Posted by MHK on April 26, 2007 at 07:56:08:|
I've cross-posted this from the other list since I believe strongly that this is a critical issue and the mistake could prove to be fatal so I hope to spark a discussion.
Let's try not to turn this into a Ken -v- me bashing thread and I think we can provide a valuable discussion here. Ken wrote in CDN the following ( BTW, let's stipulate to what I hope is an obvious typo in that I suspect he meant enter/exit with 1,000psi, not 100 psi which is what is written):
"The cave diverís rule of thirds should certainly apply here: one-third of your air for the trip in, one-third of your air for the trip out, and one-third for emergencies and contingencies. But if you started the dive with 3000 p.s.i., donít think this means youíve got 100 p.s.i. in and out. Your calculations should be based on the amount of air you have when you start your entry into the cavern. Once you hit that one-third mark, if you havenít made it halfway through, turn around. If youíre past the halfway mark, you can continue through to the end."
Ken sends a mixed message in that I believe he is correct when he suggests that the rule of thirds is applicable, but where I disagree with Ken is in his analysis regarding computing the 1/3rd's at the time you enter the "cavern" system. This is truly an overhead environment so I don't think the "cavern" name applies, but that notwithstanding you, and your team members should have discussed your "turn pressure" prior to entering the water, and the most conservative and safer approach will be to stick with the plan and not adjust calculations underwater. This is especially applicable wherein you have a team with dissimiliar tanks and varied air consumption rates. Complications such as that will lead to potential confusion at a time ( ie; 100' deep and in an OE) when you least desire it. Moreover, I also disagree with Ken when he discusses the "halfway mark", again the turn pressure of your air gauge is dispositive of the issue, not an arbitrary assessment of where you think you are penetration wise with respect to the "cavern" distance. I'm not trying to slam Ken here, but if a diver is going to dive at a 100' and put himself in an overhead environment, they need to reduce variable and confusion, so by adhering to a true rule of 1/3rd's you take the safest course of action, you exit with the most amount of reserve and you allow air pressure to control the events, not an arbitrary assessment of how much "cavern" remains.
The mistake most divers make when they try to apply a cave diver rule of thirds to an open ocean scenario is that they forget that cave rule of thirds was a methodolgy applied to linear penetration, ie; if it took me 1,000psi to penetrate 1000' into the cave system then in theory it should take 1,000psi for me to exit the same distance and reserve 1,000psi for my buddy to exit the same distance. In theory you exit the cave with no gas if the accident happens at maximum penetration.
In order to use 1/3rd's in the open ocean you need to modify it to account for the ascent. Where Ken makes his fatal mistake is that he doesn't consier this point. In numerical terms, let's assume we follow Ken's strategy and compute 1/3rd's upon entering Black Cavern and you have 2,100psi ( you can substitute any number here) but at 2,100 psi under a strict 1/3rd's you use 700 psi in and if you have a problem at max. pen. you have 700psi for you and 700psi for your buddy to exit. However that wil only get you back to the entry point, which in the instant case is 100' underwater. You need to reserve Rock Bottom/Minimum Gas and then use 1/3rd's. We call this Modified rule of 1/3rd's.
Ken, I realize Dale will nver publish anything I say so perhaps you should publish a correction in the next edition since the consequences of following the advice could prove fatal.
Hope that helps.