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GFHi - practical meaning?

Discussion in 'Advanced Scuba Discussions' started by Jay, Nov 25, 2019.

  1. Jay

    Jay Need to dive more!

    # of Dives: 100 - 199
    Location: Melbourne, OZ.
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    From my limited Rec diving experience, I 'know' that at depth when NDL=0, SurfGF ~~=GFHi (~~ bec of difference in ascent rate assumptions), then post ascending (at the computer's given rate), at the surface GFHi = current GF (GF99) = SurfGF.

    Because I never ascend directly to the surface (and especially when NDL = 0) I don't know for sure it all holds true. By definition it seems it should. I could set a low GFHi on a dive and give it spin ...

    From my limited Tec/deco knowledge, when stops are cleared, one should be surfacing at GFHi. The stops are designed to land you at GFHi on the surface right?

    But as you see in the quotes above (from another thread Recreational Ascent Rate in the last 15 feet ), Baker's 90m deco dive is replicated by @EFX, uses a GFHi of 75, and surfaces with a GF of 92%. How is that possible?

    The SS takes ~15 seconds to calculate.
     

    Attached Files:

  2. ChuckP

    ChuckP Manta Ray

    # of Dives: 200 - 499
    Location: Cozumel
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    Something ainā€™t right. I dive 45/80 - when ndl gets to zero and my first stop is shown at 30ā€™ or 20ā€™, when my deco is clear, my surfgf equals 80
     
    Jay likes this.
  3. rsingler

    rsingler Scuba Instructor, Tinkerer in Brass Staff Member ScubaBoard Sponsor

    # of Dives: 500 - 999
    Location: Napa, California
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    Yes, that bothered me. But I trusted @EFX knowledge of the math to accept that statement implicitly.
    @EFX , care to explain in further detail what you meant when you evaluated Baker's sample deco program? How do you think he prescribed GFHi of 75, yet had a SurGF of 92%?
     
    Jay likes this.
  4. EFX

    EFX Solo Diver

    # of Dives: 200 - 499
    Location: North Central Florida
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    Baker doesn't calculate SurfGF in his sample program. He calculates, using Shearwater's terminology, GF99. You can look at his program output yourself. I've included the file Decolessons which is his paper. Go to the end of that paper to see the output. The pertinent column is labeled Max %M-value. Here is an excerpt from his paper:

    "Next, we call the subroutine MVCALC (for M-value calculation) to determine the maximum Percent M-value (PMVMAX) across all 16 compartments UPON ARRIVAL AT THE TRIAL DEPTH. This represents a worst-case condition in terms of
    proximity to an M-value. This procedure is done as a second safety check in the process. The maximum Percent M-value is written to the output file for each segment during the ascent so that the user (the diver) will easily spot any discrepancy. In addition, this is valuable information for divers who wish to tailor their profiles according to individual disposition and physiology (i.e. set personal deco limits on a Percent M-value basis). The Percent M-value parameter is also used to gauge the effect of various Gradient Factors when using the Gradient Factor Method for conservatism."

    This comment, for me, indicates that the Max %m-value will not be the same upon arrival at the surface as the TC calculation at GFHi = 75% upon surfacing.

    BTW, I use Baker's formula in my spreadsheet to calculate %AoM using the Current option and I get the same results he does (92%) when I simulated his dive.
     

    Attached Files:

    Jay likes this.
  5. rsingler

    rsingler Scuba Instructor, Tinkerer in Brass Staff Member ScubaBoard Sponsor

    # of Dives: 500 - 999
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    Hmmm. Well, a GF99 of greater than GFHi might be of concern to any diver during the dive. I figure the diver's presumption (or at least MY presumption) is that when I specify GF 50/70, my maximum %M-value for the leading compartment at least, will never exceed 70% at any time during the dive.
    Is this an unreasonable assumption?

    And at the surface, my GF99 should = my SurGF, which I thought was <= GFHi.
     
    scubadada, markmud, Jay and 1 other person like this.
  6. EFX

    EFX Solo Diver

    # of Dives: 200 - 499
    Location: North Central Florida
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    Finishing the dive with GF99 <= GFHi is a very reasonable assumption and makes perfect sense. In my trials of the spreadsheet for both NDL and deco dives I found some dives finish at or below GFHi and some finish higher, but all dives finish below GFHi = 100% (M0). Why? I don't know other than the calculation that uses GF computes the ceiling from which the stop time is determined which then determines the TC pressures. To get SurfGF (%AoM with Surface selected) I divide the CTC pressure by its M0 and multiply by 100 to get the percentage.
     
    Jay likes this.
  7. Jay

    Jay Need to dive more!

    # of Dives: 100 - 199
    Location: Melbourne, OZ.
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    Thanks @EFX. In that table of Baker's at the end of the dive at a depth of 0, he computes a GF of 91% (Segment 41). The SS computes the same in row 60 (0 depth also). Both are for 0.3 of a minute. Changing the SS to "surface" it then computes 63%. But since we're at the surface then those numbers should be equal (ignoring the 0.3min of time), or very close to equal. Am I missing something? If I'm not, then it would appear to be a computational issue?
     
    rjack321 and FreeFlyFreak like this.
  8. Subcooled

    Subcooled Assistant Instructor

    # of Dives: 200 - 499
    Location: Finland
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    A GF/Hi of 85 means that a friend of mine ends up in the recompression chamber because he was 15% off the maximum and succumbed to other factors that the computer was not aware of (being cold, overweight, 40+, in bad aerobic shape, carrying heavy gear and so on).

    In theory the GF Hi means how close to the theoretical limit you dare to go (in %), but there are other factors too.
    GF Low sets the deep stops. I know that a value of 30 is preset on some computers.

    Dive computers only do an educated guess but the diver must know better.

    70 was the default, 85 recommended by friends to avoid "excessive decompression stops" (avoiding decompression stops is a stupid idea, IMO), 90 used by at least one older guy who has not been too many times to the chamber...

    I am sure that there are more knowledgeable people on this topic than me but I do know for sure that a GF/Hi=85 can get you into a hospital if you ignore all the best practices. Not everybody is young and fit.
     
  9. michael-fisch

    michael-fisch Manta Ray

    # of Dives: 2,500 - 4,999
    Location: Germany
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    High GFHigh numbers are an excellent way of displaying how big your brass balls are.:banghead:
    20 years ago I was reduced to diving a GFHigh of 125% since doing a GFHigh of 150% like Buchaly and Waldbrenner were doing wore me out.
    Now I use a GFHigh of 85% for single dives and drop it to 80% for multiple dives over a few days.
     
  10. Fastmarc

    Fastmarc Just drifting along... ScubaBoard Supporter

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    Location: Kingston, Jamaica
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    I know the replies here are more referencing deco dives when suggesting GF85 as being 'ballsie', but it seems that on basic rec computers, at least for the 1st dive they mimic GF95 (my observation with my Perdix vs my Mares RGBM. Not sure what it's like with repetitive dives yet). With this being used by people mostly ignorant of deco stress ( eg. riding NDLs), why are not more divers heading to the chambers? As was mentioned in another thread, rec divers are often experiencing more dec stress than deco divers, so shouldn't these computers be even more conservative of at least GF85?.
     
    FreeFlyFreak likes this.

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