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

[Divin'Papaw]

Massively confused about what's meant by the SurfGF -- always understood it to mean that should you immediately bolt for the surface, that will show you the percentage of the M-value. For example, mine read SurfGF=219 on the 15m/50' stop the other day. When it drops to 80 -- my GF-hi -- the deco clears.

My GFs profile is, and always has been, 50:80.

Shearwater also captures your ACTUAL end-of-dive SurfGF. What was your GF upon surfacing. I believe that is what is being discussed. The relationship of your GFHi to your End SurfGF.

The relationship between these two numbers:

 

[scubadada]

Shearwater also captures your ACTUAL end-of-dive SurfGF. What was your GF upon surfacing. I believe that is what is being discussed. The relationship of your GFHi to your End SurfGF.

The relationship between these two numbers:

View attachment 676991

Yes, exactly. I voted in the poll that my average end surface GF is 60 and my GF high is 95. I would imagine you voted 50 and 85

 

[Divin'Papaw]

Yes, exactly. I voted in the poll that my average end surface GF is 60 and my GF high is 95. I would imagine you voted 50 and 85

I voted 55 and 85. My average is closer to 55 when you remove my BHB dives. Those really don't count!

 

[Fastmarc]

I have mine set to 45/95 to match the NDL of the computers of the divers I'm likely to dive with. I don't push my NDL though. Rarely getting to within 5 mins of it and tailor the latter part of my dive with to end with as low a SurfGF I can. My average is 55. The highest I have in my log so far is 69.

 

[ChuckP]

Massively confused about what's meant by the SurfGF -- always understood it to mean that should you immediately bolt for the surface, that will show you the percentage of the M-value. For example, mine read SurfGF=219 on the 15m/50' stop the other day. When it drops to 80 -- my GF-hi -- the deco clears.

My GFs profile is, and always has been, 50:80.

What you describe is exactly what SurfGF is - deco dives can show a very large number, well over 100% and yes when it falls to you GFHi setting, deco is cleared.

They are talking about mostly NDL dives where you might be at 100' and 2 minutes to NDL, your surfgf would be very close to your gfhi but thru the dive profile, like a reef dive or something, as you shallow up, the ndl number will get larger and the surfgf number smaller - much more so for profiled dives, not square ones.

You can have a gfhi of 80, run all the way up against your NDL but still with your profile finish the dive well below the gfhi number.

 

[Anthony A.]

What you describe is exactly what SurfGF is - deco dives can show a very large number, well over 100% and yes when it falls to you GFHi setting, deco is cleared.

They are talking about mostly NDL dives where you might be at 100' and 2 minutes to NDL, your surfgf would be very close to your gfhi but thru the dive profile, like a reef dive or something, as you shallow up, the ndl number will get larger and the surfgf number smaller - much more so for profiled dives, not square ones.

You can have a gfhi of 80, run all the way up against your NDL but still with your profile finish the dive well below the gfhi number.

So in your scenario of a rec/reef dive at 100' where you stay above NDL (say 2 mins above), then your sfgf will ALWAYS be at or slightly below your gfHi, correct?

The sfgf will only ever be higher than your gfHi if you go into devo, even 1min past?

 

[rsingler]

Yes, that is essentially correct, if you follow your computer's required ascent rate.
@Anthony A. , what the last few posters have described is a workaround designed to
1) match their Shearwater to more aggressive computers, and
2) maximize their NDL time. With a high GF Hi, you will have the longest NDL time before going into deco.

If you are diving GF Hi 95, and pushing your dive to near the NDL limit but are planning a "safety stop" until your Sur GF gets to a level you consider safe, some might say you're actually don't "light deco". For had you set your computer for GF 50/70 on that dive, you would have exceeded the NDL you were allowed at GF95.

I don't see a problem with this, apart from "diving beyond your training", but if you understand this much you are ahead of 95% of divers, and if you carry enough gas, you are getting the best of both worlds: the longest possible NDL time and a safe surface GF. That might be important if the boat crew is checking to make sure folks haven't gone into deco. It's a cheat, but gas allowing, it's perfectly reasonable.

 

[ChuckP]

So in your scenario of a rec/reef dive at 100' where you stay above NDL (say 2 mins above), then your sfgf will ALWAYS be at or slightly below your gfHi, correct?

The sfgf will only ever be higher than your gfHi if you go into devo, even 1min past?

I believe that is correct. SurGF is based on a direct ascent to surface from where you are at. It is assuming correct ascent rates and realize your actual GF will decrease as you ascend - I've only had one bowl of cereal and my brain can't split the hairs doing the math but it's dang close........

 

[rsingler]

Better have one more bowl
Almost correct.
Yes SurfGF implies an immediate ascent to the surface (or possibly instantaneous).
But "actual GF", or GF 99 as Shearwater calls it, increases on the way up.
Your SurGF decreases with time, but coming up from 100' your GF 99 will go
1, 1, 1, 2, 2, 2, 30, 70 as you approach the surface.
That last bloom is why many of us are slowing our final ascent.

 

[Divin'Papaw]

That might be important if the boat crew is checking to make sure folks haven't gone into deco. It's a cheat, but gas allowing, it's perfectly reasonable.

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