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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. stuartv

    stuartv Seeking the Light

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    Thanks for the heads up! LOLOLOL :D
     
  2. dmaziuk

    dmaziuk Orca

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    Not to be confused with the Gradient Factor Low and Gradient Factor High that you set in your computer.
     
  3. stuartv

    stuartv Seeking the Light

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    Exactly. Or the GF99 that a Shearwater shows you. Described as "The current gradient factor as a percentage (i.e. super-saturation percent gradient) 0% means the leading tissue super-saturation is equal to ambient pressure. Displays “On Gas” when tissue tension is less than the inspired inert gas pressure. 100% means the leading tissue super-saturation is equal to the original M-Value limit in the Bühlmann ZHL-16C model." [from the Teric manual]

    Or the SurfGF (Surfacing Gradient Factor) that a Teric will show you. Described as "The surfacing gradient factor expected if the diver instantaneously surfaced."
     
  4. dmaziuk

    dmaziuk Orca

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    Ewww. I can sort of see it: if you want to show them super-saturation relative to their set conservatism and relative to Buhlmann's original M-value at the same time, you have invent some language to describe it all... But really, this it the name of the sad knight's song territory.
     
  5. stuartv

    stuartv Seeking the Light

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    They aren't showing anything relative to the user's set conservatism. A GF is a GF. All GFs are relative to an M-value. A GF is a percentage of an M-value, to be more specific about said relationship.

    Shearwater is showing the user:

    the current GF (handy)
    the GF the user would have if you surfaced immediately (even more handy)
    the user's setting for the GF the user wants at their first stop (thus why it wouldn't be used if they have no stops)
    the user's setting for the GF the user wants when they surface

    They are all just GFs.

    Not really any different than if you were talking about engine oil temperature. You could talk about the current oil temp, the oil temp you'd expect if you shut off the engine and let it heat soak, the minimum oil temp you want to operate the engine at, and the max oil temp you'd operate the engine at.

    Inventing specific terms to use when referring to those different GFs doesn't seem all that radical to me.
     
  6. Jay

    Jay Need to dive more!

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    I think my choice of words "relevant/irrelevant" could cause definition issues/nuances so I think it prudent to alter the headings below.

    GFLo does not affect NDL on a NDL dive

    GFLo does not typically affect NDL on a NDL dive and/or the effect is minimal

    GF Lo affects NDL on a NDL Dive.


    Anyone have one of these computers that allow a custom GF to see if GFLo has an effect (by keeping GFHi a constant and seeing if NDLs change)?
    Divesoft
    Mares Genius


    And the cool thing about SurfGF is that it is displayed in Yellow when the current (user input) gradient factor modified M-Value is exceeded, and in Red when 100% (un-modified M-Value) is exceeded.
     
    couv and stuartv like this.
  7. KenGordon

    KenGordon Rebreather Pilot

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    Lucky this GF scheme is an open source industry standard or people might be unclear about how their computers will behave.
     
    dmaziuk and stuartv like this.
  8. KWS

    KWS ScubaBoard Supporter ScubaBoard Supporter

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    My very crude explaination.


    Tough explainations to follow but it looks like stewrtv has it right per my understanding. The part that is missing form all the write ups is that (someone correct me if I am wrong) haldean determined that a 2:1 pressure ratio (cell to ambient pressure) was the max pressure differential that could be allowed or bubbling would exist. That 2:1 ratio limit is the M line or M value. the gf's are a percentage of that ambient to M line limit you are allowing yourself to be exposed to. so for a moment lets forget about ATA and only look at it as depth for explaination purposes. If you tissues are at 75 ft tissue pressure, and you are at 75ft then the ratio is 1:1. when you go to 37.5 ft the tissue @ 75 to ambient pressure 37.5 is a ratio of 2:1. If you go to 25 ft then it is 3:1 ect. So haldean says do not exceed a 2:1. Now later technologies have now said not to exceed around 1.5:1 because of micro bubbles formed are detected with ultrasound. Still the 2:1 ratio is used to determine the M value limit. If you use a 50/x GF then the computer (if youexceed ndl) will will stop you when you get to 50% of the difference between ambient and the M value. if you set it to 30/x then it stops you at 30% of the M value. the smaller the number the sooner you reach that limit and the sooner the computer tells you to stop. 50% should put you at the 1.5:1 ratio. The numbers I used are only for explaination only and are not accurate in any means. Once you use ATA and not just depth the numbers change greatly but the general principle is the same.

    Now If you ascend slowly enough by theory your cells can keep up with you but that is not reality. Say you dive to 100 ft if the fastest compartment is 5 minutes then in 5 minutes the fastest compartment is at 50 ft 5 min later it is at 75 ft. now you go up to 30 ft. you cells are at 75 and your ambient is at 30 in 5 min the cells catch up to 1/2 the difference or go form 75 to (30+75) /2 or about 53 ft. now using depth only again 53/30 is about 1.5:1 ratio. In reality it is more like 85 to 65 ( corrected for ATA) and that gives you a roughly 1.30:1 ratio +-.

    Now lets go back to using only depth again. you can go from 100 to 50 and get a 2:1 but starting at 30 you can only go to 15 and have a 2:1. the shallow really needs the aATA accuracy adn not depth only. Since 30 ft is 2 atm and surface is 1 atm there is you 2:1 ratio. You do a safety stop at 15 ft and the difference now reduces to about a 1.5:1 ratio. you tissues will always lag your actual depth. The surface interval is the period to allowthe tissues to catch up while on the surface. Your gf for the surface is always greater than the the deep gf so you have gf's like 30/85. Your computer has levels of conservatism max agressive would be 100/100 max cautious would be 1/1. actual gf's would be conservative low = 50/90 med 40/80 and hi pg 30/70. On a no ndl dive the low number is not used. the hi number is used to determine the NDL time the lower the number the shorter the NDL time is. At 100 ft low may equal 20 min med =15 and hi is 10 minutes. these numbers are not correct but it is good enough for cause and effect demonstration purposes. Now a no ndl dive allows you to go directly to the surface. If you use a gf of 30/60 the ndl tme is such that if you go directly to the surface at prescribed rates you will not exceed the 60 in the 30/60 gf set. Different makers use different gf settings for the conservatism mechanism levels and that should explain why different makers and users all get different time to NDL on their computers. For grins if you are at 100 ft 4 atm to hit the 2:1 ratio you have to go instantly to 33 ft or 2 atm. by the time you get to 33 ft it is 2 minutes and you cells have moved to perhaps 65 ft. using atm when you get to 20 ft you are looking at 3 atm vs 1.6 atm. well under the 2:1 ratio and with in the 1.5:1 ratio for micro bubbles. 3 min at safety stop and the micro's are gone and the cells are probably at 30 ft so you can go straight up and not exceed the 1/5:1 ratio or the 70 setting in the gf 30/70.

    Hope this very gross explaination helps.
     
  9. dmaziuk

    dmaziuk Orca

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    Here's the problem: there is no formula for NDL.

    1. You simply add a minute of gas loading and calculate "safe" ascent to the surface. If you hit the M-value in one of your tissues, that minute is your NDL. If you don't: add another minute and repeat. Until you do.

    2. Buhlmann model calculates M-value as the "ascent ceiling", or the deco stop depth. (It's simply more convenient than using Haldane-style 2:1's. Baker's "M values" paper shows how to trasnform one to the other: the underlying equation's the same.)

    3. So when you're computing the NDL as per #1, using Buhlmann's formula as per #2, you are actually looking for the first decompression stop depth.

    4. By Baker's definition, First Stop is where GF Lo applies.

    Ergo, you you are using ZH-L + GF model to compute the NDL, and not using GF Lo, you are doing it wrong.

    5. Also by Baker's definition, GF Hi is what applies at the surface. So if you can ascend all the way there without violating GF Hi, you are within NDL.

    Obviously, the only way you get the same NDL from #1..4 and from #5 is if your GF Hi = GF Lo.

    :popcorn:
     
  10. tursiops

    tursiops Marine Scientist and Master Instructor ScubaBoard Supporter

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    Nonsense, and you know it. Hence your popcorn emoji. Why do you post this crap?
     
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