a condom, which has about a 2% statistical risk. You can combine the two, which will theoretically lower the risk to 0.02%. I think that most people would agree that that's being pretty careful. If a woman who scrupulously uses both methods becomes pregnant, do we then call her names because, obviously, she must have screwed something up?
Sounds pretty absurd in that context.

 

[ucfdiver]

If a woman who scrupulously uses both methods becomes pregnant, do we then call her names because, obviously, she must have screwed something up?
Sounds pretty absurd in that context.

mommy?

 

[liuk3]

My comment referred to the vast majority of people who have not yet been bent or for a chamber ride




Like I said, you don't understand statistics

Let's talk dice, since you mentioned them... if you shoot craps, the odds or risk of rolling 'snake eyes' is 35:1 or 1/36. That doesn't mean that if you roll 36 times or 100 times or 1000 you will ever get snake eyes, because the outcome of each roll is independant, like tossing a coin. While the probability of the outcome of subsequent rolls changes, the odds don't. If you've made a thousand rolls without coming up double 1, there is no point at which that outcome is gauranteed

On every dive you have a risk of getting bent, or not. The risk isn't contant because it changes according to the profiles & physiological factors etc which have already been mentioned in this thread. Some dives might have a 99% chance of you getting bent, others might be 0.01 or 0.001% to use the odds you mentioned

The statistical "undeserved" rate doesn't mean that every person is equally subjected to the same risk, or that everyone is going to get bent sooner or later if they do enough dives. That's just not true, and I don't think having an instructor telling students that is constructive

The amount of 'experience' I've had is irrelevant

I'm not sure you understand statistics and probability well. Maybe I don't either. How about I bet you $1000 that if you roll the dice 1000 times, snakes eyes will show up at least once. You should have the clear advantage since each throw is independant and only has 1/36 chance of hitting snake eyes.

 

[ianr33]

If a woman who scrupulously uses both methods becomes pregnant, do we then call her names because, obviously, she must have screwed something up?

Something (or someone )

 

[Slamfire]

Well, here's an example of where "do more deco" is not a clear prescription. Marroni's studies showed, in the dives they did, that a 10 fpm ascent from 70 was one of the worst ascent profiles for bubbling. Simply doing a very slow ascent is not enough...

:shocked2: Really... and here I am thinking I'm being more conservative than Rush Limbaugh... BTW, I don't just simply do a very slow ascent. I still do the stops. Anyways, I could be pig headed and start arguing that bubbling and DCS don't necessarily correlate, but no. Seriously, thanks for bringing this up. I was not aware of this one. I assume I can find the Marroni research in Rubicon, correct? I'll study it after work hours and see what can I come up with to adjust accordingly.

 

[boulderjohn]

:shocked2: Really... and here I am thinking I'm being more conservative than Rush Limbaugh... BTW, I don't just simply do a very slow ascent. I still do the stops. Anyways, I could be pig headed and start arguing that bubbling and DCS don't necessarily correlate, but no. Seriously, thanks for bringing this up. I was not aware of this one. I assume I can find the Marroni research in Rubicon, correct? I'll study it after work hours and see what can I come up with to adjust accordingly.

You can find the same results in a 2004 study on the DAN web site--perhaps they were referring to the same data. They found that in terms of bubbling, 30 fpm had the best results, 60 fpm the next best results, and 10 fpm the worst. They also found that stops were more important than ascent rates.

You have to remember that in an ascent, you are balancing offgassing in some compartments with ongassing in others. If your ascent is too slow, you are essentially just adding to your bottom time, or, rather, turning it into a multilevel dive.