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What's your SurfGF and how does it compare to your (Rec) GFHi?

Discussion in 'Advanced Scuba Discussions' started by Jay, Aug 12, 2019.

1/ What's your average SurfGF? 2/What's your GFHi?

  1. Q1/ Average SurfGF = 40

    2.3%
  2. Avg. SurfGF = 45

    6.8%
  3. Avg. SurfGF = 50

    13.6%
  4. Avg. SurfGF = 55

    4.5%
  5. Avg. SurfGF = 60

    18.2%
  6. Q2/ My Rec GFHi = 75

    15.9%
  7. GFHi = 80

    15.9%
  8. GFHi = 85

    38.6%
  9. GFHi = 90

    11.4%
  10. GFHi = 95

    11.4%
Multiple votes are allowed.
  1. CandiveOz

    CandiveOz Barracuda

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    Hi @Jay ,
    I'm still trying to get my head around the change Shearwater made to SurfGF so I thought a graphical representation of would help explain my logic.

    Below are pair of letters on a gradient factor chart. Where the letters land on the chart is what I think the colours of GF99 and SurfGF should be:
    1. For A-A, both GF99 and SurfGF are above Buhlmann's M-value line and are both in the red.
    2. For B-B, GF99 is above GFhi so its yellow. But if you surface (instantaneously) at this point, your GF would be over 100 so SurfGF should be red in my opinion.
    3. For C-C, GF99 is below GFhi so no warning colour, but again SurfGF should be red.
    4. For D-D, the diver is at GFlo so no GF99 warning, but SurfGF should be red since the diver only commenced decompression.
    5. For E-E, the diver is close to finishing deco and SurfGF is still above GFhi so should be yellow.
    6. For F-F, the diver's GF is well below GFhi and GF100 - therefore no warning colour.
    Unless there is a misunderstanding, I don't understand the rational Shearwater made to SurfGF from your post #22.
    SurfGF.PNG
    image.png
     
    Jay likes this.
  2. stuartv

    stuartv Seeking the Light

    # of Dives: 200 - 499
    Location: Manassas, VA
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    @CandiveOz I don't know how the colors were done for GF99 and SurfGF before, so I don't know why the change. But, we can talk about how they work now.

    First, it seems a lot simpler to just say that SurfGF is now always the same color as GF99.

    Second, bear in mind that Shearwater computers are generally designed primarily for people doing deco dives. Rec diving is a limited subset. For deco dives, the SurfGF is going to be above GFHi as soon as you get almost to NDL = 0 and then keep going up from there for the remainder of the diver's time on the bottom.

    So, if SurfGF coloring behaved as you're suggesting, for deco dives, SurfGF would just be red pretty much all the time. Which is really not very helpful.

    Other random thoughts:

    Conceptually, GF99 would be a negative number when you're on-gassing. It only becomes a positive number when you are ascending and starting to off-gas (i.e. when the tissue tension in your leading compartment becomes higher than ambient pressure - and I'm being a bit imprecise here for the sake of simplicity). In practical terms, we never use negative values. And we only look at the value of the leading compartment. You could be on-gassing in one compartment while off-gassing in another. So, some compartments positive and some negative, at the same time.

    When you are on the bottom and have been there long enough for your tissues to be saturated, your GF99 is 0. It doesn't go positive until you start to ascend.

    So, this color-coding of GF99 will only have any effect when you are ascending.

    If your GF99 is higher than GFHi, you have missed a mandatory deco stop. But, maybe you are doing that on purpose. E.g. perhaps you got in with GF30/70 but decided to ascend based on GF95/95 because something happened and you need to get out as quickly as possible. You could do so by monitoring GF99 and making sure it never goes above 95. When you get to your last stop, look at SurfGF and wait for it to drop below 95 to make your final ascent to the surface.

    If you did that, your GF99 (and SurfGF) would be red almost the whole way up - which CAN be okay, if you're doing it on purpose.

    If you are not doing it on purpose, the color-coding as defined is actually a bit too liberal, I think. Imagine that your GFLo is a lower number than your GFHi. E.g. you are using, for example, GF50/80 and not GF80/80. If your GF99 is higher than your GFHi, then it is way higher than your GFLo. It is way higher than the prescribed GF for every 10'/3m stop you were supposed to make on the way up to reaching your GFHi.

    In other words, if you use a more traditional GF setting, by the time GF99 shows Yellow, you have skipped at least one mandatory deco stop and possibly more than one.

    It might be more useful to make GF99 turn yellow if you have missed any deco stops. It would always be yellow when the current algorithm turns it yellow, but it would potentially turn yellow sooner.
     
    Jay likes this.
  3. CandiveOz

    CandiveOz Barracuda

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    Hi @stuartv ,
    Thank you for you comprehensive feedback.
    It may be red all the time because in reality that is what it essentially is for deco dives. To make it yellow implies that the diver is above GFhi but below GF100 which may be considered misleading (although the SurfGF number itself will be greater than 100!)

    But I take your point. For simplicity, changing the numbers together makes sense. If you look at the C-C ascent profile; both GF99 and SurfGF would start off white, then both would turn yellow as the diver ascends, then both would turn red as the diver passes above Buhlmann's M-value line.

    I agree with your suggestion/recommendation and have adjusted the chart to reflect your line of thinking.
    Thanks again :)
    SurfGF2.png
     
    Jay and stuartv like this.
  4. EFX

    EFX Solo Diver

    # of Dives: 200 - 499
    Location: North Central Florida
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    I don't understand the change in colors Shearwater made for SurfGF. Regardless of the color change I think the concept is great. I think the first graph is best. GF99 should not be yellow when above your planned deco stops (your dotted line in the graph). That information is already shown by the deco stop depth and time flashing red on the dive computer. A yellow GF99, which is your current GF (not the planned GF) means that not only did you exceed your current stop but you exceeded the GFhi. The SurfGF shows what would happen if you could instantly ascend to the surface. It is not showing you your current CTC (Controlling Tissue Compartment) status. It's a future status based on a what-if scenario.

    Running different profiles on my ss (spreadsheet) I noticed that for light deco the %AoM (SurfGF) would be in the yellow (like the first graph). It was only for more deco time that %AoM was red and then generally only for the initial ascents and early deco stops. Of course, this varies with the profiles. As StuartV said above as you progress through the deco stops and the tissues continue to offgas the SurfGF will decrease toward your GF99. For those of you who are new to some of the dissolved gas decompression theory the GF changes incrementally from GFLo to GFHi as deco progesses. If you are following the computer's deco ascent schedule the GF99 should match the GF for the planned deco stop.

    For those of you using my ss there is no GF99 because there is no real-time calculations -- only the segment GF which is based on the calculated deco ascent schedule. The %AoM (like SurfGF) is based on an instantaneous (or, if you like, worst case emergency) ascent, blowing through all the deco stops.
     
    CandiveOz and Jay like this.
  5. Jay

    Jay Need to dive more!

    # of Dives: 100 - 199
    Location: Melbourne, OZ.
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    The colouring of SurfGF was as per the OP attachment, then changed as per post 22. The manual is on RevC which is the old colouring methodology.

    It's as Stuart says in his post above. The original colouring didn't aid Deco divers. In SW speak, the colour red is reserved for things that are serious and bad. The use of red in in SurfGF did not align with the use of red elsewhere (for Deco divers). I queried this earlier and here's SW reply:

    "Surface GF shows the gradient of your controlling compartment if you were to instantaneously ascend to the surface. It is not a gradient that the user ever experiences (except when actually at the surface). On decompression dives, it is quite common for this number to exceed 100% (by quite a lot in some cases). This number would then drop to a reasonable level during decompression prior to surfacing. In the previous implementation, SurfGF would turn red when over 100%. This was inconsistent with the meaning of colors on shearwater computers. Generally, red means "really really bad". In this case, decompression divers expect this number to be over 100%, so this situation doesn't fit that criterion.

    GF99, however, is the current controlling compartment gradient. That is, the gradient the diver is experiencing right now. If this goes over 100%, that definitely meets the criterion of "really really bad" as the diver's chance of getting bent is super high in this case.

    So, if GF99 is over 100% both SurfGF and GF99 will turn red. If GF99 is less than GF high, SurfGF will remain white, no matter how high it climbs. As it is not an emergency and is in fact expected on almost all decompression dives."
     
  6. CandiveOz

    CandiveOz Barracuda

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    Ok, here is my attempt :confused: to graphically represent Shearwater's colour logic for GF99 and SurfGF.
    SurfGF3.png
     
    Jay likes this.
  7. EFX

    EFX Solo Diver

    # of Dives: 200 - 499
    Location: North Central Florida
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    Thanks for the explanation. If I remember the SW manual stated that red means you better fix this now or you're going to get hurt. The SurfGF is what could happen so yeah given the SW philosophy on alarms that makes sense.
     
    Jay likes this.
  8. Fastmarc

    Fastmarc Just drifting along... ScubaBoard Supporter

    # of Dives: 200 - 499
    Location: Kingston, Jamaica
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    Question: For the computers that have a rec mode, is the shown NDL based on GF100 or the GFHI that is set (eg. GF85)? For example if you see 0 NDL and you are running GF85, is it when the GF99 (GFNOW) reaches 85?
    Or would it be when SURFGF reaches GF85?
     
  9. stuartv

    stuartv Seeking the Light

    # of Dives: 200 - 499
    Location: Manassas, VA
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    The NDL is based on your GF Hi setting.

    If NDL is 0, your GF99 could be most anything, depending on your depth - but will be less than GF Hi.

    When NDL is 0, SurfGF will be pretty close to GF Hi, depending on your depth. Slightly above GF Hi, but pretty close. The difference between the 2 represents how much you will off-gas during your ascent (if you were to make a direct ascent at 33'/10m per min).
     
    Fastmarc likes this.
  10. Fastmarc

    Fastmarc Just drifting along... ScubaBoard Supporter

    # of Dives: 200 - 499
    Location: Kingston, Jamaica
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    Is this why some who have this type of computer seem to 'ride' the NDL as they are using GF99/SURFGF to determine their exposure risk?
     

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