Thirds

[coldsmoke]

Assuming a bottle failure, if rock bottom (figured as Richard did) is 65CF, you're only really going to need something like 20CF to make it to 70 ([65-2*{1+150/33}*.75/2]).

So if double deco (due to loss of 50%) takes 54CF, and ascending to 70' takes 20CF, and you began your ascent with 65CF, you may be in gas-share city.

I assumed the discussion was rock bottom + deco on backgas based on your previous post.



Hunter

 

[Blackwood]

Answering the question: "why does it take 20CF to go from 150 to 70?"


I just made a rough assumpion about richards calcs. Starting at 65CF, I subtracted 1 minute times 2 divers at 150 to get ascending volume, then ratioed that to a single calm diver. The conservatism of RB and rounding of in-my-head math inflated the numbers over an actual depth by depth accounting.

 

[rjack321]

I assumed the discussion was rock bottom + deco on backgas based on your previous post.



Hunter

Rock bottom is defined as the amount of gas it takes to get you and an OOG buddy up to your next breathable gas (which may be the surface).

You don't add in the amount of gas it takes to deco out on backgas.

Rockbottom minus the amount of gas it takes you to ascend normally to your switch may leave enough to deco out on backgas (it typically ends up leaving enough due to the reshaping). You should plug in some realistic values for you to double check though.

So
65cf gets you and a buddy from 150ft up to 70ft (only) @ 1cf/min each.
adding gas enough to compensate for lost 50% would be 2 major failures which we typically don't plan for.

 

[rjack321]

getting back to 1/3rds...

If rock bottom is 65cf
and you have a set of al80s
rock bottom psi = 1300psi (more than a 1/3rd!)
remaining gas is 1700psi
1700/3 = 566
rounds to 500 psi

you want to descend and penetrate some wreck (not really a good idea in this case but roll with it)
3000 start
2500 turn
buddy OOG at max penetration
1000psi used to exit (unrealistically low)
exit with 1500 assuming you can exit just as fast as you entered and your consumption doesn't go up (not part of your RB or turn calcs)
ascend with your new stressed consumption
gas switch at 200psi

As you can see, 1/3rds with an ascent reserve in this ocean wreck scenario ends up being pretty impractical.

 

[Blackwood]

yah, that's an oh **** scenario, and it also neglects the gas used getting to the wreck. If it takes a while, you may want to re-figure thirds on the fly based on remaining gas (minus rock bottom).

 

[rjack321]

For the few substantive wreck penetrations I have done, we generally use a bottom stage (1300psi in, 1300psi out) and leave the entire backgas as reserve. An al80 works out to roughly MDL limits at 80-100ft. Here we are goofing around in/on the Mackenzie in Sidney BC last year.
http://www.nickambrose.com/diving/movies/2008-4-victoriabc-mckenzie-web.mov

 

[coldsmoke]

Answering the question: "why does it take 20CF to go from 150 to 70?"


I just made a rough assumpion about richards calcs. Starting at 65CF, I subtracted 1 minute times 2 divers at 150 to get ascending volume, then ratioed that to a single calm diver. The conservatism of RB and rounding of in-my-head math inflated the numbers over an actual depth by depth accounting.

I guess what added to the confusion for me was that I wasn't planning a 10ft/min ascent starting from 150'. Once I saw RJacks maths I understood where it was coming from. I didn't learn that way though.

Rock bottom is defined as the amount of gas it takes to get you and an OOG buddy up to your next breathable gas (which may be the surface).

You don't add in the amount of gas it takes to deco out on backgas.

Rockbottom minus the amount of gas it takes you to ascend normally to your switch may leave enough to deco out on backgas (it typically ends up leaving enough due to the reshaping). You should plug in some realistic values for you to double check though.

So
65cf gets you and a buddy from 150ft up to 70ft (only) @ 1cf/min each.
adding gas enough to compensate for lost 50% would be 2 major failures which we typically don't plan for.

Yes, I know. That's why I wasn't following the discussion. I misread early on which lead me to believe that people were talking about allowing for RB/Min Gas and Lost Gas at the same time.

Hunter

 

[Blackwood]

Yes, I know. That's why I wasn't following the discussion. I misread early on which lead me to believe that people were talking about allowing for RB/Min Gas and Lost Gas at the same time.

Hunter

I think the jist of the question was: does Rock Bottom necessarily ensure that you have enough for either failure (backgas OR lost deco gas)?

 

[b1gcountry]

I think the jist of the question was: does Rock Bottom necessarily ensure that you have enough for either failure (backgas OR lost deco gas)?

It seems like the answer to that is that it simply depends. Everyone uses their own formula for rock bottom depending on how much conservatism they want to calculate in. Some people like GUE deep stops, others don't. The take away message for me is just to look at all the numbers if you're diving a plan you aren't familiar with.

Tom

 

[MonkSeal]

I think the jist of the question was: does Rock Bottom necessarily ensure that you have enough for either failure (backgas OR lost deco gas)?

RB for backgas is RB for going up to next breathable gas. Next available gas has its own RB. However, the picture is not black & white, combining available gases and sharing within team gives whole picture.