What's your SurfGF and how does it compare to your (Rec) GFHi?
- Thread starter Jay
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OP
Jay
Need to dive more!
FWIW I'm diving with 99/99 for GF. My SurfGF is usually in the 50's, sometimes 60's, seldom in the 70's. Which mean's I am diving a reverse GF*. In practical terms it's probably averaging 80/55. I tend to take it very easy from 10m and shallower, and super-slow from my 6m SS (5 min) to the surface. On deeper squarish dives to ~40m, gas isn't an issue, so as long as I achieve my SurfGF I'm not too worried about seeing NDL=0. It's dives with a shallower average depth that I need to check I've got enough gas to get my SurfGF down even though NDL is farther from the limit; with 50bar reserve, my NDL and gas limits intersect at an average depth of 16m on GFHi of 99 (SAC is 1bar/min for AL80).
I wanted to somehow assess the wisdom of this seemingly aggressive GFHi and for the occasional dives where NDL get's pushed below 5min remaining. Here's my thought process:
At max Rec depths (or slightly beyond) on Air, a 40m dive for 20min produces stops from 9m for GFLo's between 60-83 with 24-27mins of deco using a GFHi of 70. GFLo83+ is 6m stop depth.
Reducing the deco time to something closer to 'light' deco, say a 40m dive for 15mins, with a GFHi of 70, a GFLo of 65+ results in 12min of total deco starting at 6m. With a GFHi of 60, 6m stops are produced from GFLo of 70+
Given GLo has less of an effect on the depth of the first stop as max depth becomes shallower, at 30m for 25min produces 15mins of deco starting from 6m for GFLo 53+
To me, this reinforces SurfGF targeting, long SS and super slow ascent as per Recreational Ascent Rate in the last 15 feet
That in turn (perhaps) defocuses NDL at depth ... because:
The difference between GFHi of 90 & 99 is 3 mins at 30m, 2min at 40m. Between GFHi 80 & 99 it's 6 and 3mins respectively. i.e. not massive differences at depth and it's just a question of gas supply and time availability at/near SS depths. Secondly there's seems almost/no difference between doing what I'm doing and say very 'light-deco' on a lower GFHi and a higher GFLo provided I'm slow from 10m and shallower.
re Reverse GFs: *The question of whether the actual (self adjusted) m_value line I'm following is a reverse hockey-stick (like merely padding the SS), or is a modified linear mayn't be too relevant for Rec depths.
(p.s. I realise/agree on a NDL dive GFLo plays no effective role).
Thoughts? Holes in my logic? and who else dives a similar NDL/SurfGF style?
I wanted to somehow assess the wisdom of this seemingly aggressive GFHi and for the occasional dives where NDL get's pushed below 5min remaining. Here's my thought process:
At max Rec depths (or slightly beyond) on Air, a 40m dive for 20min produces stops from 9m for GFLo's between 60-83 with 24-27mins of deco using a GFHi of 70. GFLo83+ is 6m stop depth.
Reducing the deco time to something closer to 'light' deco, say a 40m dive for 15mins, with a GFHi of 70, a GFLo of 65+ results in 12min of total deco starting at 6m. With a GFHi of 60, 6m stops are produced from GFLo of 70+
Given GLo has less of an effect on the depth of the first stop as max depth becomes shallower, at 30m for 25min produces 15mins of deco starting from 6m for GFLo 53+
To me, this reinforces SurfGF targeting, long SS and super slow ascent as per Recreational Ascent Rate in the last 15 feet
That in turn (perhaps) defocuses NDL at depth ... because:
The difference between GFHi of 90 & 99 is 3 mins at 30m, 2min at 40m. Between GFHi 80 & 99 it's 6 and 3mins respectively. i.e. not massive differences at depth and it's just a question of gas supply and time availability at/near SS depths. Secondly there's seems almost/no difference between doing what I'm doing and say very 'light-deco' on a lower GFHi and a higher GFLo provided I'm slow from 10m and shallower.
re Reverse GFs: *The question of whether the actual (self adjusted) m_value line I'm following is a reverse hockey-stick (like merely padding the SS), or is a modified linear mayn't be too relevant for Rec depths.
(p.s. I realise/agree on a NDL dive GFLo plays no effective role).
Thoughts? Holes in my logic? and who else dives a similar NDL/SurfGF style?
Fastmarc
Just drifting along...
If I am diving with my local operator, it is easier for me to stay a bit longer at SS and slowly ascend, getting on the boat last. However, when I'm diving otherwise with some of my buddies, it usually from a canoe which is not anchored, so they tend to pop up from the safety stop to get to the boat and hang on, especially if the sea is choppy and there is wind. I didn't realise how quickly until I started attempting to lengthen my SS and go up slowly. I still surface last, but not as slowly as I would have liked. I'll just have see how quickly the boat is drifting and if I can follow while slowly coming up, I'll do so.
This Is an absolutely fascinating post! Well done! It's beginning to reinforce my suspicion that reverse GF's, or at least GFLo=GFHi dives with numbers from 70-85 are not an unreasonable approach to diving, especially if you have the gas and time stretch out the last stop to further decrease SurGF.
I need a little bit more of your experience, since you have measured so much of this. From your first post you stated
Now that I've followed this entire thread through to the end, I realize that
is not strictly true, since GFHi presumes a 9m/min ascent, and SurGF presumes an instantaneous ascent. It's probably insignificant for the terms of this discussion, but do you know how big the difference actually is?
Yes, I should have added that, of course.
@Jay , pick a middle of the road aggressive Rec dive; say 40m to NDL.
I was really thinking conceptually, as in
"OMG, you actually have to plan gas for that", vs
"a few minutes at best", vs
"don't even bother thinking about it."
For example, in the graphic in post #1, there is a dive to 41 m which went to an NDL of 0 minutes. If you were indeed diving GF99/99, your SurGF at the moment you left the bottom was somewhere at or above 99. Yet for that dive, your actual SurGF upon reaching the surface was in the 50s. I presume there had to have been a substantially slower average ascent than 9 m/min, when accounting for the safety stop.
While I think experimenting on yourself with GFHi's of 99 to determine this would not be the wisest course, do you have a feel for the GFHi/SurGF difference on dives where your GFHi was more conservative?
Yes, it was your post that prompted mine. Since Jay had been collecting data, I wondered if he could validate your observations and presumed conclusion, especially for long ascents, like a square dive to Rec limits.
What you said makes sense. Thanks!
FWIW, @Jay already commented on this here: What's your SurfGF and how does it compare to your (Rec) GFHi? , calling it "trivial". Just wondering if that's been tested.
surf GF is nothing more than an existing calculation based on existing data but is not displaying it. GF99 is the current tissue pressure ratio minus what is lefto0f hte cedimal point. ire a raitio of 1.73 would show as 73 if not in entirity. hte gf showed is teh same thing as teh gf n hte gf lo/gfhi only it is the current value and not a limit. It is only good when tissue is greater than ambient pressure. so if you were satuated at 66 ft ( 3atm and you wer at a safety stop 20 ft or 1.6 atm then gf99 should show 3/1.6 or 88. so you wait till you get that reading down to say under 40 so what when it reads 38 you can head to teh surface where the ratio willagain rise because of reduced abient pressure to again about 95. which is 95 percent the was from ambient to the M line. surf GF is the calculated or expected GF99 value you should see at the surfce if you chose to surface. so looking at your computer tissues are at 30 ft andyouaere at 20. the ratio is roughly 1.9 atm/1.6 for a gf99 of 1.25 the corosponding surf GF would be 1.9/1. still under teh 2.0:1 limit. this may change adn probably will if this was a repreditive dive etc. I dont know I only suspect that.
When it comes time for the safety stop you no longer have to do 20 ft and count down 3-5 minutes you go to 20 ft and count down the GF99 or surfgF untill it says you have met the tisssue pressure that is safe to surface.
IN SHORT YOUARE TGRADING IN A RULE OF THUMB FOR DATA TO EVALUATE.
You also can use this as somewhat of a replacement for thelevel of conservitism. if you dont want to get to teh surface with a gf higher than 85 then sit at safety stop until the surfGF says less than 85. This way with out changing computer settings you can just change your surface gf need on the fly instead of before each dive in the settings.
perhaps someone can explain how one can get a a surface GF of less than 60.
If you were saturated at 20 ft the ratios would be 1.6:1.6 or 1:1 and then when you hit the surface you would now be 1.6:1? unless you safety stop was at 10 ft. I can see that being able to happen but not probably by tech divers but not rec divers..
If you were saturated at 20 ft the ratios would be 1.6:1.6 or 1:1 and then when you hit the surface you would now be 1.6:1? unless you safety stop was at 10 ft. I can see that being able to happen but not probably by tech divers but not rec divers..
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