HowManyCubicFeet

[Dominick Gheesling]

Anyone want to discuss the differences between managing stages by Thirds vs. 1/2+200?

[JohnnyC]

Bobby's got a good article on his stage paradigm. It's worth taking a look. Should be able to find it here with a brief search.

[grantctobin]

http://www.cavediver.net/forum/showt...ht=Bobby+stage

[kieren]

Thirds on the stage leaves the reserve exposed to loss from just a single failure and is also not shareable. 1/2+ leaves the reserve on your back so you?ve always got it with you and protected by more redundancy.

Never any shame in leaving a bit extra in the stage as well (especially for more complex dives), but you should always remove the volume you?ve used from your available backgas before calculating thirds.


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[PfcAJ]

1/2+200 keeps the reserves with you at all times. Empty stages can be jettisoned rather than ending up with a bunch with 1000psi in them slowing you down.

[FW]

1/2+200 keeps the reserves with you at all times. Empty stages can be jettisoned rather than ending up with a bunch with 1000psi in them slowing you down.

What he said...

[rddvet]

Thirds on the stage leaves the reserve exposed to loss from just a single failure and is also not shareable. 1/2+ leaves the reserve on your back so you?ve always got it with you and protected by more redundancy.

Never any shame in leaving a bit extra in the stage as well (especially for more complex dives), but you should always remove the volume you?ve used from your available backgas before calculating thirds.


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The problem is I find many, many cave divers are either trained or assume 1/2+ 200 is just that. You still dive 1/3s on backgas and dive the stage to 1/2+200.
More times than I can count when I?ve had discussions with people pre-dive about stage management they dont realize they?re supposed to subtract from their backgas. I was taught the appropriate way by a gue diver and it seems all the gue divers subtract from back gas since I?m sure its a gue standard procedure. But people I know who took a stage class from other agencies are hit or miss on that point. And those that are ?self-taught? to dive a stage pretty ubiquitously dont subtract from backgas

Obviously thats all based on my interactions with people I?ve dove with or overheard in the parking lot. Others may have a different experience.


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[gschaut]

How about 1/2 + 200 on the stage PROVIDED, you calculate 1/3'ds of TOTAL (stage + back/side) gas for turn point?

80cuft in stage, two 120cuft (cave filled) doubles. 320cuft total. Turn at 106cuft.

That would be 40 cuft from stage going in, plus 66 cuft of doubles. That would be 27.5% of start pressure in back gas. Starting at 3600, turn at 2700 on doubles.

Did I forget to carry the 1?

[JahJahwarrior]

It wasn't explained well to me the first time either,
and I hate doing math.
Fortunately, if you are diving consistent tank sizes, then you just kind of memorize stuff, but the math work is good to stretch the 'ol gray matter from time to time.

I had a gas matching calculation for a while, I didn't think to add one for 1/2+200 math. I've added one for 1 or 2 stages, shows you how many cubic feet you need to subtract from available cubic feet on backgas and your resulting new turn pressure.

It's at the very bottom, happy scrolling!

http://divebhb.com/gas-planning/

[jcr]

The problem is I find many, many cave divers are either trained or assume 1/2+ 200 is just that. You still dive 1/3s on backgas and dive the stage to 1/2+200.
More times than I can count when I?ve had discussions with people pre-dive about stage management they dont realize they?re supposed to subtract from their backgas. I was taught the appropriate way by a gue diver and it seems all the gue divers subtract from back gas since I?m sure its a gue standard procedure. But people I know who took a stage class from other agencies are hit or miss on that point. And those that are ?self-taught? to dive a stage pretty ubiquitously dont subtract from backgas

If the point of subtracting the difference from your usable backgas is added conservatism then fine I'm all for added conservatism but there are better ways to add conservatism that don't require lots of math such as diving fourths, sixths, or keeping all your backgas in reserve. The whole point of thirds is establishing the minimum amount of gas that two divers need to exit given a catastrophic gas loss at maximum penetration. There are many situations where this would not be enough gas and frankly I question whether it would ever be enough. Nevertheless, the premise is to determine minimum gas needed to exit and then add conservatism as needed. Calculating thirds of all gas supplies including stages conflates minimum gas requirements with added conservatism. Assuming the worst case scenario of catastrophic gas loss at maximum penetration each diver needs a third to get back to their stages, which have just enough gas (1/2 + 200 psi) for each diver to exit on their own stage. 1/2+200 on stages and thirds of backgas is the absolute minimum gas needed. Now add conservatism.

Let the floggings begin.