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

[Jay]

The average GFHi vs SurfGF difference is 24. (24.2 excluding the two widest inputs; myself & ChuckP).

 

[Jay]

I dive some longer dives my first dive using EAN32, a GF set at 45/80 and almost always head to safety stop with a surgf higher than 75

If I have no redundant gas, I do not let SURGF get above 90, once it was at 94 before I realized what that truly represented but no more of that - it's what I feel comfortable safe with...

On first dives, I can easily be in the surf GF 85 range.
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Am I correct in saying that you're often running or perhaps riding NDL=0 or in 'deco' given that your SurfGF (easily in ~85 range) is greater than your GFHi (of 80)?

 

[CWK]

My Rec GF Hi is 99, but I surface when my Surface GF is 75 or less.

 

[Jay]

SurfGF 'capping' makes a lot of sense; much more sense than worrying about if your NDL is getting low for a given GF.

I guess this would alter the fast tissue / slow tissue balance in these two cases: in both cases you surface with a GF of 75% (leading tissue), but in case a/ GFHi=75%, and case b/GFHi=100%. (on a NDL dive).

On a dive, it would be interesting to see how / if the computer could recommend the most effect path to reduce you SurfGF to the desired 'cap' (if it was above). Like off-gassing efficiency. Or would the answer simply be get shallower (to 10m) and then super slow it from there (judging from the GF graph).

 

[ChuckP]

Am I correct in saying that you're often running or perhaps riding NDL=0 or in 'deco' given that your SurfGF (easily in ~85 range) is greater than your GFHi (of 80)?

I wouldn't call it riding it - I just dive and see what I want to see, pay attention to NDL, how much deco, surgf, gas remaining and where we are at in the dive. Based on past experience, as long as I feel good and diving daily, I'm comfortable in an emergency with a surfgf of 90, but certainly try and surface much lower than that.

I think it's all relative to computer settings and actual computer. I used to use a Scubapro G2 set at MB=0 and dove a little less aggressive, I think I had one dive where the NDL got below 5 minutes and I see why now. That computer set to those settings is at least 10 minutes off what a Shearwater at 45/80 is telling you.....

I gave the G2 to my son, so I've been diving it more and more trying to see where it's really at in settings - I think computer settings revolving around GF's and the addition of SurfGF are a much more accurate way to judge where your body is at in any given dive.

I use Divelog 6 as a logbook and have only played with Shearwaters Cloud. If I wanted to analyze a dive - gas consumption, ascent rates and such - I'd use Shearwater Desktop..... I looked briefly at the GF graph in the cloud and it doesn't make sense really - I have to read up on what it's telling you.... I wish there was an easy way to import from Dive Log to Cloud...

 

[Jay]

Thanks for the detail @ChuckP

The SW GF graph is graphing GF99; the GF at that particular point in time of the dive. GF99 doesn't give you much clue what value it will ultimately be at the surface, hence SW introduced SurfGF (which isn't yet included in Cloud, but will be ...).

 

[Fastmarc]

If your SurfGF is higher than your GF high, wouldn't that mean you would be exceeding the NDL for this GF setting? Would the computer show you into deco?

 

[stuartv]

If your SurfGF is higher than your GF high, wouldn't that mean you would be exceeding the NDL for this GF setting? Would the computer show you into deco?

The Teric manual describes SurfGF as "The surfacing gradient factor expected if the diver instantaneously surfaced."

I infer this to mean that it does not factor in ascent time. So, your SurfGF could be higher than your GFHi and you still not be into deco because the computer knows that, at the expected (pre-programmed) ascent rate (30 ft/min or 10m/min), you will off-gas some as you ascend, so your current GF (your GF99) will drop some and be less than or equal to GFHi by the time you actually reach the surface.

That said, SurfGF won't go very much above GFHi before you'll cross over into deco.

 

[Fastmarc]

The Teric manual describes SurfGF as "The surfacing gradient factor expected if the diver instantaneously surfaced."

I infer this to mean that it does not factor in ascent time. So, your SurfGF could be higher than your GFHi and you still not be into deco because the computer knows that, at the expected (pre-programmed) ascent rate (30 ft/min or 10m/min), you will off-gas some as you ascend, so your current GF (your GF99) will drop some and be less than or equal to GFHi by the time you actually reach the surface.

That said, SurfGF won't go very much above GFHi before you'll cross over into deco.

Thanks for the explantion @stuartv .

I've been eyeing the Genius and read this in the manual. It seems this computer may be treating SurfGF differently unless I'm not understanding it correctly.

"The current gradient factor (GF NOW) is the highest value of inert gas supersaturation among all 16 tissues of the algorithm at the present moment. The gradient factor at the surface if the diver ascends now (GF @ SURF) is the value of supersaturation that at least one tissue will reach if you were to ascend now at the allowed ascent rate disregarding any deco and safety stop"

 

[stuartv]

Thanks for the explantion @stuartv .

I've been eyeing the Genius and read this in the manual. It seems this computer may be treating SurfGF differently unless I'm not understanding it correctly.

"The current gradient factor (GF NOW) is the highest value of inert gas supersaturation among all 16 tissues of the algorithm at the present moment. The gradient factor at the surface if the diver ascends now (GF @ SURF) is the value of supersaturation that at least one tissue will reach if you were to ascend now at the allowed ascent rate disregarding any deco and safety stop"

I agree. It does sound like the Genius is doing it a little differently.

I'm not sure it really makes any practical difference, though.

Even if you were all the way down at 132'/40m of depth, the ascent time is 4 minutes. So, the difference between the Shearwater and the Genius would be however much you would off-gas during that 4 minute ascent. I.e. not much.

The shallower you are when comparing the two, the closer they should be.

Personally, my main use for SurfGF is what I'm at my last/safety stop. I look at my SurfGF while I'm hanging there and generally try to let it drop below 75 (or even 70) before I begin my final ascent to the surface. My last stop is generally going to be no deeper than 20'. So, the difference in what is shown for SurfGF between the Shearwater and the Genius at that last stop is going to be VERY small.

I also note that what the Shearwater shows results in "erring on the side of caution". Whatever it says, you will be lower than that when you actually surface (even if by the absolute tiniest of amounts), as you cannot instantaneously go to the surface (unless you can get the Enterprise to beam you there).