dive plan

[aw]

Diver 1's turn pressure would be 1700 psi on stage and doubles.

Diver 2's turn pressure would be 2400 psi on doubles.

Both would use 72 cf for penetration and another 72 cf for exit. This would leave Diver 1 with 72 cf for an emergency and Diver 2 with 94 cf (roughly).

In theory only, it appears Diver 1 could get both out of the cave alive even if Diver 1 dropped his stage early and then Diver 2 had a catastrophic gas loss at max penetration. In reality, however, 1 or both divers would most likely die.

Here's why - Diver 1 has 144 cf available in his doubles. If he uses 1/3 for penetration (after dropping stage), then that leaves only 96 cf in backgas. Diver 1 needs another 48 cf to get back to his stage. That leaves 48 cf for Diver 2. That last 48 cf is sufficient to get Diver 2 back to Diver 1's stage. But then Diver 1's doubles are completely empty. And all they have is a stage bottle with 48 cf left (which is enough to get them both out, theoretically). BUT UNLESS they were both VERY good at traditional buddy breathing, you'd have 2 divers fighting over one stage bottle. And you'd have 1 if not 2 fatalities. So many other variables not taken into account here (exit flow, stressed SAC rates, faster exit swimming rates, fumbling at stage pickup).

Best option would be to carry the stage the entire dive. Ditch the stage on exit when it goes empty, and run slick for the door sharing 7 ft hose.

[OneBrightGator]

Very good point, Alan. So you think it's safer to breathe the stage empty as your third and leave your BG as your exit/reserve? I suppose it's really no more dangerous then the traditional dive, but breathing the stage and BG down a third leaves 3 sources of gas instead of two.

There are plenty of people who dive 85s and will sling a stage to match up with 104s.

[DeWayne]

That is why I would opt to breathe the stage down (without dropping it unless empty and an emergency is at hand) and recalculate the reserves in my back gas. With the reserve in your back mounted cylinders you maintain the redundancy of having that backup reg drawing from your emergency third without having to play musical cylinders in a #### storm.

[Gary]

Sounds like a little gas matching leaves them equal or so close that 100 psi off either's thirds is basically correct. (However at least one reference indicates that this steel cylinder held 71.2 cubic feet at 2475 and became know as the standard 72 so perhaps holds less gas then assumed.)

Whether it comes as 3x79 or 2x119 the amount of air used is the same and weigh's the same. Dropping the empty 72 will cause very little difference from a buoyancy point of view since the tank is probably very close to neutral when empty. Buoyancy wise either diver is in about the same situation and should have the same concerns about becoming buoyant as their tanks are near empty.

Whether he drops the 3rd tank or not he's got reserve air for both me and himself and if he's more comfortable swimming without the stage bottle, has manifolded tanks and a long hose, etc. then he's in the same situation at that point as the guy with 119's.

However...

A diver breathing the 72 empty and dropping it is of slight concern to me because it is possible to get water in the tank and even the regulator. If he is the type of diver that typically mistreats his equipment, dives ancient and outdated 72 cuft tanks likely banded together with the wrong sized bands and who knows what sort of manifold... dives an unusual combination of tanks that are somewhat suspect in current condition (and I may have my doubts as to his skills and experience). Unless I know more about this guy and his equipment I may have other concerns about diving with him then whether he is dropping the stage or not.

Not breathing the stage empty and keeping it close does ballance out the air reserves better then dropping an empty but I don't see the dropped stage as any more of a problem then the 119's diving with a manifold and no stage and the same air reserves and dive plan - they are in nearly the exact same situation once the empty stage is dropped.

[skip]

quote: "If he is the type of diver that typically mistreats his equipment, dives ancient and outdated 72 cuft tanks likely banded together with the wrong sized bands and who knows what sort of manifold... dives an unusual combination of tanks that are somewhat suspect in current condition (and I may have my doubts as to his skills and experience). Unless I know more about this guy and his equipment I may have other concerns about diving with him then whether he is dropping the stage or not. "

Hey! I resemble that remark! Dang. you young guys always think anything more than 10 years old is ancient and outdated. a set of 72's is nice for us old skinny guys who love to squeeze into slits where only side-mounters dare to go in mudholes typically reserved for dairy cows. we don't get much deeper than 40 feet and sip air like a princess. and what's that about bands? we use rope to hold em together, so's we can just cut and run ifin we have ta. And whut manifold? I like the good ole reliable j-valves so we know when it's time to look for the way out. I her'd some folks once talk about lines or reels or sumpin like dat; must be some new-fangled thang for them girlee-divers what'ens fraid they might get lost in the dark.

-skip

[aw]

So you think it's safer to breathe the stage empty as your third and leave your BG as your exit/reserve?

Well, I'm still advocating breathing stage and doubles down to thirds. But carrying the stage throughout the dive since the other diver is not stage diving, to avoid the above scenario. In the event of that emergency, Diver 1 could donate his 7 ft hose and return to his stage until the remaining 2/3 is gone. Then dump the empty tank and continue out on his reserve backgas reg.

But you could do as you're thinking and just breathe the stage to zero for the penetration portion, if you wanted to deal with a completely empty tank from max penetration when there is no emergency. Guess you could do it either way and still have resolved the original problem.

[skip]

I don't like the idea of a completely empty tank or near empty. Seems best to leave 2/3's and drop it. It's back there waiting and you know where it is. Two second stages on it might help with a fight over the gas scenario. As long as you made sure it was dropped with adequate gas for both divers to exit from that point, both divers know it and should be relieved. If divers are air sharing already, it's really nice to know and to find a tank on the way out with enough gas for both (and only one needs it).

I'm thinking of the probability of failure of 5 tanks versus 4. If at any part of the dive, you have fewer possibilities, it seems you are not as safe?

-skip

[aw]

I don't like the idea of a completely empty tank or near empty.

If you do any real stage diving (where both are staging and you drop at 1/2 plus 200), then a near-empty stage is a norm.

If divers are air sharing already, it's really nice to know and to find a tank on the way out with enough gas for both (and only one needs it).

As I stated above, if Diver 2 is sharing off Diver 1's gas from max penetration, they will both need that stage bottle by the time they reach it.

More than one way to skin a cat . . . have Diver 2 drop a safety stage along with Diver 1's partially empty stage.

[adam0321]

reading this makes my head hurt

[tflaris]

one diver has three lp72's (full at 2475; 2250 with plus rating) and the other diver has a set of doubled 119's (full at 3442). The first diver wants to use a third of one 72 and drop it, switch to back gas (doubled 72's), or should he/she carry it all the way?

how much usable gas for thirds for both divers, what is team turn pressure? how and why? no deco, no depth concerns. we can assume all tanks are full at start of dive, but that really makes no difference, right?

a third independent stage tank for one diver (while other uses doubles) has me questioning my math....

just curious, don't try this at home!

-skip

Diver 1
72's baseline 2.7 cu/ft for every 100 psi x 2 = 5.4 for back gas
psi @ 2475

72 baseline 2.7 cu/ft for every 100 psi = 2.7 for stage
psi @ 2250

Useable gas:

Back gas
2475/3=825 round down to 800
(800/100) x 5.4 = 43.2 cu/ft

Stage
2250/3=750 round down to 700
(700/100) x 2.7 = 18.9 cu/ft

Total 62.1 cu/ft


Diver 2
119's baseline 3.45 cu/ft for every 100 psi x 2 = 6.91 for
psi @ 3442

Useable gas:

Back gas
3442/3=1147 round down to 1100
(1100/100) x 6.91 = 76.01 cu/ft

Total 76.01 cu/ft

Therefore Diver 1 is the controlling gas volume.

To convert 62.01 cu/ft for Diver 2's 119:

(62.01/6.91) x 100 = 897.39 psi

So Diver 2 w/ 119's will use no more than 897 psi or you can round down to 800 psi for safety in order to gas match with Diver 1.

Diver 1 turn pressure:

Back gas 1600 psi
Stage 1500 psi

Please feel free to double check my math. I was using the NACD Cavern/Cave student workbook as a reference.

Hope this helps.

Tony Flaris



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