Re-Evaluating My GF
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I've had some vey similar conversations recently. When I was in FL around New Years I talked to 3 different very experienced tech and CCR instructors. One dives GF85/85, one uses GF95/95, and one dives straight Buhlmann (i.e. GF100/100).
All of them said that the purpose was to get shallow as quickly as possible. But then they all hang at their last stop until the actual Surfacing GF is much lower. In effect, diving Reverse GFs.
I actually emailed Shearwater a couple of weeks ago requesting this capability as a feature request. I.e. add the ability to set a Shearwater computer to reverse GFs. You can do it (dive a reverse GF profile) manually, but having the computer set for it would allow it to predict a more accurate TTS.
I asked a well-respected deco researcher a while back "why not dive with the GF Lo set the same as the GF Hi?" His response was that there is no data to suggest that (for example) GF80/80 is any less safe than GF50/80. But, he dives GF50/80 because it is a smaller departure from previously accepted best practices. Therefore, it seems safer. I.e. safer to make smaller, incremental changes in the way we dive.
I re-evaluated my other GF recently, too. She has now moved out.... LOL
I'm sure they are very experienced and all, but it doesn't sound like great practice to me, particularly the last two, where you're riding close to or at M value all the way to the last stop and then becoming more conservative. Did they share their surfacing gradient?
That makes a lot more sense to me. Seems to me that the data suggests that it is counterproductive to force deeper stops with a very low GF low, and that is a growing consensus. But, that does not seem to me to be the same thing as saying "you should dive straight Buhlman until your last stop."
No, I don't recall them saying exactly what their normal SurfGF would be.
According to the Shearwater manual (v65) you can only change the GFHi while diving. You cannot change the GFLo. The description they give is lowering GFHi to make the dive more conservative. I don't know if you can lower it below GFLo or be able to raise it (making the dive less conservative). Anyone have experience with this?
I can understand occasionally hanging longer than what the preset GFHi would indicate. You might want to do this to add additional conservatism, i.e. if you were working harder. I hope they are not doing it as standard practice. If they are then why not set GFHi to a lower setting?
I haven't verified this myself (because I am uninterested at this juncture in flipping my GFs), but I believe @stuartv is noting that you can't set the GFHi below the GFLo on Shearwaters.
These guys want to have a high GFLo so they have absolute minimum of deeper stops. That forces them to keep the GFHi way up there because that is just the way the computer is programmed. In other words, if you want a GFLo of 95, you are forced to set the GFHi at at least 95. You can't program a 95/75. It's then up to you to add conservatism manually by extending your hang on the last stop or two and watching your Surfacing Gradient to judge when you have padded enough. At least, that is how I interpret Stuartv's explanation.
As I said before, I'm not a fan, but I think that is why they set a 95/95 but end up diving something that is more conservative, but doesn't likely line up with any straight gradient line, since it sounds like they are only padding last stop.
At 95/95 you should be padding every stop with extra 5% conservatism on top of M-values that were good enough for Dr. Buhlmann. That nitpick aside, it does indeed look like the implementation lets you either dive straight Buhlmann profile, or add extra padding to deeper stops, but you can't pad shallow ones except by watching the graphs on-the-fly.
Right. You can't set the GFHi to less than the GFLo at any time.
I have never changed my GFs while in the water, but I have read people who post about potentially switching it (from a lower value) to GF 95 in the water in a case where some emergency dictates that you get out of the water as quickly as possible.
Allowing a Reverse GF setting (e.g. GF95/50) would mean (in my mind, anyway) that the computer gives you an ascent plan that corresponds to what GF95/95 would give you - until you get to your last stop (10 or 20 feet). Then, it would hold you there until your SurfGF drops all the way to 50.
@EFX asked "why not just set the GF Hi lower?"
Well, imagine if your normal thing is the example I just gave - GF95/50. i.e. you ascend to your last stop while riding the M-line at 95%. But then you stay there until your SurfGF drops to 50. Your question is "why not just use a GF Hi of 50?" The answer is that the resulting ascent would have you spend a lot more time at deeper depths. And the whole point is to get shallow as quickly as possible.
If you use the dive planner in Subsurface to compare a dive using GF50/50 to a dive using GF95/95 and then staying at the last stop until SurfGF drops to 50, and you turn on the Heat Map display for tissue loading, I think you will see that the result is GF50/50 has you at the surface with much more inert gas loading in your slow compartments compared to using GF95/95, then staying in until your SurfGF drops to 50.
I have never changed my GFs while in the water, but I have read people who post about potentially switching it (from a lower value) to GF 95 in the water in a case where some emergency dictates that you get out of the water as quickly as possible.
Allowing a Reverse GF setting (e.g. GF95/50) would mean (in my mind, anyway) that the computer gives you an ascent plan that corresponds to what GF95/95 would give you - until you get to your last stop (10 or 20 feet). Then, it would hold you there until your SurfGF drops all the way to 50.
@EFX asked "why not just set the GF Hi lower?"
Well, imagine if your normal thing is the example I just gave - GF95/50. i.e. you ascend to your last stop while riding the M-line at 95%. But then you stay there until your SurfGF drops to 50. Your question is "why not just use a GF Hi of 50?" The answer is that the resulting ascent would have you spend a lot more time at deeper depths. And the whole point is to get shallow as quickly as possible.
If you use the dive planner in Subsurface to compare a dive using GF50/50 to a dive using GF95/95 and then staying at the last stop until SurfGF drops to 50, and you turn on the Heat Map display for tissue loading, I think you will see that the result is GF50/50 has you at the surface with much more inert gas loading in your slow compartments compared to using GF95/95, then staying in until your SurfGF drops to 50.
I would amend this a bit (and you probably didn't mean it literally). I don't think you'd ride 95 all the way to the last stop.
While the GFHi is the "surfacing" gradient, it affects more than just your last stop.
A 95/95 would simply create a parallel line offset from the M Value, offset by 5%.
But, when the GFHi it is not the same as the GFLo, it is creating a "line" with a different slope than the M value line, and it will change more than just the shallowest stop. A 95/50 would, like a "normal" GF, draw a line from 95% of M value at first stop to 50% of M value at surface. So, the actual limiting GF (the % of the M value that controls when you clear the stop) at a given depth on deco changes linearly as you ascend between the GF lo and the GF Hi.
Thus, while the biggest difference between a 95/50 and 95/95 would be at the last stop, there would be some changes to the intermediate stops as well, very small at first, but progressively larger until you got to the surface.
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