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GFHi - practical meaning?

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

  1. biggpa

    biggpa Angel Fish

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    Going back to the original post, I started to look at one of my dives in Shearwater Desktop Cloud and noticed that the surfacing GF99 was 77%, which is strange since my gradient factors in my Perdix AI are set to 50/85 and I did not pad the stop. I also imported the dive into Subsurface to get a second opinion.

    For those not familiar with Subsurface, the green is the calculated ceiling by Subsurface while the red is the ceiling from the computer. The red in Subsurface matches what is displayed in Shearwater Cloud Desktop.

    I have labeled some points on the dive profile and gotten the info from both Subsurface and Shearwater Cloud Desktop. Note that SurfGF comes from Subsurface while GF99 Shearwater.


    A: Start of deco obligation. SurfGF(87), GF99(On-Gassing)​

    B: End of Ceiling from Subsurface. SurfGF(85), GF99(26)​

    C: End of Ceiling from Sheatwater: SurfGF(71), GF99(8)​

    D: Surface. SurfGF(69), GF99(77)​


    The values of A and B make sense to me – beginning and end of deco obligations would mean that the SurfGF ~= GFHi (as said in the OP). However, I do not understand C or D.

    - Why did Perdix have a deco obligation end at C /SurfGF(71) instead of B/SurfGF(85)?
    - Why are Subsurface and Shearwater GFs so different upon surfacing (69% vs 77%)?

    Maybe I am misinterpreting or not understanding what I am really looking at, but any insight would be appreciated
     
  2. KWS

    KWS ScubaBoard Supporter ScubaBoard Supporter

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    To inject ,,, the surf GF is dependant on following a set ascent protocol. If that is 30 fpm and you comply,,, then the surf GF should become a reality. so if you are looking at a surf gf at 50 ft you can expect it to change if you alter deviate form the 30 fpm. I see it as a close loop of some sorts. you enter a gf hi and that will generate at say 80 ft a NDL for that depth under the understanding that you ascend as expected. If your (dont take me literall on this) if your GF Hi is 85 and you do the full NDL and asscend at 30 fpm you will hit the surface at 1.85 to 1 ratio or surf GF 1.85 or 85 if you slow your ascent to say 20 the expected Surf GF will decrease and will not be a match to the GF Hi you entered for conservatism. come up too fast and your SURF GF as corrected will be higher than expected. to starta a dive teh NDL is a calculation using expectations. during the surface some of those calculations become past and there fore a fixed value to use when calculating the surf GF. Yes all these values recalculate every few seconds but it is the change that is not shown that occures when you deviate form decent bottom and ascent times or rates. i love teh idea of using surf GF. it is a final product on how well I complied with the assumed plan. if it is high i came up too fast if it is low i came up slow or did SS time that may not have been considered.
     
  3. rsingler

    rsingler Scuba Instructor, Tinkerer in Brass Staff Member ScubaBoard Sponsor

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    I'm not sure that is completely accurate.
    As I understand it, Shearwater SurGF assumes an instantaneous ascent.
    Other manufacturers may follow your thoughts of a 30 fpm ascent.
    Therefore, your GF upon arriving at the surface should be at or below the SurGF you saw before you started up.
    In either case, SurGF remains a valuable look-ahead metric, and I'm glad computer makers are climbing on the bandwagon.
    As far as the thread topic (GFHi) goes, I have concluded that for Recreational dives, I can configure my "conservatism" with a GFHi that implies a very liberal dive, thus extending my bottom time, and then use SurGF to extend the shallow portion of my dive until my gradient factor is as low (as "conservative") as I'd like it to be.
    For rec dives, GFHi is only relevant in giving me as long a dive as I'd like to have, while gas planning based on that bottom time then allows me to be as conservative as I want to be when I'm offgassing. In other words, I'm diving "reverse GF's" recreationally. I program for 99/99, then not surface until I'm <70. My Rock Bottom Gas planning thus includes an extra ~20 min of shallow'ish fool-around time to drop from 99 to <70. If I hit Rock Bottom before NDL is "0", it's time to explore shallower. Again - Recreational dives.

    For deco dives, I think there's more data needed. NEDU and Spisni only started us down the road we should explore.
     
  4. KWS

    KWS ScubaBoard Supporter ScubaBoard Supporter

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    I havnt got the surf GF questions resolved yet as far as how other settings may or may not effect its computation, however tonight i did use the NDL planner with SS off and SS at 3 minutes. the NDL values did not change on my petrel TWO. That supports that the NDL ascent protocol is still direct ascent and does not include the SS as it should be. I had some concern that if I had SS set to 3 then from 100 ft it would use the additional 3 min SS as deco time and increase the NDL making the SS no longer a safety but a mandatory stop if you stayed the full NDL time. So now I am working on the effects of using the SS on surf Gf. If turned off would the puter take the SS as a multi depth portion of the dive resulting in being well below the direct ascent expected surf Gf. IF that is so then doing the SS is a big conservatism element in its own right allowing perhaps the dive to use a safety stop in lue of higher conservative settings. As I almost always do the safety stop I am rethinking my use of medium conservatism and gong to low. Seams that the SS is an uninended factor when it comes to computing NDL and Surf Gf. It appears that if you do a SS then you will never come near your max allowed GF art the surface. Especially if you do the recommended 5 minutes for greater than 100 ft or with in 5 min of NDL. That is fine wit me , but I think making the decision of the SS will be done by using the gf99 and surf Gf values.
     
    rsingler likes this.
  5. KWS

    KWS ScubaBoard Supporter ScubaBoard Supporter

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    I looked up the ascent rate for shearwater and it is 33 FPM so that rate should be the assumed rate for computatiions

    If you meant instantanious surface and not ascent,,,, I could believe that also since that will happen normally shallow.

    I have a few questions left for shearwater to clear up a few aspects.
     
  6. Jay

    Jay Need to dive more!

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    All answered in this thread What's your SurfGF and how does it compare to your (Rec) GFHi?
     
    rsingler likes this.
  7. KWS

    KWS ScubaBoard Supporter ScubaBoard Supporter

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    My questions rest in the other settings such as SS settings and how they affect the data. For instance a low conservative setting with a SS vs a medium setting with no SS. It appears that as the NDL planner goes there is no difference in the NDL calculations with SS on or off, so that means that off gassing only uses the 33 fpm for the direct ascent to the surface. I will have to wait to see what the surf GF says when i am at say 100 ft to see if surf GF is an instantanious magical surface number at that point or whether it considers the deco at the 33 fpm to calculate the surf Gf. That would be the case if surf Gf is considered of no use untill the safety stop depth. It follows the same general thinking that gf Hi is only achievable if you comply with the prescribed ascent rate. If you do that then Gf hi will be our surface Gf. however if you were at say 100 ft and rocketed to towards the surface at say 100 fpm would you not hit the gf Hi value prior to the surface and will hitting GF Hi va;lue call for a stop somilar to how GF Lo dies when in deco.. If the ascent is done properly if cant happen. but what if improperly done???? For me it is just insight on how the processing works. With tables the SS provided the safety buffer for you. Now we have computers. There are safety settings in force in addition to the archaic safety stop rules of thumb. Are they becoming unnecessary parts of the dive by being replaced by conservatism and how is the best way to implement both at the same time. I will be looking at surf GF to make that decision for me.
     
  8. njdiverjoe

    njdiverjoe Nassau Grouper

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    No they are absolutely not becoming unnecessary. When tables are cut, the people who make the table are deciding how conservative they want you to dive. When you select low/med/high conservativeness on your computer, you are making a decision on the maximum acceptable risk of DCS that you are willing to tolerate in the event you need to make a direct ascent to the surface. It should not include the (optional but highly recommended) safety stop, if it did then it would be a mandatory stop and now we are quickly moving away from recreational diving.
     
    rsingler likes this.
  9. dmaziuk

    dmaziuk Orca

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    Except that the empirical data doesn't really support the "more conservatism: less risk" conjecture. It is probably true, up to a point, but as far as the recorded numbers are concerned, you have X in Y risk at low conservatism, and you still have X in Y risk at low conservatism. Well, maybe X in Y+1.
     
  10. dmaziuk

    dmaziuk Orca

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    It's the other way around, actually: deco dives are the ones studied from Haldane on. It's the no-stop profiles that suck to program for and nobody quite knows how they fit into the models.
     
    rsingler likes this.

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