• 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_Antipodean, Jan 15, 2019.

  1. huwporter

    huwporter DIR Practitioner

    # of Dives: 1,000 - 2,499
    Location: Sydney, Australia.
    190
    165
    43
    Hi @stuartv , OK, firstly 20/85 was used with those profiles as a good illustration of some of the peculiarities and less-than-optimum profile features that the current GF linear steps implementation can throw up with certain profiles, not as a recommendation. The point I was actually making was that spending all that time getting up to the deco gas switch doesn't make a whole lot of sense.

    Generally, on a significant multi-stage deco ascent, the faster compartments are controlling (i.e. are closest to M-value, or have the highest gradient factor) at the start of the deco, medium-fast are controlling during the middle and medium/medium-slow by the end. Using a GF-Hi of 85 means that no compartment will be more than 85% of the M-value at the surface; the faster your ascent up to that point, then the faster the controlling compartment will be at the end of deco - the slower your ascent up to that point, the slower the controlling compartment will be.

    Personally, my preference is to limit the maximum GF experienced by the fastest tissues, while balanced against avoiding spending 'excessive' time deep to additionally load the medium-slow tissues; my entirely subjective anecdotal experience is that this leaves me less fatigued after bigger dives. The initial proposal I'd put to my team when planning 40 mins @ 65m for real would look closer to my third profile example than the first, more of a controlling of ascent rate than 'deep stops'.

    ...I also think that many of the people who say they dive, e.g. 85/85, in actual truth are not really diving 85/85 because I don't believe they achieve 9m/min ascents all the way from the bottom to the first stop. Whenever you hear comments like "my computer was set to 85/85 but the first stop had disappeared by the time I got there" - what this actually means is that the extra time they had spent on their slower ascent had effectively inserted 'deep stops' into the profile which had cleared the first 'calculated' stop during their ascent; if they'd actually done 9m/min (or, whatever their ascent rate setting was) all the way, first stop wouldn't have cleared.
     
    stuartv likes this.
  2. stuartv

    stuartv Seeking the Light

    # of Dives: 200 - 499
    Location: Manassas, VA
    7,206
    3,075
    113
    I think that depends somewhat on the computer. But, I believe your statement is correct for Shearwaters.

    Regardless, the hardcore tech divers that I know that are diving GF85/85 or higher seem to generally be aware of the need to ascend at an actual 30'/min. Actually, I think it is pretty normal for most of them to ascend much more quickly than that, from the bottom up to somewhere near their first deco stop.

    Anyway, I think we're on the same page. No further questions, your Honor. :wink: :D
     
    RayfromTX and huwporter like this.
  3. KenGordon

    KenGordon Rebreather Pilot

    2,154
    958
    113
    Stuart,

    The m-limit applies to the ratio of the inert gas partial pressure, 3.2in your example, and ABSOLUTE pressure, 2.0 here. So the over pressure ratio is only 1.6, not 2.

    Consider a gas switch to 50%, now the ratio as per your calculation would be 3.2 to 1 (50% n2 at 2bar absolute). Does the diver suddenly get bent because it isn’t o2 and not n2 being breathed?
     
  4. KenGordon

    KenGordon Rebreather Pilot

    2,154
    958
    113
    That is not the algorithm’s fault, that is a poor choice of gases given the algorithm.

    Setting GF per compartment. That Bühlmann bloke, what did he ever know about deco eh?
     
  5. huwporter

    huwporter DIR Practitioner

    # of Dives: 1,000 - 2,499
    Location: Sydney, Australia.
    190
    165
    43
    Well yeah, to repeat what I said elsewhere, that was an example designed to illustrate an extreme case, not a recommendation for how to plan that dive. I think it's still a valid observation even for less extreme cases. See also: 10,000 forum posts about how deep stops have probably been overemphasised in the past.

    (Also, that is kind of a weird response - do you suggest picking your deco gases to work around quirks in your algorithm? Particularly when it is kind of obvious that there isn't any real physiological basis behind it, it's just a mathematical quirk?)
    Well, Bühlmann didn't talk about gradient factors at all, that was that Erik Baker bloke. :D

    Since GFs are already a graft on top of Prof. Bühlmann's work, why not discuss other approaches of grafting...
     
  6. Jay_Antipodean

    Jay_Antipodean Need to dive more!

    # of Dives: 50 - 99
    Location: Melbourne, OZ.
    428
    168
    43
    Updated list. Adding Garmin Descent.

    GFLo basically does not affect NDL on a NDL dive
    Shearwater (all)
    Dive Rite's discontinued Nitek - Q (post#2)
    Deep 6 (post#93 and via PM)
    Garmin Descent (from a Descent diver via PM)

    TDC-3 <-- very provisional (see post#69)

    GF Lo affects NDL on a NDL Dive
    none yet
     
  7. dmaziuk

    dmaziuk Great White

    4,276
    1,367
    113
    Because the model is built on the assumption that fast tissues can tolerate much greater overpressure that the slow ones.

    That's what happens when you learn stuff from reading code: you gain very deep, detailed understanding of how it is done. Not so much of why or what for.
     
    huwporter likes this.
  8. stuartv

    stuartv Seeking the Light

    # of Dives: 200 - 499
    Location: Manassas, VA
    7,206
    3,075
    113
    Isn't that why they have different M-values?
     
    scubadada likes this.
  9. scubadada

    scubadada Diver Staff Member ScubaBoard Supporter

    # of Dives: 1,000 - 2,499
    Location: Philadelphia and Boynton Beach
    9,194
    4,572
    113
    Exactly
     
  10. dmaziuk

    dmaziuk Great White

    4,276
    1,367
    113
    Yes, and if you think your leading compartment at the first stop does not need more protection you set your GFs to 85/85. That is up to you.

    But what GFs were designed for is ("Confusion about deep stops", page 2, "illustrating the problem"):
    The reason he invented GF Low is to add more conservatism to this initial phase of ascent; you are not willing to let your body get up to 85% of the M-value on the way up to your first stop because you subscribe to his reasoning and believe your fast compartments need extra protection.
     

Share This Page