"Undeserved" DCS hits

[Doppler]

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.

John: something else we have to consider is that during our ascent -- at all stages of it once we bridge the {theoretical} off-gassing ceiling -- we are also setting into motion the mechanics that drive gas out of solution and into a free phase state... or at least, that's what the maths seems to be telling us.

Of course, who knows. The decompression dog and pony show is anything but stable... or predictable.

Be well.

 

[Tortuga68]

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

Agreed, and I don't know if the instructor that coined the phrase you mentioned meant it literally, or was tryng to make another point, or perhaps wanted to avoid the 'blame game' and ensure prompt diagnosis and treatment of potential DCS cases

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.

Only if you bet me $1000 per roll

A better way to think about it would be for you to bet $1000 that the next roll out of a series would come up snake eyes. Would you place that bet on the first roll, or the 36th, or the 100th or the 999th? Would the odds be better on the 1000th roll than on the 1st, or would they be the same?

In order for it to be true that every diver will get bent sooner or later, there must come a point where the next dive will be the one that bends them. Otherwise you can always do 'just one more dive' without getting bent

In one way, this is just numbers and doesn't have much to do with the actual discussion - but there are people that have done a hell of a lot of dives without being bent, so saying they will eventually get bent is unprovable at best and unsupported at worst

 

[liuk3]

Only if you bet me $1000 per roll

A better way to think about it would be for you to bet $1000 that the next roll out of a series would come up snake eyes. Would you place that bet on the first roll, or the 36th, or the 100th or the 999th? Would the odds be better on the 1000th roll than on the 1st, or would they be the same?

In order for it to be true that every diver will get bent sooner or later, there must come a point where the next dive will be the one that bends them. Otherwise you can always do 'just one more dive' without getting bent

In one way, this is just numbers and doesn't have much to do with the actual discussion - but there are people that have done a hell of a lot of dives without being bent, so saying they will eventually get bent is unprovable at best and unsupported at worst

Your unwillingness to take the bet supports the "dive long enough, you may get bent" argument.

 

[Tortuga68]

Maybe, but the argument was 'dive long enough, you WILL get bent'

 

[jtivat]

Let us say, that we have two identical divers making two identical dives with the same equipment. Diver 1 uses 2500psi of gas, while Diver 2 uses 1500psi of gas. So, Diver 1 breathed more gas, specifically more molecules of inert gas? So, would it not follow that Diver one had more molecules of inert gas in his/her body to get rid of? Duh????


Cheers

So then if a breath my deco gas faster it will offset this right???

 

[karstdvr]

Your unwillingness to take the bet supports the "dive long enough, you may get bent" argument.

This may be fatalisitic,but I have to give this some validity. This thread has gone on for some time, with a lot of responses,but how many posters on this thread have gotten bent,and from an "unearned hit"? I am not bashful to admit that a couple months ago I took a hit that exceeded what most cave divers blow off as a little skin bends. I developed subcutaneous emphysema along with large region of skin bends. The dive involved staged decompression,but nothing any different than I have done many times before. So the comment,you do this long enough you will get bent probably has some validity,just took me 1400 cave dives before I got the wake up call.

 

[HowardE]

So then if a breath my deco gas faster it will offset this right???

It doesn't say that - does it? But maybe you should try it out eh?

His assertion is that someone with double the breathing rate may need more time to offgas. Not that breathing more gas speeds up the gas transfer rate.

I think his point is...

That even a marginal difference in dive profile at great depths, or a differential in gas consumption as great depth could cause you to basically exceed your "plan" or "table" or "computer" and therefore... following your computer, plan, table, etc might not be enough to offgas completely. Thus... if this were the case, and you complained of "doing everything right, and there was no reason why I got bent" - you'd just be another guy complaining of an "undeserved hit", because you did "everything right"...

Some people think that following their computer is a roadmap to safe diving (I did everything right), but realistically, we have to consider the factors at hand, and make adjustments accordingly, including the possibility of hanging longer than our computers tell us to, or longer than our dive table or what have you - based on factors that don't necessarily go into the dive plan.

 

[Chatterton]

So then if a breath my deco gas faster it will offset this right???

Actually, yes.

If you use more deco gas because your respiratory and cardiac rates are elevated, that will increase the elimination of inert gas acquired during the bottom phase. Obviously, intentional hyperventilation is different, and bad, for several reasons involving body pH, PCO2, and WOB, maybe even O2.

Pre-WWII, USN hardhat divers used to do jumping jacks on the dive stage during decompression to facilitate off gassing. Read some of Edward Ellsberg's works on diving in the 1920's. However, when it came to working this into tables it was too difficult to calculate. How much work is work? It resists quantitative analysis, which leads us back to the beginning of this thread.

This is not new science? It is basic decompression physiology, and if you don't understand it, you probably should not be conducting deco dives? You could earn yourself an unearned hit.

Cheers

JC

 

[boulderjohn]

This is not new science? It is basic decompression physiology, and if you don't understand it, you probably should not be conducting deco dives? You could earn yourself an unearned hit.

Michael Powell (Dr. Decompression) does not understand it. When I asked about this a number of years ago he said flat out that breathing rate does not affect decompression.

 

[The Chairman]

I think it was Bob who once opined, "Decompression Science is like measuring with a micrometer, marking with chalk and cutting with an ax." There are a lot of factors that we don't understand completely, if at all. Because of this, most of the algorithms include a sizable "fudge factor" to keep us safe.

The result? Very, very few DCS hits for the amount of diving we do. Of course, the aside to this is that a DCS hit is so rare that we assume that the guy getting it screwed up royally. While this is a possible scenario, there are enough people citing hits where the tables were followed that it is good to rethink our paradigms. Being smug about those "others" getting bent will disappear pretty quickly if it were to happen to you. It's important not to let bravado or hubris keep you from recognizing when you do get hit. It's not a crime, but it's certainly unpleasant enough from what I have seen. Getting help quickly can reduce the pain and the injury, so don't let your judgement be clouded by the negative connotations some associate with DCS.