a hypothetical for discussion

[bletso]

If I remember correctly, the line arrows at the jump point to the nearest exit, Olsen. Head for Olsen. I never dive a known, popular, mapped system without knowing the the geography. Always stack the cards in your favor, not Murphy's. That is where all the hard work of the cartographers pays off. One can see the Olsen exit from the end off the line, at least during the daylight hours.

Dale

[RN]

Yes, the end of the line ends in OW at Olsen, but the natural light from Olsen cannot be seen from the Crossover line. Cartographer or not, I am not going to trust a line arrow if I've been that way. While at Peacock the line arrows on the gold lines are marked with distances, that isn't the case in all systems. I've been on lines in JB where the line arrows point deeper into the system. Following the line arrows adds an additional 500' or more of passage. It makes sense there, though, because coming the way I came from cannot be exited by someone in backmount. I would be hesitant to use Olsen as an emergency egress unless I've been there that day, or even possibly within the past week. The opening from the passage to the basin is small enough that a small cave in could make it impossible to pass through and now you've just added a couple of hundred feet more to your exit and raised your stress level.

[Line Squirrel]

switch to the other tank and exit.

Exactly

[BabyDuck]

tim, i wanted to head off someone saying that if the pothole leg of the circuit was a bit longer or deeper, you'd hit thirds from that side before you reached your cookie. what i was trying to say was that if you had some super-sensitive way to read every psi, you might have broken thirds to get to your cookie, but that in reality reading an spg in 100s you would not have.

[apitkin]

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

[lamont]

We had this scenario in our C2 course, and one buddy turned the dive right after the cookie and got a stern lecture (underwater) from the instructor. Once you pass the cookie you are supposed to be committed to exiting on the other side of the circuit.

In reality, if the cookie was substantially closer to the entrance of the circuit and the issue occurred right after the cookie, and we hadn't started pulling gear (other than the cookie), and I knew that we hadn't passed thirds on the penetration side, then I would consider turning it as an option -- but it would need to be obviously a better choice, and not require a not of debate underwater. The closer the cookie is to the midpoint, and the further the problem occurs after the cookie, the more we're simply committed to exiting the other side.

Excluding this kind of massive hypothetical, the point of picking up the cookie is to commit to the circuit and exiting the other side. That means that you have to rapidly go from the whole team being okay, to the whole team being in a world of hurt in a hurry -- and those kinds of situations are very unlikely. In that kind of situation, your training becomes secondary to being able to draw on experience and adaptability. If you really lost a burst disc, did you just create a huge mess in the cave? How did you exit that mess? In the confusion did you exit the silt cloud towards your entrance so that now your preferred exit on the circuit is on the other side of a huge mess? Might be better not to go back into the mess just to complete the circuit if you're still closer to your entrance -- sure. I'm not going to armchair quarterback anyone who really gets into a situation with that quantity of soiled underwear and manages to get out of it.

But you have to be careful in confusing yourself with hypotheticals that are highly unlikely to occur, which then cause you to have poor reactions for the more likely scenarios. The 99% most likely scenarios are that turning and swimming towards your original entrance on a circuit is a really bad idea past the halfway point, and that is more likely to get you killed. So the cookie means that you need to focus on committing to the other exit. It is much more likely that holding open the option of running for your original entrance on a circuit is going to confuse you and kill you, than it is going to save you from some bizarre hypothetical accident right in the middle of the circuit after picking up the cookie.

[Gary]

"switch to the other tank and exit."

Yes that is my choice also (independent backmount).


But assuming full gas loss of both (manifolded) tanks - right at thirds in both directions - and facing an air sharing exit... In a hypothetical situation where there isn't enough extra efficiency to squeeze out of the dive then there is likely no way the two divers can share air to either of the planned exits. Lost all gas on one diver - unless weighted properly for zero gas in tanks many divers in that situation would be buoyant. They barely made it from either direction when swimming each separate and weighted normally - now floating, excited, tied together with a 7' hose, and already "past thirds" by a bit. ---- Then Olsen would be the best choice.

If Olsen wasn't an option divers would be depending on flow, efficiency and luck to make it to Peacock by either route. But once past the cookie - and thirds - continuing in the planned direction to the exit would be the default (shouldn't require discussion). Pot Hole is a terrible option for trying to surface but maybe their best hope. Maybe better handholds for pull-and-glide on the Pot-hole line for a more gas efficient exit than pure swimming... but with both divers tied together "extra efficiency" is a vain hope at best.

Being weighted properly would obviously be a good idea. I think both "buoyant Out-of-Air diver" situations I'm thinking of were with Aluminum tanks - steel tanks less of a problem for most. But with Aluminum tanks actually wearing enough lead to be neutral with empty tanks would actually be important in that situation.

Sidemount or independents offers additional redundancy vs the "Completely out of air" situation. The "Completely out of air" with a manifold situation is pretty unlikely anyway. Sidemount or independents you'd have to postulate both regulators failing simultaneously - and even then you can still get to the gas.


If the system were significantly deeper then deco gas might become an issue. Olsen might get one alive to an exit but with insufficient gas remaining to complete deco. Bent would be better than dead - but if the gas situation were less obviously dismal to reach the original planned exit(s) taking a chance on the farther exit starts looking more attractive.

[Gary]

None of the above. I don't push my gas management that much. The most I'll push anything is 1/3s + 100.

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.


One of the issues I was thinking was that pre-dive planning discussing the potential exit points for various points of the dive would likely point out the need for more gas on the dive BUT I realized that you wouldn't really know in this situation how tight the gas was until you were actually in the situation:

You set up the dive to thirds on the one side and exit. You come back and dive to thirds, if you hit thirds before the cookie you turn if you hit the cookie first you continue. But you see the cookie ahead you go forward to get it and you are already at/past your planned turn gas - you'd find out really only once you were already in the situation.

It has never made much sense to me to set up a circuit like this anyway. You can dive it on one side - and return. You can dive it on the other side - and return. You see both sides going in and coming out.

Instead as a circuit/traverse, in order to "prove" it can really be done you set up one side do the other, don't see the exit on the 2nd side possibly leave a cookie stuck in the middle where you couldn't actually reach it as you thought. And then unless you did a visual jump you have to go back on another dive to get the reels/cookies you left in the cave.

On a double gap-jump like that a circuit/traverse seems like a silly idea to me. If the circuit/traverse has just a single jump and the jump can be set up with the cookie as the "halfway" marker with continuous lines to both exits and so there isn't any reel left in to have to retrieve later. Or if the whole circuit/traverse can be done thirds with no gaps it doesn't require all the setup and mess.

I'd rather just do the dives from each end and skip the extra work and risks.

[phillip1]

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.

+1
I don't understand why some people will push to thirds or even beyond (I know I know it's hypothetical). Thirds is NOT conservative enough.

[FW]

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.

Like Tegg etc., I am sidemount, and odds of loosing all my gas would be slim. Like RN etc., I never push my gas to thirds.