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  1. #21

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    Quote Originally Posted by gschaut View Post

    Not that I have ever been tempted to bust thirds, but this just re-enforces the lesson.
    Thirds was developed with a 3 person team in mind. In that regard, with all three divers at or near thirds at the turn point, the team would have two times as much reserve gas available than was used by an out of gas diver to get to the turn point. That made "thirds" a conservative strategy even when the failure occurred right at thirds.

    Unfortunately that's not how most people dive "thirds". A large percentage of the time a two person team is involved, and with just two people if a total gas loss occurs with a diver right at thirds, the remaining reserve gas is just equal to the amount used to get there. Unless the team is just as efficient exiting as they were entering (which is unlikely) it is setting the stage for a multiple fatality.

    Consequently, I dive thirds in a two person team like I dove the old US Navy Tables - with multiple fudge factors. Even if I have a 3900 psi fill, I plan 1200 psi thirds. Anything over 3600 psi then puts 100-300 psi more in the reserve. I'll also normally turn the dive about 100 psi early, which adds another 200 psi to the reserve (100 psi less used in penetration and 100 psi less used on the exit). That brings the total reserve gas to somewhere between 300 and 500 psi. If we're in small passage cave, I'll turn even earlier based on anticipated low viz plus any delays due to restrictions. How much earlier depends on the actual dive conditions, but turning another 200 psi early means another 400 psi in the reserve, which is now around 700-900 psi.

    So when I dive thirds in a two person team I end the dive with somewhere between 1500 and 2000 psi per side. And that's in side mount where total gas loss is very unlikely.

    If you are diving anywhere near thirds in back mount, in a two person team, you're already pressing your luck and betting your life that a total gas loss will not occur anywhere near your planned max P.

    NACD Cave DPV Cert # 666: Cave DPV Anti-christ

  2. #22
    Moderator CDF-STAFF Member
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    Quote Originally Posted by Sludge View Post
    According to my instructor it's close to 100%.
    Quote Originally Posted by Nitrogenius View Post
    Not sure how to take this.. completely sarcastic or something else?
    Quote Originally Posted by MORGAN View Post
    you should plan for failure to occur at the worst possible time - at the point of maximal penetration.
    What Morgan said.

    Whoever said money can't buy love never bought a puppy.

  3. #23

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    I just planned a 160 min dive to burn test my scooter. I expected 180 min, so planned 80 out and 80 back. It died at 90 min. Luckily I was in open water, so could go shallow and cut straight across the reef to shorten the swim back, but it was still long and still sucked.

    Jim

    Sent from my SM-G930F using Tapatalk


  4. #24
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    Jun 2011
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    St Petersburg, FL
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    2,883

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    All of my gear failures(leaking hoses and whatnot) have been on the surface except for one. On my first or second scooter dive after class my HP hose ruptured within 30 seconds of turning. It was a HP hose, so no big deal, but it was at max penetration, and makes your butthole pucker for that split second you're figuring out what it is.



 

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