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What’s the current trend in gradient factors?

Discussion in 'Technical Diving' started by Zubar, Jan 8, 2020.

  1. Zubar

    Zubar Active Member
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    I’ve been away from things for a bit but wondered what the current trendy GFs were. There used to be some debate on what was best. My Baltic planner and I therefore assume my ostc is set at 20/80. Will I die or just be laughed at for not keeping up with the Mark Powell’s? Yea it’s know there is always ratio but that means thinking and I’m not sure I’m up to that.
     
  2. Wibble

    Wibble Fish don't talk
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    I dive 50:80. Worked well for me and is what certain well known divers recommend.

    However... On some longish and deepish dives (~50m, 2ish hours) I’ve found myself to be somewhat shagged out and knackered afterwards. Was becoming quite hard for the drive home. So I’ve Now adjusted my deco to go shallower for the latter part of the dive, or extend the deco by 10 mins if that’s not possible. I set my plans and Shearwater to a 3m final depth. If it’s rough, even moving up to 4.5m for the last 10 mins seems to help.
     
  3. Vanny

    Vanny Well-Known Member

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    50/85. Some good info on the likes of you tube from Simon Mitchell and co.
     
  4. splinter

    splinter Active Member

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    50/80 for me. Dives down to 45m and around 90 mins or so.

    Sent from my SM-G950F using Tapatalk
     
  5. Zubar

    Zubar Active Member
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    Cheers fellas. I’ll do a bit of research.
     
  6. Wibble

    Wibble Fish don't talk
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    What have you been using? It seems that the 50:80 "consensus" differs from those who like to stay deeper, longer , e.g. 30:90 or even higher.

    For me it was the maffs wot did it. Did some calculations for a couple of dives and it was obvious that 50:80 was optimal in terms of getting you up shallower, quicker, but leaving you decompressing sufficiently. The bottom line was my question to a well-known expert who does deep, long dives... "what GF do you use?". "50:80". If it's good enough for him in the extreme dives, it's certainly good enough for me in my far less challenging dives.
     
  7. Vanny

    Vanny Well-Known Member

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    Wibble ; to clarify are you saying current divers doing deeper longer are still utilising GF’s of 30/90 ? Can you quantify deeper / longer.

    I would say divers in the normixic (60 mtr) and poss into shallower hypoxic (60+ mtr) range particularly in the uk are more aware of the latest thinking / research into deco theory. hence I would expect many have reviewed GF choices, resulting in higher low numbers and a considered approach to the high GF.
     
  8. Wibble

    Wibble Fish don't talk
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    Have seen some divers still on the deeper GFs, at least that’s what they’ve posted online. Nice to see the consensus here.
     
  9. Vanny

    Vanny Well-Known Member

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    From diving Croatia the past few summer hols , meeting divers from across Europe it’s surprising how some practices vary, soooper slow ascents , really deep stops on sub 50 mtr dives , air dil at 40-50 mtr.
     
  10. Doomanic

    Doomanic Dinosaur Wrangler
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    100/100 and a pack of these;
    [​IMG]
     
  11. Nick Ward

    Nick Ward Active Member

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    65/85 for me... there was an interesting article I read about the ratio between the two values... will see if I can find the link...
     
  12. Nick Ward

    Nick Ward Active Member

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  13. Wibble

    Wibble Fish don't talk
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    What a poor article that seems to be opinion and not research. Although published in May 2019, it's not as up-to-date as the information the well-known experts in the field are now saying.

    Seems to promote 70:83 which is a higher GF-low than most use.

    This statement isn't comparing like with like: a computer that's been logging the exact dive profile + with an algorithm which will continuously adjust the stop times, compared with a rough ready-reckoner.
     
  14. Nick Ward

    Nick Ward Active Member

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    opinion - possibly, but I think the amount of research he has done more than qualifies him to give an opinion...

    yes - it does seems a bit high... I tend to prefer a GFlow of 65... but given his knowledge and experience, I suspect he's not just chosen it on a whim...
     
  15. Wibble

    Wibble Fish don't talk
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    Mention of VPM / bubble models. Might be mapping their results (which probably influenced his favoured "tried and true" deco tables) to GFs which ends up with an odd GF-low.
     
    #15 Wibble, Jan 10, 2020
    Last edited: Jan 10, 2020
  16. Tribal Chestnut

    Tribal Chestnut Well-Known Member
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  17. Wibble

    Wibble Fish don't talk
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    As I understand it, the higher the numbers are, the closer they are to "pure Buhllmann" (100%), and therefore are statistically *more likely* to result in a bend.

    The effect of changing GF-low is a lower value keeps you deeper for the first stop (and therefore the argument goes that you're still ongassing which will increase your deco time). Increasing GF-low puts the first stop shallower, but could statistically put you closer to bubble formation.

    The effect of changing GF-high is changing the final stop time, e.g. 6m. Reducing it increases the final stop time, reducing increasing it reduces the time.

    So, for example,
    • 30:70 will keep you deeper longer which increases your overall deco time. The 70 will lengthen the final stop.
    • 50:80 will get you up to the first stop to start offgassing, but gets you out quicker
    • 70:83 gets you up but means you are reducing your safety margins for bubble formation. You still have to hang around to let the slow compartments off-gas.
    • 95:95 is basically getting out as quickly as possible but with minimal statistical safety margins. If in the shit, better to take the chance of a bend than drown.
    There must be some spreadsheets around which have run the numbers to compare the resulting deco times. Certainly when I did my training and ran the numbers I found 50:80 was about the optimum.

    At the end of the day, Buhllmann generally works even though it's a pure mathematical model that doesn't take into account your personal circumstances nor conditions. Alas sometimes it doesn't always work as people still get "undeserved bends". Hence most people consider extending their final stop or take a very slow final ascent, especially if they consider there's extenuating factors (cold, difficulties, fitness, etc.)

    Edit: reducing/increasing!
     
    #17 Wibble, Jan 10, 2020
    Last edited by a moderator: Jan 10, 2020
  18. Nick Ward

    Nick Ward Active Member

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    a GFlow of 50 supposedly still allows stops that are deeper than optimal - certainly for deeper dives...
     
  19. jb2cool

    jb2cool Moderator
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    Clear as mud, thanks.

    I think you mean a lower GF-high number increases the final stop time and a higher GH-high figure reduces final stop time.

    They all get you up to the first stop to start offgassing, the difference is where this first stop is.

    Define optimum, this is the difficulty. 100/100 will give you the shortest deco and you can plumb in a GF like 5/50 which would give you a ridiculously long deco, 'optimum' is somewhere in between, it's all a matter of taste and what risk appetite you have.

    If you've never been bent then the GFs you currently use are working well enough for you, if you have been bent after surfacing then maybe lower your GF-high a bit to extend the final stop could be sensible. If you felt a bend coming on while you were still in the water then maybe lower your GF-low to extend the intermediate stops a bit but you could otherwise try raising this to speed up your intermediate stops.

    I currently dive 30/85 and think i might drop my GF-High a bit. I think i've been bent a few times (never confirmed) so think a slightly longer final stop would be worthwhile, i'll likely drop this to 80 to see how i get on but wouldn't rule out dropping this further. I'll likely leave my GF-Low untouched as i mainly ignore this and dive to GUE/Ratio Deco rules instead and regularly see my intermediate stops on the computer clear before the GUE/RD rules say to ascend. I think being able to work deco out in my head is a valuable backup so i'll accept a slightly longer deco for this benefit.

    I think Wibble's/Splinters suggestion of 50/80 is a pretty good one.
    I think Nick's suggestion of 65/85 is a little too aggressive for my liking.
    I think Tribal Chestnut's suggestion is 50/70 is a little to conservative for me at the minute but if solo diving then i can see the merit in this.

    Maybe Suunto was right all along.

    This is all the waffling of a madman (me) though and i've started on the wine so take it with a pinch of salt.
     
    #19 jb2cool, Jan 10, 2020
    Last edited: Jan 10, 2020
  20. nickb

    nickb Well-Known Member

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    70/85 for everything.

    Ascent rates are critical for this too. A lot of people don't get anywhere near achieving the GF they think they're diving 'cos they don't get up quick enough. I try to get up the first part of the ascent at 12-15m/min at a minimum. Pissing about at 6m/min or even less is just making that low GF even lower.

    I would love to see dive computers output the actual GF in a dive log. I think it would shock a lot of people. Shearwater now offer a live 'Surface GF' which is great and I have it configured right there on my first screen.
     
    splinter, Vanny and jb2cool like this.

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