a hypothetical for discussion

[BabyDuck]

ok - you and your familiar, similarly experienced full-cave buddy with similar sac rate plan a circuit. you are at a system very much like peacock (and with the same names and same relative distances) but either it is deeper or the distances between openings are longer or your sac is not as good as it is.

you set up one side by going up the peanut tunnel & putting in a jump at crossover in your usual manner. you go up crossover to somewhere in the middle, hit thirds, drop your cookie(s), and turn the dive. you exit without incident and have a surface interval.

you both now go up the pothole/olsen line and put in the jump to crossover. just as you get to your cookie (which might be a hair past thirds in a theoretical way that can't be measured, but is at thirds by the tolerance of your spg), there's some disasterous happening & one of you loses your gas.

you quickly & smoothly start air-sharing, right at 2/3 gas. viz is intact, the line is fine.

do you:

1) keep going to complete your circuit. it is verified line, and shallower than turning around, but will be at the ragged edge of your gas.

2) turn around and go back out the way you came. it is also verified line, but a bit deeper, and might be past your available gas. you could possibly go out pothole. that line is unverified.

3) turn around and head for olsen. you would go back over your jump then turn left instead of right. you have been to olsen before, but not on this trip/not for several months. it is a closer exit by far, but the line is unverified by you recently.

what would you do? assume a small amount of time/gas lost for discussion, but not more than a couple of minutes.

would it matter if as your team was getting in, another was getting out talking about a nice dive to olsen? or not?

would it matter if you had each put a tank of o2 for deco in the cavern?

aaand...go!

[stairman]

Id definetely head for the pothole jump and evaluate from there weather to surface in Olsen or continue toward pothole depending on remaining gas and stress levels.

[cavediver256]

I would turn and attempt exit at Olsen......if exit from Olsen wasn't possible, then Pothole would be my second choice. I would want to get to the nearest exit as cleanly and quickly as possible. End of story !!!

[RN]

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.

[netmage]

1) keep going to complete your circuit. it is verified line, and shallower than turning around, but will be at the ragged edge of your gas.

2) turn around and go back out the way you came. it is also verified line, but a bit deeper, and might be past your available gas. you could possibly go out pothole. that line is unverified.

3) turn around and head for olsen. you would go back over your jump then turn left instead of right. you have been to olsen before, but not on this trip/not for several months. it is a closer exit by far, but the line is unverified by you recently.

would it matter if as your team was getting in, another was getting out talking about a nice dive to olsen? or not?

would it matter if you had each put a tank of o2 for deco in the cavern?

aaand...go!

I don't agree with, or understand the "might be past your available gas" comment.... 3rds is 3rds is 3rds...

I havn't been to Olsen lately - last time I recalled - I thought the sink was visible from the Crossover jump... so from that perspective - I would consider it verified and egress there. Knowledge of the cave comes to bear here. I may have to look tomorrow - on the map it looks further...

I'm also w/ Rob - when calc'ing gas we round down, so there is always a little gravy.

On points #1 & #2 - 3rds is 3rds is 3rds... The only parameter shifting balance towards Peanut as being preferred - is if deco gas is not in the basin... If you do NOT have deco gas - you will want the final shallow exit the Peanut profile provides (say that 3 times fast...)....

But I'm also leaning heavily towards path of most recently known egress...

[tecterri]

Marcie,

Tim (netmage) and I had a rousing discussion about this, we just did a 3 hour dive in Peacock today . . . it continues to put the "what do we do if" scenarios as part of our dive planning. Thanks for the post !!

My gut reaction to the scenario is our training tells us to return to the known/verified/gas requirement exit.

My real world response (only because we dive this cave so often) would be to get to Olsen, if Olsen was not available, get to Pot Hole.

[RN]

I don't agree with, or understand the "might be past your available gas" comment.... 3rds is 3rds is 3rds...

I havn't been to Olsen lately - last time I recalled - I thought the sink was visible from the Crossover jump... so from that perspective - I would consider it verified and egress there. Knowledge of the cave comes to bear here. I may have to look tomorrow - on the map it looks further...

I'm also w/ Rob - when calc'ing gas we round down, so there is always a little gravy.

On points #1 & #2 - 3rds is 3rds is 3rds... The only parameter shifting balance towards Peanut as being preferred - is if deco gas is not in the basin... If you do NOT have deco gas - you will want the final shallow exit the Peanut profile provides (say that 3 times fast...)....

But I'm also leaning heavily towards path of most recently known egress...

Olsen is about 100' from the Crossover jump and I've never been able to see it from there.

switch to the other tank and exit.

Gotta love sidemount!

[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.

[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.