Recalculating Thirds

[victorzamora]

I don't think recalculating thirds is quite as simple or as dangerous as it's made out to sound. Someone mentioned Andrew Ainslie saying you should always have twice as much gas as necessary to get out, right? Can we all agree that "standard thirds" in a cave with no flow that this would be true at the point of maximum penetration?

You start with both tanks at 3600psi in a cave with no flow (high flow makes everything more conservative). At 400psi, you notice a side passage you want to check out and write it down. You need 800psi to exit from there. Starting a dive TO there with 1200psi in those same tanks (or 3600psi in smaller tanks for equivalent volume) would mean turning at thirds there. You turn your dive at 2400psi being aggressive (800psi farther in), and exit to the point of the side passage. You reach the side passage with 1600psi left in your tanks. You need 800psi to exit, as was mentioned earlier. (1600-800)/3 means you've got at least 200psi to penetrate into that side passage. You are diving thirds of "non-reserve" gas into that side-passage, giving you twice the necessary gas to get back to gold where you'll have twice the necessary gas to get back to the surface.

Your reserves, through the whole dive, are constantly greater than or equal to twice the gas needed to exit.....and the dive was as aggressive as one could make it. Turning earlier (2600psi) would mean starting the side passage with 2000psi and having 400psi to explore if diving to recalculated thirds. I was not taught this, but would have no problems doing so depending on the cave and the day and how the rest of the dive was going. I've done this a bunch and always exited with plenty of gas left. I normally dive quarters when doing this. I know that past success is no guarantee of it being fail-proof (as we all know, people get away with bad habits for too long).

Let me reiterate that I'm a newish cave diver and clearly don't have the experience of the more veteran amongst us. This might be one reason why I'm so eager to continue the dive, to continue learning as much as possible. Maybe after a few hundred more cave dives I'll be more likely to be more conservative.

[MandM]

I appreciate your illustration,but I have concern for things like basing a plan on things such as planning for a 600psi exit or depending on "light flow and speedy exit" . There have been numerous dives that I've done where the exit seems so routine that I get lulled into complacency,and then something out of the ordinary occurs ,which causes a delay. For example, everyone from the Hill 400 jump exiting assumes this will be a simple exit out,with moderate flow boosting you along. I remember someone using the clay bottom near Harry's crack to simulate zero viz,thinking if done late in the day no one would be around. I remember having to do a zero viz,line drill all the way to the Ear because that is how much sediment was kicked up. I recall burning an extra 300-400 psi on the way out from my normal amount. Maybe I am completely wrong,but I plan my dive and gas needed,and stick to the plan. If there is a side excursion that I want to see on my exit,then that has to be contained within the original gas plan,no recalculations,assumptions for flow etc. If there isn't enough safe gas reserves,then that side excursion is skipped pending another trip which has this as part of the dive/gas plan. In cave diving our accident analysis rules are based on numerous fatalities,just read "Blue Print to Survival",each rule is prefaced with an accident that occurred. History will show that when we monkey with gas planning it really can bite us,and that is borne in the fact that we lost two world class cave divers who used some voo-doo gas planning. Be safe and have a good holiday

In the example, the "light flow and speedy exit" was what occurred, not what was planned for, resulting in less gas usage out than in (certainly plausible). The key here is that thirds for the side trip are calculated based on actual gas volumes required for exit while still maintaining sufficient reserve, hence the calculation being done underwater and not as part of the initial gas plan. If there is sufficient gas for the side trip do it, if there isn't you don't.


That said, I don't disagree that 600psi could very likely be light on gas to exit with for many sites. As stated the numbers were chosen for simplicity for the example and I was not advocating usage for any particular dive plan. Gas plans need to account for many variables that are site and team dependent, and personal conservatism in gas planning needs to be added to any dive. Obviously you're not wrong in being conservative, better to be sitting on the surface wishing you had spent more time in the cave than sitting in the cave wishing you had more gas to get out


Recalculating thirds is definitely reducing safety margins, there is no denying that, but so is using thirds as opposed to quarters, or sixths. At what point does the risk become to great, that is for each individual/team to determine.

[Pavão]

A lot have commented that a given number (500-600 PSI) is too little gas to exit a cave, while at same time agreeing that 1000-1200 PSI is sufficient, but with twice the distance, wouldn't this scenario make the 500 PSI amount at half the distance safer?
For simplicity of math, I too, am excluding additional safety margins, which would be added to both scenarios anyways.

I have never recalculated 3rds and not getting into the discussion, just pointing out the above detail.

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Bobby pointed out an interesting fact about flow, in that it is not always working in your favor, in case of a blind exit, the strong flow on your back would work against you and likely delaying an exit as it would be always throwing you around, into the wall, crevices, away from line and messing with your body position and putting you in places you don't intend to be, this makes sense that coming out in zero viz on a low/no flow cave might be faster than on a high flow one.
I believe a lot of people have a more aggressive plan when diving in high flow caves than no flow, something to think about.

[Greenwood_60]

Very relevant.

http://www.cavediver.net/forum/showt...eam-and-Thirds

[RGU]

well there was definatley some miscalculations involved then..

Not by you on this thread! Your calculations and understanding of the problem are correct.

[Sludge]

Several people are going on and on about pressure versus volume. I have been cave diving for almost nineteen years and have always been under the impression that the relationship of pressure and volume is linear. In other words, at 3600psi my 95s have 260 cubic feet of gas and at 1800 they have 130. If this is not true please enlighten me.

[victorzamora]

Several people are going on and on about pressure versus volume. I have been cave diving for almost nineteen years and have always been under the impression that the relationship of pressure and volume is linear. In other words, at 3600psi my 95s have 260 cubic feet of gas and at 1800 they have 130. If this is not true please enlighten me.

That's mostly true. At high pressures it's not completely accurate but kind of close enough.

I think the distinction being made is that 600psi in lp50s is a while lot less has than 600psi in lp125s. It's not pressure that will save your life.... it's volume.

[Danseur]

That's mostly true. At high pressures it's not completely accurate but kind of close enough.

I think the distinction being made is that 600psi in lp50s is a while lot less has than 600psi in lp125s. It's not pressure that will save your life.... it's volume.

At the end of the day, this is all that matters.

[JillHeinerth]

Always better to have gas you don't need than need gas you don't have.

[Sludge]

That's mostly true. At high pressures it's not completely accurate but kind of close enough.

Yes, the compressibility of gases in the Ideal Gas Law. At the pressures we use, the difference won't ever approach one whole cubic foot.

I think the distinction being made is that 600psi in lp50s is a while lot less has than 600psi in lp125s.

I believe anybody that has the ability to find the "on" button of their computer can understand that already.