When thirds are not enough....

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I am NOT suggesting that you need to carry an extra tank with you through the entire dive. What I am saying is that diving a stage (breathing it to roughly half, then dropping it) is better than dropping a single safety bottle at x point in the dive.

Let's build a dive plan. We want to go see some cool room 2300' into the cave. Its a little tight leading up to it, but not "titties and tanks" small, or anything approaching that. We have a few options:

a) Do the whole thing on backgas, but it puts us real close to "thirds".
b) We could do it on backgas, but drag a safety to the halfway point, and still dive "thirds' out of our backgas
c) Dive a stage, drop it at half plus a bit, and dive to the cool room on backgas with a modified "thirds" turn pressure.

If I'm interpreting your posts correctly, option B is how you would approach this dive, while C is how I would do it. We both recognize that option A doesn't give enough wriggle room.

Diving a stage gives you WAY more than enough gas to complete your dive (with 104s, you get 365cuft to play with vs 288). That excess translates into extra reserve PAST your "two thirds" for exit . If you were going to carry the bottle anyways (in the form of a safety), you might as well breath it, carry it a shorter distance, and end up with extra reserves WITH YOU farther in the cave (in the cool 2300' room). You were already going to be there with your 104s (or whatever youre using), but by diving a stage, your tanks have more gas in them, anyways. Gas in the front doesn't do you any good if you need it in the back.

As far as swimming an extra (full) bottle, you not only content with the added bulk of the tank, but the added gas in your wing needed to compensate for the gas in the bottle. Its more than just a streamlining issue, too, as you're physically having to push more mass through the water.

You also can't share a single safety bottle. A single safety bottle greatly reduces your options compared to a proper stage, or even a pair of safeties.

If your dive plan puts you close to not having enough gas to complete the dive, use your resources in a manner than gives you the MOST options in the water. Filling and carrying one...more...tank... isn't really an issue. If it is, save your nickels and hit the gym :wink:
I think we agree on the major points. The difference is that you are promoting the concepts that:
1. WAY more than enough gas is better than just "way" more gas than is needed for the dive, and
2. it's better to have the maximum possible reserve during the entire dive.

I don't disagree with either point, and all things being equal I am more than ok with that kind of over kill. There is nothing more useless than gas where you can't use it and no one will argue with that.

But everything is not neccesarily equal. In the tourist caves where this applies, "thirds" of a set of doubles is often not enough reserve, but "thirds" plus 72 or 77 cu ft is going to more than cover it. There is nothing wrong with staging the dive, except (assuming I do not want to hit the shop at lunch) I've now got 3 more stages in an already full vehicle and I am moving more equipment to and from the sink than I need to move.

Don't get me wrong, if the dive is bigger the cave is not as well known, is silty, tight, has old line, etc, I am going to lay up, stage the dive and ensure I have as much gas as possible available for exactly the reasons you suggest. I'm just not inclined to do that on every dive.

-----

The one point where we do disagree is in the concept that carrying the full safety bottle is in some way less efficient than carrying a stage due to the assumption the wing will have to be fuller to lift the heavier safety bottle and/or that the diver is carrying/pushing more mass.

Consider A and B in your example. In both cases a diver is going to need X cubic feet of gas to get to the 2300' room and back.

Since it is close to thirds with double 104's pumped to 3600 psi I'll assume we'd plan on using 94 cu ft to get there, and 94 cu ft to get back - basically diving to "thirds". That means if all goes as planned we will breath 188 cu ft of gas on the dive and that gas will weigh 14.1 pounds, all of it coming out of the backgas in the case of the safety bottle.

If we instead do it with a stage and breathe the stage to 1/2 plus 200, we'll breath 33 cu ft from the stage going in and 33 cu ft from the stage going out, with the balance of the 188 cu ft of gas needed on the dive coming from back gas (122 cu ft). In this case the 188 cu ft of gas still weighs the same 14.1 pounds.

In both cases we started with the same volume and weight of gas (carried in back gas and stage or in back gas and safety bottle) 360 cu ft, weighing 27 pounds and we are using the same 14.1 pounds of it during the dive.

The only difference is where the weight of that gas is distributed, In the case of the safety bottle dive, at the end of the dive 5.7 pounds of the gas is in the safety bottle and 7.2 pounds is in the back gas tanks. With the staged dive, 11 cu ft and .8 pounds is in the stage and 12.1 pounds is in the back gas tanks. In both case the ending gasis the same 12.9 pounds as the total weight of gas taken and used on the dive is the same.

It is true the stage is lighter than the saftey bottle, but the back gas tanks in that case are heavier by an equal amount, so as far as the weight the wing has to carry or the total mass is concerned, it is a wash. Combine identicial mass and wing drag with the arguably lower drag from a non butt light stage and the greater drag efficiency of the stage is highly suspect.

Now, consider that one diver can haul the safety bottle in and the other diver can haul it out, meaning each diver only carries the bottle for about 1/4 of the dive (assuming the safety is left half way to the 2300' room.) That's a bit more efficient than both divers carrying a stage a third of the way to the 2300' room and a third of the way back (a third of the dive in total) and it leave the safety bottle at about 1150' rather than at about 800 feet where you'd drop the stage in this scenario.

-----

Still, it just comes down to how much reserve overkill you are comfortable with and much much equipment you want to lug to increase that overkill. What matters is that you have sufficent reserve for the particular dive.
 
<snip> Combine identicial mass and wing drag with the arguably lower drag from a non butt light stage and the greater drag efficiency of the stage is highly suspect.
<snip>

I think you meant: Combine identicial mass and wing drag with the arguably lower drag from a non butt light safety bottle and the greater drag efficiency of the stage is highly suspect.
 
Sounds like something with this story isn't being told-- I find it difficult to believe that two divers can't find the mainline in JB (run better than basically anywhere else in the state), and waste almost an hours worth of gas looking. Doesn't it sound like they're not telling the whole story to you, too?

Well, for one, the story I heard didn't have it happen in King's Canyon. It was about twice that distance in. For another, they took scooters where they shouldn't have and dusted a lot of the passage out. Rather than turn around and back track, they kept pushing forward. In the area of the cave this happened, it is quite possible to be lost long enough to deplete your gas. Others have died back in this area. Finally, not being "frequent" cave divers, they're RMV likely shot through the roof when all the $h!t started rolling down hill. Yes, there are a lot of factors that played a roll in this. Would/could this situation have happened to you or me? Probably not. We wouldn't do a dive like this to begin with. If I was concerned about something like this for me, I'd probably dive 1/4s or 1/6s, but instead I cut back by 100-200 psi just because of other factors, such as SPG inaccuracy. And I cut back more as factors increase in number - DPV, solo, sidemount, sidemount cylinder removal, all of the above, etc.
 
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I was in a penetration on the wrong side of the bag.


:w-t-f:

Can you describe the logic and reasoning behind your decision to completely block your exit? I cannot think of a reason (I am after all, not a commercial diver :wink:) but I am sure you had a good reason.
 
If I understand correctly, the rule of thirds is to assure the safe retreat of two divers even if one diver suffers complete gas loss at maximum penetration.

not quite. that's sort of the shorthand explanation.

the extra gas is for emergencies - whatever the emergency may be. lost line, lost buddy, lost vis, massive gas failure, etc. it is *not* a guarantee of safe retreat of 2 from max penetration.

you'd need considerably more than 2/3 for most dive teams - i'm sure there are a few who could keep their sac before & after such an event the same, but nowhere near all, plus the difficulties of keeping your pace up tethered together like that. yes, we practice, but no, i have no illusions that one total gas loss at max penetration at thirds will mean both escape unscathed.

as an aside, thank you, pfcaj, for your calm discussion. it's nice to read.
 
To All Posters,
As a long time lurker this is possibly the single most useuful thread I've read on SB. Thank you for all the experienced opinions offered. Sorry to spam the thread.
 
To All Posters,
As a long time lurker this is possibly the single most useuful thread I've read on SB. Thank you for all the experienced opinions offered. Sorry to spam the thread.

When I started this thread, I had no idea it would take off the way it did. I have been very busy the past few days and so did not participate after the initial post, but I am not sure I would have anyway. I really enjoyed and profited from reading the posts of people who already have more experience than I will ever have, so anything I added would have just been noise in the system.

Thanks again to everyone!
 
Well, for one, the story I heard didn't have it happen in King's Canyon. It was about twice that distance in. For another, they took scooters where they shouldn't have and dusted a lot of the passage out. Rather than turn around and back track, they kept pushing forward. In the area of the cave this happened, it is quite possible to be lost long enough to deplete your gas. Others have died back in this area. Finally, not being "frequent" cave divers, they're RMV likely shot through the roof when all the $h!t started rolling down hill. Yes, there are a lot of factors that played a roll in this. Would/could this situation have happened to you or me? Probably not. We wouldn't do a dive like this to begin with. If I was concerned about something like this for me, I'd probably dive 1/4s or 1/6s, but instead I cut back by 100-200 psi just because of other factors, such as SPG inaccuracy. And I cut back more as factors increase in number - DPV, solo, sidemount, sidemount cylinder removal, all of the above, etc.
Where did the story you heard take place? Last time we were in JB, 1300psi (<40 cuft of gas) of a stage got us nearly 3500ft in the system, and that was low flow. Exiting would be faster in high flow, they still SERIOUSLY effed up if they had stage+backgas+dpv as I heard.

Wish we could get all the details on this one, it sounds like they planned to die (or maybe didn't plan at all). One downside to scootering high flow caves is that it gives a false sense of security, and people take more time on exit than they should, killing their reserves.
 
Middle Grounds/Crinoid Glory area. They also had a scooter failure during the dive, IIRC, tried to tow out but had issues with that. Yes, it was a complete CF of a dive. Poor gas planning, poor gas management (or no gas management), poor dive plan and execution.

I don't know what the gas plan was. I don't think they told anyone. But they basically went back to an area of the cave on scooter, kept the scooters with them, tried to push through a small passage that wouldn't be an issue in certain gear configurations, but they were backmount and pushing scooters ahead of them. Rather than turning around and heading back the way they came, they continued to push on through zero visibility. I'm sure there are marks all over the cave bottom where this happened if you want to find the area. They "thought" the gold line was on the other end of this small passage and it wasn't exactly there. It was during the zero visibility portion of the dive that they became separated. They spent time looking for the line and each other, eventually finding both. By that time they had already blown past their turn pressure. For some reason they chose to push their scooters out rather than leave them behind. They made it to the chimney, one made it to the top, the other was found on the bottom. The diver that found them was on a scooter, shared air with the diver at the top of the chimney and got him back to his deco cylinder then went back to find the other diver at the bottom of the chimney and did the same. That's pretty much all there is to it. At least what they admitted to.
 
They made it to the chimney, one made it to the top, the other was found on the bottom. The diver that found them was on a scooter, shared air with the diver at the top of the chimney and got him back to his deco cylinder then went back to find the other diver at the bottom of the chimney and did the same. That's pretty much all there is to it. At least what they admitted to.

Damn. I hope they bought lottery tickets.
 
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