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Is this Bühlmann or do Garmin have it wrong?

Discussion in 'Computers, Gauges, Watches and Analyzers' started by ATJ, Feb 22, 2020.

  1. ATJ

    ATJ Barracuda

    # of Dives: 1,000 - 2,499
    Location: Sydney, NSW, Australia
    I was playing with the Dive Planner feature on my Garmin Descent Mk1. I'm using GF40/85.

    If I put in a 30m dive on 21% it tells me the NDL is 12 minutes. Seems low, but I can accept that as I have medium conservatism.

    I can then plan a second dive. If I put in a 60 minute surface interval and then dive to 30m on 21% again, the NDL is still 12 minutes. How can that be? Surely I still have quite a bit of residual nitrogen and the NDL should be significantly less.

    Do Garmin have it wrong, or does Bühlmann shorten the NDL on the first dive to allow for a longer NDL on a second dive.

    Interestingly, if I was to plan with DSAT tables and did a 12 minute dive to 30m on air, had a 60 minute surface interval the NDL for my second dive on air to 30m would be 14 minutes. The end result is similar even though DSAT has an NDL of 20 minutes for the first dive.
    wnissen likes this.
  2. dmaziuk

    dmaziuk Regular of the Pub

    DSAT will have NDLs similar to ZH-L16 at 95+ GFHi. ZH-L is strictly symmetrical, as in: you on-gas for 30 minutes, then you off-gas for 30 minutes, you're back at square 1. Which is how it normally works in a dive computer "plan" mode (it assumes square profile). I think DSAT is the same, with the caveat that you shouldn't do 6 tanks/day.

    60-minute SI is the key: I don't get much NDL reduction even on my scary conservative RGBM computer with those.

    Edit: Interesting: subsurface planner gives about 14 minutes at 40/85 w/o a mandatory stop @ 3m, then 18 minutes after the 3/3 stop and 60-minute SI. I'd have to think about why that is.

    Or ask someone: @atdotde? @KenGordon?

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