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Perdix, Deco plan (real time)

Discussion in 'Shearwater Research' started by npole, Nov 23, 2019.

  1. NothingClever

    NothingClever Barracuda

    # of Dives: 200 - 499
    Location: Red Sea and Atlantic Ocean
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    I agree with you. I tried to reflect that paramount consideration when I wrote “(and have adequate back and deco gas to linger)”. I use the rule of thirds.
     
  2. JohnnyC

    JohnnyC PADI Pro

    # of Dives: 1,000 - 2,499
    Location: United States
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    A lot of people subscribe to the rule of 3rds without actually understanding where it came from and why it's used in certain circumstances. It became the de-facto gas management paradigm for technical diving, but it is woefully inadequate in many circumstances. In fact, I would posit that it really isn't conservative enough for any real situation beyond the specific, benign situation it was designed to cover.

    1/3rds came about based on a team of cave divers having similar gas volumes, equivalent entrance/exit times, speeds, etc., and was designed to provide adequate gas to exit a cave/reach a stage/deco bottle at max penetration with one team member experiencing a catastrophic loss of gas. There are quite a few circumstances where this is nowhere near conservative enough to safely accomplish a dive, as it doesn't account for flow, visibility, restriction, entanglement, decompression obligation, etc.

    Doing any sort of technical diving should be based on actual required gas calculations, which may or may not be anywhere close to 3rds. Example, you and a teammate hit 3rds on a wreck and your teammate loses all of their gas instantaneously. Assuming you have the same SAC rate, you've used 1/3rd of your gas, now you each have 1/3rd to get to your first deco gas. IF you turn exactly at 1/3rds you'll make it. What if there's an entanglement? What if you lose your first deco gas, left on the anchor line and now you have to blue water deco? Point is, relying on 3rds can get woefully inadequate really really quickly.

    Just something to think about.
     

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