Modifying rule of thirds

[aainslie]

The first guy I heard of pushing thirds with with creative math was a guy at Little River who convieniently explained his method to a cave class before getting lost off the line and dieing nowhere near the exit. The last was John Robonson who pushed thirds without telling me as his buddy and nearly died as a result.

Both these guys pushed thirds, both thought they had justification to do so, both died in a cave.

You can follow Andrew's math spreadsheet on this if you want to but history shows this line of thinking is deeply flawed.

....because..... ?

I hate debates based on "History tells us". Where's the flaw? you're merely being intellectually lazy. it makes way more sense to keep a consistent reserve than an inconsistent one. Always diving thirds under different flows will give you inconsistent safety margins - sometimes ridiculously high, sometimes ridiculously low.

Both this time and last time you guys (Gary in particular) debated this you got focused on the "when to go low" and ignored the "when to go high". Theres' some sense to this actually - some guys called Kahnemann and Tversky won a Nobel prize inter alia for demonstrating a form of irrationality in subjects' behavior called "loss aversion" and you're showing us a nice example of it.

There are two sides to this.

If nothing else, take away the fact that diving thirds in a low flow cave is stupidly irresponsible.

If you are willing to do the (mental) work, you can safely go below thirds on high flow caves. The simple, conservative thing to do is to stick to thirds.

One more time however:

IT IS UNSAFE TO DVIE THIRDS IN LOW FLOW CAVES.

Ah - and I'd never push this on a buddy.

[OneBrightGator]

Dude... everyone knows the Rule of Thirds is the most liberal rule in cave diving and that if you push it right to the edge in the wrong conditions and the silt hits the fan, you're probably not gonna make it.

Advocating using math to "beat the system" is reckless. It's pretty hard to mess up thirds and I typically don't take a computer to the dive site, let alone punch in numbers right before I get in the water.

And hell yes it's intellectually lazy, it's recreation, not rocket surgery.

[RN]

Thanks for the math, Andrew. Although I'm pretty sure I'd still never push thirds, even in high flow, your low flow example has answered another question. This came to mind during a dive in which I was sidemounting AL80s. I didn't push thirds. In fact, I turned the dive at thirds plus 200. But, for some reason (probably the fact that 80s pumped to 3200psi only give me 160cf of gas and my usual 108s pumped to 3600 give me more than 100cf more than that!), the thought of how little gas I had with me suddenly occurred to me on this dive. I was already on the egress, but I didn't like having so little gas so far from the exit. That was the last time I soloed on 80s.

[FW]

About pushing thirds in high flow....

High flow can actually slow your exit under adverse conditions.

I used to run a drill in Devil's Eye. The students closed their eyes, and followed the line out. I had my light on, so I could observe them. It they cheated, and opened their eyes, it was obvious, because they just moved their hands over tie-offs. A *large* precentage of them got pushed around at tie-offs, and some actually went back into the cave as a result!

Total light failure? Impossible, right? I have seen three cases of total light failure, one caused a fatality. A lot of divers negelect their backup lights, so when theri main goes out, they are in real trouble.

Another way high flow can slow you down is buddy breathing (which is the main reason for 3rds anyway). You have to stay close to your budddy, and the flow tries to separate you, so you have to slow down, loosing the advantage of high flow. Add a restriction or two, and you probably won't make it out if you didn't turn on thirds.

[Line Squirrel]

Good stuff Forrest, Thank you!!!

[aainslie]

Dude... everyone knows the Rule of Thirds is the most liberal rule in cave diving and that if you push it right to the edge in the wrong conditions and the silt hits the fan, you're probably not gonna make it.

Advocating using math to "beat the system" is reckless. It's pretty hard to mess up thirds and I typically don't take a computer to the dive site, let alone punch in numbers right before I get in the water.

And hell yes it's intellectually lazy, it's recreation, not rocket surgery.

...Once more focusing on only one side of the equation...

Thirds is NOT liberal in some circumstances.

Forrest, some good points but they only complicate the debate. First, it brings me back to why I really wish sidemount would take over from backmount. BM - and particularly gas sharing - is an inherently silly way to handle OOA situations. Secondly, the chance of a TOTAL light failure IN CONJUNCTION with OOA is astronomical. The probability of a set of independent events is the product of each probability. Let's say lights fail 1 in 100 dives, and you have 2 backups. Let's also say that you lose gas once every thousand dives. Then the joint probability is 1/(100*100*100*1000) or .0000000001. Not a frequent occurrence, particularly since for TOTAL light failure all three of your buddy's lights need to fail too, decreasing that number by another 6 zeros to .0000000000000001! Put another way round, you'd need to do 1,000,000,000,000,000 dives between such an event, on average. In fact, has anyone ever heard of two properly equipped divers losing all their lights, and one losing half their gas?

GREAT point on checking backup lights! Rich Courtney has a really good story about this issue, and about how carrying TOO MANY backups can lead to complacency. Checking backup lights often is critical. I actually keep mine switched ON (on my helmet), and change the batteries regularly - not only does it help with gauge reading but with verifying functionality.

On the issue of lights out and tie-offs, I did a fantastic class with Steve Bogaerts recently. I asked him to do a refresher on stuff he thought I needed work on. One was lights out drills, sidemount style. The idea is to use "bump and push" - at each tie-off, yu move one body length beyond the tie and wait. Your buddy then bumps into you, and pushes you to indicate you should keep moving forward. We first practiced in open water. After about one minute he got a pissed off expression and motioned for me to surface. "What do you think you're doing?" he asked. "You're moving so slowly that in a real situation you'll run out of air!". We practiced a bit more, went into the cave and essentially exited on the line, swimming as fast as we'd swim with lights on. It was an incredibly powerful lesson. BUT... it was in Mexico with no flow. I'm going to give it a try in Florida as well, and see how it goes. But here's the key point - try swimming flat out in a lights out situation on your own, and then try doing it while sharing air. That alone might convince you that SM is the only safe way to exit in a gas emergency.

Anyway... back to the key point. i agree, thirds is fine, as long as you adjust it by at least 200 psi in no flow situations. 'Nuff said. And if you want to do a bit of arithmetic and trust logic, I'd suggest you can safely go a bit lower. And I know that I'll always have people telling me I'm a lunactic when I do it - but until someone gives me a VALID reason, I'll be doing it.

One more point - KNOWING your rock bottom doesn't mean you have to go there. Although I've calculated that I can turn at 2000 with a huge safety margin in Cow, I'll be the first to admit that I've never actually gone that low. But it's nice to have an exact feel for your safety margin.

Thirds gives you a highly variable safety margin. Once more Ben, NOT ALWAYS CONSERVATIVE.

[Sludge]

it's recreation, not rocket surgery.

Or brain science, either.

[Ben M]

Andrew,

I understand what you are saying in high flow caves, but still disagree. For the low flow caves, you are absolutely right. If someone is diving to thirds they are absolutely fooling themselves about what reserves they have. I agree that very few have done the math for many of their dives. That's why 3rds is the starting point for ideal conditions and you should be more conservative for anything else, not less.

While in high flow caves, the math proves you have plenty to get out, but the extra gas is there to also give you time to deal with a variety of issues that can occur and compound. So the math says you will not likely have a gas and light failure at the same time. How about if you have a gas failure and in dealing with the issue you siltout the place and the flow pushes you off the line? Same end result, you cannot see even though your lights still work. Granted you may not have to do the entire cave exit in the silt cloud. The problem here was human error, not math.

Too much gas is not likely to be an issue, too little gas can be a big problem.

I'm an engineer and not a professor; in my field when you cannot account for something or it is impractical to account for all of the variables one option when testing to failure is impractical, you give yourself a larger margin of safety. Many engineering failures are from pushing the math for one case too far without properly accounting for all the other variables or cases.

[aainslie]

Andrew,

I understand what you are saying in high flow caves, but still disagree. For the low flow caves, you are absolutely right. If someone is diving to thirds they are absolutely fooling themselves about what reserves they have. I agree that very few have done the math for many of their dives. That's why 3rds is the starting point for ideal conditions and you should be more conservative for anything else, not less.

While in high flow caves, the math proves you have plenty to get out, but the extra gas is there to also give you time to deal with a variety of issues that can occur and compound. So the math says you will not likely have a gas and light failure at the same time. How about if you have a gas failure and in dealing with the issue you siltout the place and the flow pushes you off the line? Same end result, you cannot see even though your lights still work. Granted you may not have to do the entire cave exit in the silt cloud. The problem here was human error, not math.

Too much gas is not likely to be an issue, too little gas can be a big problem.

I'm an engineer and not a professor; in my field when you cannot account for something or it is impractical to account for all of the variables one option when testing to failure is impractical, you give yourself a larger margin of safety. Many engineering failures are from pushing the math for one case too far without properly accounting for all the other variables or cases.

Then dive sixths. Hell, dive twelfths. They you'll REALLY be safe.

If you're an engineer then you should REALLY realize what I'm suggesting. A constant safety margin. Thirds, once more, gives people an inconsistent margin because people are too lazy to compute it properly.

Extra stops, extra problems, all that stuff can happen in high flow, low flow and reverse flow caves. None of that cahnges the argument for calculating a consistent safety margin.

Sheesh. Ben, you're right - this isn't rocket surgery (thanks for that nugget, I'd forgotten about it!) It's simple arithmetic. I bet i could explain it to a 15 year old... If they hadn't been indoctrinated into thirds as being sacrosanct.

[OneBrightGator]

See... if I was a consultant I could have made $1,000 off that...

Every dive is different, just because you are reserving the same percentage/amount/whatever of gas does not mean you have the same safety margin. Conditions, flow, buddies, emotions, physical fitness change dive to dive.

Thirds is a great place to start, modifying conservatively is a good way to go. Anything else is unadvisable, and to condone it on the internet is reckless.