a hypothetical for discussion

[Squirrel Girl]

I never push my gas to thirds.

That's cuz you dive with me, and I have a worse SAC rate than you!

[RN]

That would help (ALOT! - 1400 out vs 1100 in!) but for two divers exiting together shared gas in the extreme proposed hypothetical scenario and the other issues it still might not be enough.

That's my high flow gas management. In low flow, it's 1600 out vs 1000 in.

One thing nobody has mentioned is the inevitable fact that both of your SAC rates are going to jump way up if something like that happened.

In most cases yes. However, I've had stuff happen and my response always ends up in a lower consumption rate. It's pretty much the only time I intentionally skip breathe.

[Gary]

With all the attention always paid to the "one buddy suddenly out of air at thirds" situation and 2 buddies coming back on a single air supply it is worth reminding everyone that although it gives a concrete reason that makes sense for creating "thirds" as a rule in actuality it never happens that way.

Generally sudden equipment failure / air loss is not involved in fatal accidents. Most fatalities involve situations where persons violated thirds in some way - or that some other accident occurred that thirds was not enough gas to solve the problem.

While a lot of attention is paid to the manifold/non-manifold issue I don't think it has ever been an issue either way in a fatal accident.

Thirds simply provides a sufficient buffer for "most" accidents. There are however, examples of fatal accidents where quarters, fifths, or even sixths proved insufficient.

[chimie007]

With all the attention always paid to the "one buddy suddenly out of air at thirds" situation and 2 buddies coming back on a single air supply it is worth reminding everyone that although it gives a concrete reason that makes sense for creating "thirds" as a rule in actuality it never happens that way.

Generally sudden equipment failure / air loss is not involved in fatal accidents. Most fatalities involve situations where persons violated thirds in some way - or that some other accident occurred that thirds was not enough gas to solve the problem.

While a lot of attention is paid to the manifold/non-manifold issue I don't think it has ever been an issue either way in a fatal accident.

Thirds simply provides a sufficient buffer for "most" accidents. There are however, examples of fatal accidents where quarters, fifths, or even sixths proved insufficient.

Well said !

[RN]

With all the attention always paid to the "one buddy suddenly out of air at thirds" situation and 2 buddies coming back on a single air supply it is worth reminding everyone that although it gives a concrete reason that makes sense for creating "thirds" as a rule in actuality it never happens that way.

Generally sudden equipment failure / air loss is not involved in fatal accidents. Most fatalities involve situations where persons violated thirds in some way - or that some other accident occurred that thirds was not enough gas to solve the problem.

While a lot of attention is paid to the manifold/non-manifold issue I don't think it has ever been an issue either way in a fatal accident.

Thirds simply provides a sufficient buffer for "most" accidents. There are however, examples of fatal accidents where quarters, fifths, or even sixths proved insufficient.

I recall reading about an incident in one of Sheck's books where there was a sudden complete gas loss at maximum penetration and he and his buddy barely made it back to their stage bottles and out alive. While this is a rare event, it's enough for me to not ever push air management all the way to 1/3s. An extra 100-200 psi isn't going to get me that much farther and can easily be compensated for with a stage bottle. No one ever died from having too much gas.

[FW]

....No one ever died from having too much gas.

+1 !!

[BabyDuck]

i was more interested in the routes & reasonings people would choose.

[DA Aquamaster]

One thing nobody has mentioned is the inevitable fact that both of your SAC rates are going to jump way up if something like that happened.

FWIW, in the *real* Peacock, Olsen would be the best bet, because there is so much traffic, and line maintenance there that the line would probably be in good shape. I wouldn't make that assumption in less traveled caves.

I think that is Marci's point - if you are right at (or close to) thirds and one of you experiences a total gas loss, the theory that the other diver still has a third to get out and another third to get the OOG diver out is just that - a theory. The reality is:

1. stress and an elevation on SAC rate,
2. sharing gas on a long hose,
3. the high probability of large amounts of silt when the gas venting from a ruptured burst disc a) slams the OOG diver into the silt in the low tunnel and b) percolates more silt off the ceiling,
4. the possibility of being off the line and having to find the line before you start to exit, and
5. if you do not immediately get on the line before the viz goes and lose the line, when you find the line will you immediately know which way is out?

In light of that reality, it is almost certain that "thirds won't be enough gas to cut it in terms of either exiting as planned or returning to your starting point if both require virtually a full third under ideal conditions.

If you were to end up in that situation, Olse is in my opinion the best bet in the theoretical Peacock. It is perhaps 250 ft from the middle of the cross over tunnel, compared to about 1150-1200 feet to P1 via peanut and about the same via the deeper Olsen/Pothole side. Olsen has been open for decades and the probability of a geologic event is small - smaller than the probability of thirds leaving you a couple hundred feet short of P1 by either of the other routes. And, if the very unlikely event of a collapse did occur, you still have pothole and that buys you another 400 feet or so, plus you still have 200' in the bank even if the Olsen attempt fails.

The preferrable thing is to avoid that situation in the first place by not pressing thirds. When you do the math, turning 100-200 psi early costs you very little in terms of penetration, but leaves you 200-600 psi more gas in reserve (just 100 psi less penetration gas means 21.6 cu ft more reserve gas in double 95's or 130s, 200 psi increases this to 43.3 cu ft additional reserve.)

Other alternatives are a three person team, using a stage on the dive if you have a two person team to maintain a greater back gas reserve, or taking a buddy bottle as an additional reserve on a two person team. All of those options add more reserve gas and allow you to make the dive in far greater safety regardless of what happens.

[phillip1]

I recall reading about an incident in one of Sheck's books where there was a sudden complete gas loss at maximum penetration and he and his buddy barely made it back to their stage bottles and out alive. While this is a rare event, it's enough for me to not ever push air management all the way to 1/3s. An extra 100-200 psi isn't going to get me that much farther and can easily be compensated for with a stage bottle. No one ever died from having too much gas.

I think anyone who disagrees with your post is 100% WRONG.
Well said.

[aainslie]

None of the above. I don't push my gas management that much. The most I'll push anything is 1/3s + 100. So if I start with 3600, my turn pressure is 2500 leaving me 1100 for exit and 1400 for emergencies. I do this in systems like JB or Devil's. In a system like Peacock, Twin, Hole in the Wall, I dive 1/3s + 200...3600 starting, 2600 turn, 1000 for exit, 1600 for emergencies. If it's a new system or involves advanced stuff, like really tight sidemount passage, I dive 1/4s. This is also what I teach my students because of the possibility of something like what you describe in your scenario. If you need more gas to do the dive, then get larger cylinders or bring a stage.

What you have illustrated is that 3rds may not conservative enough as a gas planning rule, especially in low flow systems like Peacock.

Where's Ainslie? He must be busy or I would have expected him to chip in by now...

Andy

He's here...

I just did a dive in Ginnie on CC where I reserved 400 cubic feet to travel 4800 feet. I made it out with under 100 cubic feet left. In theory, I used about 3 times the amount of gas that I should have. It's a long story why, involving some restrictions that took longer to get past than I was expecting, an unintended jump and a flooded rebreather... but that's exactly the sort of hellish scenario that reserves are designed for. An even more amazing statistic - of those 300 cu ft used, I used 200 cu ft "navigating" (poorly, with a very screwed up head) from 4800 ft back to 3200 ft.. THe next 100 cu ft got me to the exit - and even that was due to a ridiculous SAC rate under pressure.

Let me emphasise this - if I had dived with only double the gas theoretically needed to exit, I would have run out somewhere around the Henkel, and drowned. If I had dived with triple the reserves, I would have barely made it. I NEEDED a 4x reserve.

My (somewhat misunderstood) philosophy is to a) work out the bare minimum needed to exit, and b) add whopping reserves onto that bare minimum. In this case, I had 4x the reserves theoretically needed (or, in OC terms, I was diving fifths) and it was nearly not enough.

Ah, the naughty word censors... doo-dah happens. Prepare for it.

BD, I think what a lot of people are telling you is that it's not a matter of routes, its a matter of never going anywhere near thirds in a low flow scenario like Peacock. SPG errors are one of many very good reasons NOT to go to thirds.