• Welcome to ScubaBoard


  1. Welcome to ScubaBoard, the world's largest scuba diving community. Registration is not required to read the forums, but we encourage you to join. Joining has its benefits and enables you to participate in the discussions.

    Benefits of registering include

    • Ability to post and comment on topics and discussions.
    • A Free photo gallery to share your dive photos with the world.
    • You can make this box go away

    Joining is quick and easy. Login or Register now by clicking on the button

On a NDL dive, which computers' NDLs are not affected by GFLo?

Discussion in 'Computers, Gauges, Watches and Analyzers' started by Jay, Jan 15, 2019.

  1. KenGordon

    KenGordon Rebreather Pilot

    2,311
    1,051
    113
    Because it is true.

    There is fudging going on. Previously it was claimed that that first stop is instantly gone as the next step is the surface and so the next limit is GF High. So an implementer could choose to ignore such a stop.
     
  2. dmaziuk

    dmaziuk Great White

    4,822
    1,570
    113
    Because I actually read Erik Baker's papers and Buhlmann's "Decompression" and a few sources for NDL calculation, and that's what they say. It is indeed nonsense, that is the whole point.

    And you can read them yourself, at least Erik's papers are available on-line free of charge.
     
  3. Jay

    Jay Need to dive more!

    # of Dives: 50 - 99
    Location: Melbourne, OZ.
    611
    277
    63
    Time out for 1 sec please .....

    I created this thread to simply answer the question of if NDL is affected (on a NDL dive) by GFLo, not how.

    The 'how' question is interesting, and I do enjoy reading informed views, but please in another thread.

    thank you.
     
    scubadada likes this.
  4. Jay

    Jay Need to dive more!

    # of Dives: 50 - 99
    Location: Melbourne, OZ.
    611
    277
    63
    To answer the IF question. I've emailed Mares and their initial response was "a very 'basic' answer for NDL dives the GF low affects the total ascent time to the surface, so length and number of deco stops, and the GF High controls the NDL limits" so I've gone back and asked if running two dive plans on the Genius one being 10/90 the other being 90/90 if it produces a different NDL, to which I've not yet received a response to.

    I emailed Suunto, and received a 'non-answer' so have re-asked and am still waiting.

    The TDC-3 computer does what multi-deco does.
    We know what Shearwater does and also Dive Rite's (discontinued) Nitek Q.

    I've not got around to asking other manufacturers on the list.
     
  5. scubadada

    scubadada Diver Staff Member ScubaBoard Supporter

    # of Dives: 1,000 - 2,499
    Location: Philadelphia and Boynton Beach
    9,967
    5,279
    113
    Hi @Jay_Antipodean

    Could you please summarize what you have learned so far about whether GF low affects NDL?

    BRAND YES/NO
    Divecomputer.eu/Deep 6
    Dive Soft
    Garmin
    Heinrichs Weikamp
    Mares
    Ratio
    Scubapro
    Shearwater NO
    Suunto
    Technical Dive Computers


    Thanks, Craig

    Sorry Jay, I saw your last response just after I posted. Maybe folks that have these other computers will take the time to check the planner for 10/90 vs. 90/90, or some other easy comparison to see if GF low affects NDL
     
    Jay likes this.
  6. dmaziuk

    dmaziuk Great White

    4,822
    1,570
    113
    And the correct answer is: applying Erik Baker's gradient factors to a no-stop dive is nonsense.

    But you need to understand how ZHL works, and why GF were added, to understand why that is the correct answer.

    You are absolutely right to ask the manufacturers what they've actually implemented, and I'll be really surprised if you get any answers other than the canned "GF Hi at the surface" blurb.
     
  7. stuartv

    stuartv Seeking the Light

    # of Dives: 200 - 499
    Location: Manassas, VA
    7,775
    3,514
    113
    You left out the fact that the algorithm and Baker's mods to it apply to calculating a dive plan - not maintaining a real-time calculation on the fly.

    Some reasons that makes a difference:

    Baker's code knows exactly when your dive is over and you are starting your ascent to the surface. Because the whole premise of its operation is that you tell it your entire dive, up front, and then it gives you an ascent plan. On the other hand, your dive computer does not know when you are actually starting your ascent to the surface. When you ascend from 90' to 85', your computer doesn't know whether you just started your ascent to the surface or you were just going up and over a barrel sponge.

    Baker's code does not check to see if the dive segments you put in would violate a deco ceiling. For example, you could input a dive plan of 2 legs:

    60 min @ 100'
    5 min @ 10'

    Baker's code would not tell you that you totally busted your deco ceiling during your ascent from 100' to 10'. It would only tell you what your ascent should be starting from 10' and 65 minutes into your dive.

    However, the fundamental principle applies at all times: Give the diver an ascent that allows the diver to reach the surface without exceeding the GFHi % of the M-value in any compartment. And when you are calculating that in real-time, on the fly, with no way to know when the diver is surfacing versus just ascending 1 foot, I don't see how anyone can say that is wrong to do it the way Shearwater does it - which doesn't utilize GF Lo until such time as it detects that GF Hi would be exceeded on a continuous ascent (at 30ft/10m per minute). Why calculate the first stop when you don't even need a stop?

    The number of places in Baker's code where it checks for a condition - one that IS possible - and just aborts with an error might be surprising to some people. But, Baker's code can afford to do that because it's intended to calculate a complete plan in advance. How would you like your dive computer to do that during a dive?

    Baker's code is simply not a model that anyone (in their right mind) should hold up as an example of exactly how a dive computer should implement Buhlmann ZHL-16B/C with Gradient Factors.
     
    lv2dive likes this.
  8. stuartv

    stuartv Seeking the Light

    # of Dives: 200 - 499
    Location: Manassas, VA
    7,775
    3,514
    113
    Respect. Roger, wilco.
     
  9. Jay

    Jay Need to dive more!

    # of Dives: 50 - 99
    Location: Melbourne, OZ.
    611
    277
    63
    I think the table might need to be more like no / marginal / yes ... (reason in post #46) ... bit of chicken and egg as it depends on the results.
    no - GFLo does not affect NDL on a NDL dive
    marginal - GFLo does not typically affect NDL on a NDL dive and/or the effect is very minimal
    yes - GF Lo affects NDL on a NDL Dive.

    And you're probably totally correct, but this thread is merely what does the computer output and does it change.

    Again, both those points are interesting and dare I say are a logical way to answer my OP, but they're for another thread because this thread is merely what does the computer output and does it change, and once we've built a list, then we can confidently say in future posts that GF Lo doesn't affect NDL times on NDL dives (/re GFs: NDLs are solely determined by GF Hi) and not debate that point.
     
  10. Jay

    Jay Need to dive more!

    # of Dives: 50 - 99
    Location: Melbourne, OZ.
    611
    277
    63
    I guess someone's got to make a new OP with the 'how' topic ... just not me :)
     
    stuartv likes this.

Share This Page