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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. tursiops

    tursiops Marine Scientist and Master Instructor ScubaBoard Supporter

    # of Dives: 2,500 - 4,999
    Location: U.S. East Coast
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    I just did some NDL calcs using Subsurface. Again, no effect of GFlow.
    All the Subsurface NDLs were 2-4 mins shorter than those from Shearwater. However, Subsurface seems to have its own definition of NDL as the time remaining until you hit deco, so NDL starts very large, decreases until you get to depth, then decreases slowly as time goes on. I'm not happy with this comparison....apples to oranges.
    But the main point is: GFlow is irrelevant for a dive not going into deco.
     
    Jay likes this.
  2. elmo

    elmo DIR Practitioner

    # of Dives: 200 - 499
    Location: Melbourne
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    I'm curious what definition of NDL other than "time remaining until deco" you would prefer. Subsurface is calculating what a dive computer would show *during* a dive.
    To get the NDL as in the longest bottom time until you start getting decompression stops (to compare with tables or a dive computer NDL planner), use the Subsurface planner in recreational mode (with enough gas so that isn't the limiting factor) and take the dive duration when it says to start ascending. Assuming descent and ascent rates are the same (slower rates will give a longer NDL), and gf-hi is the same, it should be similar to the Shearwatere planner.
    The Suburface planner illustrates why gf-lo doesn't affect NDL if you show the ceiling. The ceiling determined by gf-lo is there, and gets deeper with reducing gf-lo, but since the ceiling clears before you reach it on ascent, no deco stop is required.
     
    Jay likes this.
  3. atdotde

    atdotde Solo Diver

    # of Dives: 200 - 499
    Location: Munich, Germany
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    Subsurface version of NDL is the "recreational mode", where it computes the maximal time at the current depth without running out fo gas and hitting a mandatory stop. I maintain that the first stop depth is determined by GFlow but of course as that approaches the surface the gradient factor slope diverges. So, admittedly, I had to play around a bit (including cranking up the ascent rate to 20m/min for the last bit). But I found a case that is influenced by GFlow: Do an air dive to 20m. Then with GFlow=30%, you have a maximal runtime of 49minutes while with GFlow=100% it becomes 50min. But I admit that's a small difference (I kept GFhigh=100%). Of course, the influence of GFhigh is much stronger since it is the part where the deco ceiling just goes away as you approach the surface and thus it is the near surface part of the ceiling.

    BTW, I wrote the Subsurface code for gradient factors and recreational mode.
     
  4. tursiops

    tursiops Marine Scientist and Master Instructor ScubaBoard Supporter

    # of Dives: 2,500 - 4,999
    Location: U.S. East Coast
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    I am not an experienced Subsurface user, but I read the manual and played with it and I believe the results I got. Please try it yourself and see if your NDLs are "similar" to those from Shearwater. Yes, I want to use an NDL as given by a dive planner, not time-remaining during a dive. Those should be the same when one reaches the bottom, no?
    My results from Subsurface were consistently 2-4 mins shorter than those from Shearwater. I could see no significant differences when I changed the descent rates.
     
  5. tursiops

    tursiops Marine Scientist and Master Instructor ScubaBoard Supporter

    # of Dives: 2,500 - 4,999
    Location: U.S. East Coast
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    Congratulations; an impressive piece of work!
    Is there a reason your "NDL" is not the same meaning as that from planners on dive computers?
     
  6. Jay

    Jay Need to dive more!

    # of Dives: 50 - 99
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    Updated list. For NDL dives:

    GFLo irrelevant
    Shearwater

    GFLo relevant
    Technical Dive Computers TDC-3
     
  7. scubadada

    scubadada Diver Staff Member ScubaBoard Supporter

    # of Dives: 1,000 - 2,499
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    Here is an example with MultiDeco using the @rsingler method with 1 second stop times, NDL being the longest time requiring only 1 second stop at 10 feet. An extreme example, comparing 100/100 to 5/100. My Dive Rite Nitek Q gives the same NDL at a given GFhi, regardless of the GFlo.
    upload_2019-1-17_15-52-38.png upload_2019-1-17_15-52-59.png

    Here is another example, using a 10 second stop time, with NDL being the longest time with no stop. Again, 100/100 compared to 5/100
    upload_2019-1-17_16-5-4.png upload_2019-1-17_16-5-18.png
     
    rsingler likes this.
  8. elmo

    elmo DIR Practitioner

    # of Dives: 200 - 499
    Location: Melbourne
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    I don't disagree that the NDL number displayed in the Subsurface data box when you hover over the profile plot differs from the NDL given by the Shearwater recreational planner, but I believe we are talking about two different interpretations of NDL. The data box in Subsurface displays the "live" depth, duration, NDL, etc. and is meant to replicate what a dive computer would show during the dive. The closer you get to the NDL, the lower the displayed number gets. The NDL in the Shearwater planner (and the PADI RDP and similar NDL tables) is the maximum bottom time before you need to do decompression stops. This is equivalent to the time at which the Subsurface planner says to start the ascent. Using that definition of NDL, I calculated NDLs using the Subsurface recreational planner for the Shearwater default gfs and compared them to the values from the Shearwater planner according to @tursiops above. Assuming descent rate of 20m/min and ascent rate of 10m/min, bottom depth of 24m (80').
    Gf 35/75 NDL 18min (Shearwater 18min)
    Gf 40/85 NDL 23min (Shearwater 22min)
    Gf 45/95 NDL 28min (Shearwater 27min)
    The are pretty close. Yes, gf-lo makes no difference.
     
  9. tursiops

    tursiops Marine Scientist and Master Instructor ScubaBoard Supporter

    # of Dives: 2,500 - 4,999
    Location: U.S. East Coast
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    Interesting. Thanks.
     
  10. admikar

    admikar ScubaBoard Supporter ScubaBoard Supporter

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    i have just tried both. 35m/9minutes Perdix and 35m/12 minutes Subsurface, both 50/80 for no deco. Perdix will not react to GFLo change while Subsurface will start deco at 16/80. Perdix will give deco above 9 minutes.
     

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