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ceiling/GF

Discussion in 'Shearwater Research' started by lermontov, Sep 27, 2018.

  1. lermontov

    lermontov Manta Ray

    # of Dives: 500 - 999
    Location: christchurch
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    i read somewhere you can miss the stop depth as outlined on your DC and go to the ceiling depth but what does the ceiling represent -is it the M value of your dive profile or the ceiling of the GF you have chosen.

    or is it simply the ceiling representation if your deco steps were smaller .

    eg you have 3m steps outlined on the DC but when you get to say 9m your only staying there because the DC tells you to but real GF ceiling is actually 7.5 m but you cant go there because its not a multiple of 3m
     
  2. Shearwater

    Shearwater Dive Equipment Manufacturer

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    Hi, @lermontov . The ceiling is not an M-value unless you are using 99/99 as your gradient factors. Your second scenario is correct. Perhaps the graph below clarifies things. The ceiling would be the dotted orange line.
    upload_2018-9-27_10-37-16.jpg
    We do not recommend breaking stop depths, even if in doing so you stay under the ceiling. There is little or no data comparing the risk differences between doing stops in 3 m intervals vs following a continuous ceiling as your ascent. Some people have talked about following the ceiling as a way to speed up your ascent during an emergency situation. The point being that it is not a standard practice and we do not know what risks this practice might bring.
     
    Dive Cochran and seeker242 like this.
  3. lermontov

    lermontov Manta Ray

    # of Dives: 500 - 999
    Location: christchurch
    729
    285
    63
    thanks for that all makes sense

    ps on a technical note you GF Lo scale should be reading vertically as the G/F is a ratio between AP and the M value at a given depth, the scale as youve drawn it is a ratio over a reducing depth :)
     
    Lineardiver likes this.
  4. rsingler

    rsingler Scuba Instructor, Tinkerer in Brass Staff Member ScubaBoard Sponsor

    # of Dives: 500 - 999
    Location: Napa, California
    2,874
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    Well, actually, it doesn't matter, if drawn properly. What Shearwater's horizontal scale represents is the intercept of a line drawn to an unseen vertical scale, with the top edge of the graph. It just happens to be horizontal because that fits better. A vertical scale would just waste unused space at the top of the chart.
     
    Scuba J7 likes this.
  5. lermontov

    lermontov Manta Ray

    # of Dives: 500 - 999
    Location: christchurch
    729
    285
    63
    agreed it doesnt matter if the G/F was say 50/50 as the scale would be correct but a I said its a technical point - its more efficient as an illustration drawn like that
     
  6. leadduck

    leadduck Barracuda

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    I beg to differ but it does make a difference, because the M-value line is not parallel to the ambient pressure line (b<1), particularly for the fast compartments that determine the first stop. In the plot above, the dotted orange line is probably close to a GF20/80 line, not GF30/80.
     
    Dive Cochran likes this.
  7. Shearwater

    Shearwater Dive Equipment Manufacturer

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    Point well taken. Thank you for bringing this to our attention. We corrected the chart. [​IMG]
     
  8. lermontov

    lermontov Manta Ray

    # of Dives: 500 - 999
    Location: christchurch
    729
    285
    63
    not just a great product but integrity and humility!
     
  9. KenGordon

    KenGordon Rebreather Pilot

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    Since this has turned into Pedants Corner, now you have GF as 30 at some depth below the start of the ascent.
     
    rsingler likes this.
  10. lermontov

    lermontov Manta Ray

    # of Dives: 500 - 999
    Location: christchurch
    729
    285
    63
    lol yes your right
     
    rsingler likes this.

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