Risks in Cave Diving

[Bobbert]

50% chance surviving a dive to half gas? I've yet to have a reg/post failure in a cave. Even in my one silt-out incident, it didn't take more gas coming out than going in. My Apprentice class in JB where we had portions lights out, sharing air, and portions just lights out, we didn't use more gas coming out than going in, though it was very close. Thinking about it, I don't know of many people who have had a failure such that they required their reserve air. That means that every dive, I could have gotten away with diving to 1/2. I guess this would qualify as a proof by counter example?

I would also say that it's fairly obvious that the calculation of risk would HAVE to factor in things like silt, low flow, new system, new buddy, new cave diver, frequency of dives leading up to the dive you're calculating, how well the system is marked (IE Ginnie with arrows every 100ft or a rare system), siphon or spring, etc. One thing that cave training makes VERY clear, is that diving to 1/3 in JB/LR/Manatee/Ginnie has a much greater safety reserve than diving to 1/3 in Peacock. You can easily prove that by looking at a dive log of someone who records dive end pressures, and I can almost guarentee that they end with less gas from a peacock dive than a Ginnie dive.

Before I fall for this too hard...I'm not sure if your post was serious or stirring the pot? I mean, you've got a graph and spreadsheet and haven't mentioned any confirmed source of data, so maybe I'm taking this way too serious

You’re not taking into consideration that maybe your a little more comfortable with you instructor there, plus the fact that maybe more then one thing could go wrong. and the fact that your buddy has to get you out if you have a major air failure, and if you both used half instead of thirds one of you most likely won’t make it, possibly both. My bet would be both. Breaking the thirds rule is absolutely nuts in any cave dive.

[phillip1]

The fact is is that eventually **** will hit the fan no matter what you may think or how much experience you have. I got stuck pretty bad once, caught in the line and almost all the dives I do end up in zero viz on the way out, the one thing that i never do under any circumstances is go past thirds I always turn a bit before and sometimes allot before. The one thing that is not being discussed is that when you are in trouble and it will happen to everyone to greater or lesser degree, knowing that you have enough air in reserve to sort out your problem is not only a life saver but a great mental helper too. When I got stuck it was tense enough as it was but it was cool because I knew I had enough air for a while, it could have been a very very bad situation if on top of one major problem I was breaking the thirds rule and thought that I might run out of air etc... Anyone even contemplating breaking the thirds rule is insane, it makes no sense in any situation or in any cave.
I think that people like that eventually after many dives get a false sense of security, that a major problem won't happen to them.

[Corbett]

Gary posted at the end; "And for those planning for a long career in cave diving additional safety margins might be worth considering."

I like that. I practice that. Conservative is a good word when it's used in diving.
If I'm going to a new cave or tunnel that I haven't been to before, I'll keep a full stage
on me, just because. If my computer says "have a nice day" -your done with deco, I'll
hang out a few more minutes. (maybe find another snorkel keeper)

That rock or room will still be there Tomorrow. BTW I only turn at thirds a third of the time.
I turn at quarters two thirds of the time. I know this makes me a wimp, but I need to stay
alive because I have a huge "honey-do" list, and I'm way behind....
take it easy.
Chris

[Gary]

Interesting concept. What is missing in this set of assumptions is that with each 100 dives (or pick a number), experience increases, comfort increases and thus the chances of survival increase all things being equal.

Can you add that to the numbers machine and pump a new graph?

I certain I don't have to data to calculate it that way. If the complexity of the dive grows with the diver's experience and the gas reserves are pushed to the same limit it is possible that there might be no advantage. I was actually kind of thinking that experience would tend to push one towards greater gas reserves and that would tend to increase safety. It would seem that whatever the value of experience a change in reserve gas would make much more difference.

I included the data mostly because it is easier to show the basic assumptions and methods than explain them. The data fit better than I would have expected making sense over a huge range of tank reserves (not just the interval in question). Anyone can take the numbers and do whatever they want with them to change the assumptions to fit better - or use another method.

Whatever assumptions and methods though are going to have to traverse and explain a tremendous risk factor (on the order of 1/10000 vs 1/2 = thousands of times) over a very small change in reserved gas. Any reasonable facts are going to show that small changes in reserve gas have a far higher risk multiple than I, or I think most people, have ever considered.

[aainslie]

Gary,

What exactly is your model? I can't work out how you got those graphs. Could you tell us a bit about your methodology?

[Gary]

The basic model is the "known" overall risk of cave diving death (~1/10000) assumed to be at aprox 2/3 reserve gas vs a reasonable guess of survival in no flow diving to half. 500 psi makes up the difference between these two values and needs to be distributed "somehow". Roughly 5 decades in 500 psi seemed one way to do it.

To test the theory and produce the derived graphs I assumed that for large numbers and distributions that a normal distribution curve including the two known points and risks 50% and .0001 would provide the best fit I was likely to come up with. .9999 is way out in the 4th or 5th standard deviation (Normal Distribution Curve reference)

Lacking the tools to fit a normal distribution curve to the data I produced a normal distribution curve and fitted the curve to the two points. The first column is fitted air reserves to the distribution curve and the following columns are a normal distribution curve produced by 51 coin flips (thus .5 ^51 - ignoring the .05 ^ 51 typo). [more coin flips would have provided better resolution on the graph and extended the numbers farther above 80% and 20% reserves - but the shape is the same and the values above 80% and below 20% are virtually certain anyway.]

The assumption of 1/2 gas reserve = 50% chance made the distribution very balanced over the volume of the tank which would also make good sense. But I don't think it changes much in the 1/2 to 2/3 gas range regardless of the number assumed for survival at the 1/2 reserve level.

Having the odds for a single dive it is merely a matter of raising the survival chance to any given power to determine the chance of survival over a larger number of dives.

Using the normal distribution curve for distributing the risk factor of 5000ish over the range gives a range of about 2 to 15 times per ~100 psi rather than a general 10 times risk per 100 psi.

The graph, of course, is no better than the very rough assumptions that went into making up the data. But the numbers are so stark minor variations in the method or assumptions make very little difference. I would think that to significantly change the results reapportioning the 5000 times risk factor linearly instead of geometrically would make the numbers less extreme but I don't think that model would hold up to much scrutiny.

In any case the start and end points are assumed but the trends and risk factors seem fairly reliable. I doubt many would consider the minimal gains an extra 100 psi would give is worth increasing their risk by ten times (or even 2 times).

The safety gained from even a small amount of extra gas is well worth the effort of bringing an extra bottle in most cases.

[FW]

About experience, and comfort level..........

There is an alarming trend for cavediving accident victims to be trained cavedivers. 40 years ago, it was only OW divers that didn't know any better. Today even OW divers know the dangers of overhead.

Most of the fatalities, among trained cave divers, have no gas left in their tanks. It is EXTREMELY obvious that some emergency caught them with insufficient reserve.

The accident analysis "rules" are for everyone, not just new divers.

I am like Phillip. I add 50-100 psi, or more, to turn around, for things like:
New Buddy
Low/silty passage
Major restrictions
Long dives
New exploration
Mapping
and anything else that *might* delay my exit

In the last couple of years, I have ended most of my dives with half to 2/3 of my gas remaining. If you don't believe me, ask some of my buddies.

[phillip1]

The question to ask is: why risk your life and the life of your buddy to go a bit further in a cave that is always going to be there, you can always come back later and plan the same dive with enough gas to be safe?

[deepdiverbob]

About experience, and comfort level..........

There is an alarming trend for cavediving accident victims to be trained cavedivers. 40 years ago, it was only OW divers that didn't know any better. Today even OW divers know the dangers of overhead.

Most of the fatalities among trained cave divers have no gas left in their tanks. It is EXTREMELY obvious that some emergency caught them with insufficient reserve.

The accident analysis "rules" are for everyone, not just new divers.

I am like Phillip. I add 50-100 psi, or more, to turn around, for things like:
New Buddy
Low/silty passage
Major restrictions
Long dives
New exploration
Mapping
and anything else that *might* delay my exit

In the last couple of years, I have ended most of my dives with half to 2/3 of my gas remaining. If you don't believe me, ask some of my buddies.

I am closing in on 50 cave dives now, and I have yet to call a dive on thirds. Like eberyone is saying, the caves will be there tomorrow, and I am not going to find anything that someone else hasnt already found.

What do you think about including a presentation on accident analysis for the 2009 NSS-CDS workshop? It might sound a bit morbid, but wasn't it the gruesome pictures Sheck saw of car wrecks that gave him the idea to write Blueprint for Survival?

[FW]

I am closing in on 50 cave dives now, and I have yet to call a dive on thirds. Like eberyone is saying, the caves will be there tomorrow, and I am not going to find anything that someone else hasnt already found.

What do you think about including a presentation on accident analysis for the 2009 NSS-CDS workshop? It might sound a bit morbid, but wasn't it the gruesome pictures Sheck saw of car wrecks that gave him the idea to write Blueprint for Survival?

There was an accident analysis presentation at the 2008 workshop, during the Friday evening social. That is where I got my information. I will make the suggestion to the 2009 workshop chairman.