The "New" Ginnie

[Gary]

Little River, Florida USA

93SEP-A novice cave diver ran out of gas and drowned on a solo dive in the Little River cave system. The diver was found with no air in either of his independent 104 tanks about 1300 feet back in the cave on the mainline. Though the individual frequently made solo dives he was not diving with a "buddy bottle."

The diver was known to use "creative" gas management rules outside of the basic tenets of cave diving and on at least one occasion had explained the gas management strategy he utilized to a group of cave students. Basically the diver reserved sufficient gas to exit form known points in the cave using the outflow in the system. The problem is that liberalized gas management rules such as this leave not margin for error or the unexpected compared to the golden "rule of thirds" or better (i.e. use at least 1/3 of your gas for penetration and exit on the remaining two thirds).

Members of the recovery team speculate that the diver ventured into an unfamiliar part of the cave and got lost in the low silty tunnels and "tees." Having silted out the area, the diver spent precious time searching for the main line connection and likely missed the tee on the way back. Eventually he found his way to the line but it was too late. A long time aquaCorps subscriber, he had renewed his subscription only a week before.

Using a spreadsheet and applying self devised thumbrules does not change the basic nature of what Andrew has been doing. It is dangerous and the additional risks it entails then also puts others at increased risk (in recovery) and also to the cave from damage. Nor should the idea be promoted to others.

[MengTze]

Using a spreadsheet and applying self devised thumbrules does not change the basic nature of what Andrew has been doing. It is dangerous and the additional risks it entails then also puts others at increased risk (in recovery) and also to the cave from damage. Nor should the idea be promoted to others.

It is highly dangerous yes, but so is cave diving in itself and exploration by the nature of the unknown. On the other hand, I think the risks Andrew took/takes are calculated ones and the backups he had were ample.

I do agree that this is not something that should be promoted. But if it is discussed should also not be dismissed right out.

I fail to see how gas rules have a correlation on cave damage.

[guru caver]

I fail to see how gas rules have a correlation on cave damage.

If you run out of gas...you die and the lousy bouyancy control will damage the cave...... or cause it to be closed for everyone else

[MengTze]

By that token, any gas rule used to penetrate a cave could result in that deficient buoyancy control situation......? While there may be greater risk with a modified 1/3rd plan, it does not by defacto stipulate an increased risk to cave damage. One could argue that by consciously planning alternate gas management, one also needs a deeper insight in to the risks and account for redundancies. This deeper insight leads me to believe that the net risk is lower, or equal at the most.

Should this modified 1/3rd plan be conducted without that deeper insight, yes, one could argue that there is an increased risk to cave damage as a result of the deficient buoyancy control situation. But in that instance, I put it that a standard gas plan would yield the same risk to cave damage......

[RN]

First, let me state I do not violate thirds, and in fact go more conservative most of the time. However, this discussion on thirds has raised some questions.

Unless I'm missing something the rule of thirds was born out of the need to reserve enough gas to get a diver and the diver's buddy out of a cave from the farthest penetration point in. I've always thought this wasn't a very good rule because most situations in which there's a massive loss of gas at the farthest penetration point will produce anxiety in the divers who, in turn, will increase their breathing rates, thus not allowing them to exit on the 2/3s gas left. This tells me this rule is too liberal, even if you're counting on flow to get you out faster. So why don't the agencies modify the rule? I'm guessing it's because it's a very rare case in which a massive gas loss occurs at exactly the time of maximum penetration.

To add to this, none of the agencies promote solo cave diving. Since this is the case, it makes sense that the rules aren't geared towards solo cave divers:

Training - there's no solo cave diving training.
Guideline - nothing to do with buddies/solo.
Air - the rule of 1/3s specifically states the reserve is for diver and buddy.
Depth - nothing to do with buddies/solo.
Lights - also nothing to do with buddies/solo.

For a solo sidemount diver, losing all gas from one bottle at the maximum penetration diving 1/3s leaves enough gas in the remaining bottle to get out alone. However, carrying a buddy bottle adds 80cf of unused gas. Diving cave filled 121s provides 165cf per tank. So if you dive 1/2s in sidemounted 121s or smaller with a buddy bottle, no matter which bottle you lose use of at maximum penetration, you would still have the equivalent amount of air as 2 divers diving 1/3s who has a massive gas loss at max penetration. So what's wrong with diving 1/2s in that case?

Just thinking out loud...

[Gary]

For a solo sidemount diver, losing all gas from one bottle at the maximum penetration diving 1/3s leaves enough gas in the remaining bottle to get out alone. However, carrying a buddy bottle adds 80cf of unused gas. Diving cave filled 121s provides 165cf per tank. So if you dive 1/2s in sidemounted 121s or smaller with a buddy bottle, no matter which bottle you lose use of at maximum penetration, you would still have the equivalent amount of air as 2 divers diving 1/3s who has a massive gas loss at max penetration. So what's wrong with diving 1/2s in that case?

So you've got 165x2+77 cuft = 407 cuft
If you dive each 121 to 1/2 you use 165 cuft of 407 is 40.5% thus violating thirds.
It takes you 165 cuft to get in and you have only 159.5 cuft to get out on. If this happens at max penetration unless flow is helping you out or you pick up additional efficiency somewhere you won't make it.

The buddy bottle is not really a buddy bottle for a sidemount diver because they have independent air sources. A diver with a manifold needs a "buddy" because they don't have a totally independent air source if the manifold takes their total air supply. A manifold diver diving 121's with 330 cuft of air they count - plus 77 cuft not counted in a "buddy bottle" - has a total loss of gas at max penetration diving only to 3rd's just used 110 cuft getting in and has only 77 cuft to get out : not going to make it.

The for a manifolded diver even diving only to thirds the buddy bottle would have to be as large as the backmount bottles for it to work.

For a sidemount diver the third bottle would have to be 1/2 the size of the other 2 - then if they dove to 1/2 on the two and lost one - they'd have enough in the unused tank to make up for the loss. That's not really diving "halves" though - it's just not counting all the air you started with. (The diver actually uses up to 40% of the total gas there and thus violates "thirds" but still may potentially make it on a single failure.)

There is some safety advantage to all available air sources being at near equal volume at the time of a potential failure. In the case above not breathing the 3rd tank down at all makes it better then if each tank is breathed down an equal amount. Where two tanks are breathed to half and the 3rd tank is untouched the diver has used 40% but still has as much air to get out on as it took to get there. Where if the same 40% is pulled from all 3 tanks and than the larger tank fails than the diver only has 90% of what it took to get there.

You could also say that if you had an infinite number of tanks and only lost one you could dive to halves on each then you'd still have as much gas as you came in on. Two sidemount divers could dive to 3/7ths (42.8%). 10 divers with 4 tanks apiece would be able to dive to 39/79 (49.4%).


The math doesn't matter. Just like the test done in cave class that shows that two divers sharing air in a silt out won't make it out alive diving thirds. Even without a tank failure a diver modifying thirds with these seemingly logical and mathematically correct rules will fail to make it out from a significant number of dives.

The diver in Little River in '93 "should" have made it out if there was a tank failure - according to his math. But there wasn't a tank failure and he still died. He barely made it 400' back to the main line again and still 1300' from the entrance when he ran out of gas.


There may be times to modify your thirds based on the dive and conditions or route - but not on some math rule created to justify why you can make it based on flow or a single tank failure.

If there actually is a gear failure quite frequently thirds isn't conservative enough.

There was a discussion long ago where someone said they though explorers pushed their thirds and their response was: No - they pad their thirds with extra conservatism.

[Mark Dougherty]

At the turn point the safest thing is to have approximately equal volumes of gas in each cylinder. Then a loss of one system results in a "worst case" which is still manageable.

Breathing your two sidemounts down to halves while carrying a full buddy bottle is not a good strategy. If your buddy bottle fails, you now have only just enough gas to get out. Since SPGs are not that accurate and a lot of demand valves don't work when you get down to the last few PSI/bar, you could conceivably be left without enough gas to get home.

In fact in many situations it's better to breathe the stage tanks down slightly lower, reserving more in the sidemounts. If the stage tank fails, you have the option of dumping it, which will give you less drag and difficulties on the way out.

The "thirds rule" is only a guideline. I often modify it, but nearly always to something which is more conservative, for example diving to 1/4 rather than 1/3. This is especially the case when diving with very small cylinders. The "extra third" is not just for handling an equipment failure, it's also a reserve for other emergencies.

The only times I remember deliberately going over thirds have been in systems with several siphons and large comfortable dry passages, when diving in a team with other divers. In that situation I have reasoned that the gas I was carrying was easily enough to get me to a safe place and that between the various team members we could then reconfigure and get everyone out safely. That's kind of what Gary was saying - with several divers and lots of tanks you have greater security. However I think that argument is a lot stronger if there are comfortable intermediate airspaces (with no danger of CO2 buildup), since you can take your time to switch gear without any stress, and in the absolute worst case send someone out to go and get more gas.

Concerning buoyancy control, on any dive where buoyancy control is critical, surely you should dive with redundancy? i.e. a drysuit + BC (of whatever type) or a wetsuit, redundant BC (e.g. dual bladder wing) or a drysuit with inbuilt additional buoyancy compartment. Obviously if you have 2 direct feeds they should run off different cylinders, although you do have the option of oral inflation.

RN, there is solo cave diving training. The CDG in Britain trains all of its divers to dive solo.

Obviously all of this discussion needs to be modified if you are solo diving with a DPV. In that situation failure of the DPV may be the most critical problem and you have make sure you are carrying enough gas to get out swimming (and/or tow a second machine).

Mark

[aainslie]

Well, what the hell. Since Gary started this, let's keep it going.

The questions placed above make the first, most important thing clear - thirds was a simple rule designed to ensure that if a cave diver lost half their supply, they'd have half left.

I prefer to think of this as the "rule of doubles". Always have double the gas necessary to exit.

Let's think about how CCR divers di it. Their "rule of doubles" is to have two SYSTEMS available, each of which can get them out. They carry and cache sufficient OC gas to get them out very safely, plus they have tehir CC gas to get them out. Generally, like "thirds", the OC gas goes untouched.

This got me thinking - why not do something similar while on OC? Cache a bunch of gas that is available in an emergency, but that in general isn't moved? The NEW rule becomes, always ensure you have double the gas to get to the next STAGE.

I'm not going to go any further. But using a mix of rebreather and OC techniques changes things. Massively. The downside is you need to pre-cache a bunch of gas. This is only effective if you intend to repeat the same dive over and over again. But it reduces the amount of cylinders you need to take in and out of the wateron any given dive, and those cylinders go in and out pretty empty by normal cave standards.

But at no point do I violate the rule of DOUBLES. In fact I generally have MORE than double the gas necesary to SWIM out at any time during the dive.

Gary, the problem with people like you is that cave diving is more of a religion than a science. You believe that rules descend from heaven. I don't - I'm more than willing to rethink everything and anything. Sheck et al were fairly clever people - and the greatest thing about the rule of thirds is that it's incredibly simple. But for exploration dives, a bit of thought and planning pay off hugely.

[Gary]

Let's think about how CCR divers di it. Their "rule of doubles" is to have two SYSTEMS available, each of which can get them out. They carry and cache sufficient OC gas to get them out very safely, plus they have tehir CC gas to get them out. Generally, like "thirds", the OC gas goes untouched.

This got me thinking - why not do something similar while on OC? Cache a bunch of gas that is available in an emergency, but that in general isn't moved? The NEW rule becomes, always ensure you have double the gas to get to the next STAGE.

My post had already covered a lot so I didn't venture into the CC gas method - but I figured that someone would likely bring that up.

With rebreathers + OC bailout, the ONLY thing OC bailout is to cover a CC gas system failure. As long as the rebreather is functioning the diver will generally have many times the gas they might need to handle any emergency - plus the OC bailout to cover a loop failure. For many rebreather divers this safety margin is one of the big advantages of CC diving.

For OC diving the diver has only their planed reserve (for thirds double the gas they came in with). By trimming thirds an OC diver shorts their safety on every accident possibility. They are not bringing the same safety reserve a CC diver has.


An open circuit diver shorting thirds is taking one of the greatest safety strengths of CC diving and turning it into the greatest safety weakness for OC.

[aainslie]

nonsense. rebreathet oc reserves are based on a total loop failure.