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

[dmaziuk]

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?

In theory you could get very close to NDL and ascend directly to the surface (no safety stop) faster than the safe ascent rate, and go over GF High that way.

 

[ChuckP]

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.

LOL - I never thought of that and something good to know when the time comes BUT

I just leave my GFhi set at 80 and dive that way, I'm older and smoke and chose to lower my risk factors when single gas source diving - If I feel good that day, I don't mind coming to the surface at 85 but generally try to keep it below 75 by just extending SS if I have to.. Extending your dive at 50' has very little effect on SurfGF, you need to get shallow or at SS to have an effect or deco gas.

SurfGF is a much more descriptive way of keeping track of where your body is at rather than looking at some NDL number - you can have a NDL of 99 but yet SurfGf of 75. I haven't dove much in the last year but when diving everyday here and doing more adventurous profiles, I paid a lot of attention to how fast SurfGf would bleed off while on back gas at different depths - just the knowledge or idea of what was happening helped plan deco dives based on gas amounts and TTS.

 

[Wibble]

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

Therefore it'll always be under my GF-hi.

(Was the SurfGF which was confusing - have seen it much higher than 240 before. Must remember to have a look at the end of the bottom phase of a dive just before the ascent into deco begins)

 

[ChuckP]

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.

One more bowl and I'd dip into my Lucky Charm stash LOL

That makes perfect sense - GF99 has always confused me and it's something I don't even look at because it confuses me. My ascents from last stop depth are painfully slow and will always be that way.

I don't think I will be able to see it or how much it effects but next dive day, I'll go from 130 to 60 and see what the numbers do but I bet it's not much - from 60 to surface would be much bigger change though and I don't need to see that to believe you LOL

 

[dmaziuk]

SurfGF is a much more descriptive way of keeping track of where your body is at rather than looking at some NDL number - you can have a NDL of 99 but yet SurfGf of 75.

I'd have to run the numbers to be sure, but I doubt that. NDL is back-calculated from GF Hi so when your NDL is at 99, back-calculated from GF Hi of 80, it is very unlikely that your surfacing GF can be anywhere near 80 (like 75) at that point.

 

[Wibble]

One more bowl and I'd dip into my Lucky Charm stash LOL

That makes perfect sense - GF99 has always confused me and it's something I don't even look at because it confuses me. My ascents from last stop depth are painfully slow and will always be that way.

I don't think I will be able to see it or how much it effects but next dive day, I'll go from 130 to 60 and see what the numbers do but I bet it's not much - from 60 to surface would be much bigger change though and I don't need to see that to believe you LOL

GF99... This should be between your GF-lo and GF-hi when you're in deco, but on the bottom it can be 0. It represents the 'stress' as a percentage from 0 to the GF-hi (or above if you've missed a stop and you're "above your ceiling").

It's supposed to give you an alternative view of how long your stop's going to last. It's far easier to use the ceiling!

(This is my simple numpty interpretation!)

 

[scubadada]

Here is a 63 min drift dive in Boynton Beach, FL. The dive starts on the outside reef, crosses over, and finishes on the inside reef. The max depth was 90 ft., with an average depth of 62 ft. It is the 2nd dive of the day on 33% nitrox. The NDL on my Teric got down to 5 min.

The GF/GF99 shows ongassing until the start of the ascent. At the safety stop it is about 32%. At the end of the 3 min stop, it is about 26%. After a 1 min final ascent, the end surface GF is 79%. The rapid rise in the GF/GF99 during the final ascent is nicely illustrated here.

 

[scubadada]

From the Teric manual. GF99=SurfGF upon surfacing

 

[stuartv]

No, it's not "personal", it's pretty much unknown. The "personal" part is how risk-averse the person is, and whether they believe that "less is safer" (which is a bit of an if as well in this case).

I completely disagree. Everyone's physiology is different. I.e. personal. The GF that works for one person does not necessarily work for another. When you dive enough, you can start to develop your own database of what works for you. Anything from "I used GFx/y on these dives and always felt fine. I used these other GFs on these other dives and always felt tired afterwards."

What GF settings are "safe" for an individual are very personal. The statistics that the manufacturers use are just a baseline for people to use as a guide when they get started diving.

I dive GF 50/70 for both no-stop and deco.
It's uncommon for me to have a no-stop dive where I surface with greater than 60%, and I try to take 2-3 min to ascend the last 15 feet where GF99 is blooming.
I am uncomfortable with GF Hi in the 80's, though many are not. My deep dives might be 3 min shorter than a buddy's with 50/70, but with Nitrox, it's more common to be gas limited than NDL time limited.

All you'll get is personal opinions. But it's interesting to see the number of tech divers who are now diving 50/70 when 30/85 used to be the standard. Change comes slowly.

My 2 cents?
Always dive Nitrox.
Try a lower GF Hi and see if your dive style is even cramped.

I just wanted to point out that if you have a GF Hi of 85 and you ascend to your final stop (safety stop/deco stop), and then stay there for extra time, your body will never see a GF99 of 85. It would only see 85 in that scenario (on an NDL dive) if you stayed down until NLD hit 0 and then did a direct ascent at 30 fpm all the way to the surface. Even a 3 minute safety stop at 15' will (most likely) drop your GF99 to the point where you'd surface at 75 or less. Partly because you will be a fair bit below 85 when you get to your safety stop and then because it will drop some during the safety stop.

GF99... This should be between your GF-lo and GF-hi when you're in deco, but on the bottom it can be 0.

If you are at your max depth, it WILL be 0. Your tissue tension has to be higher than ambient pressure for GF99 to have any meaning.

 

[rsingler]

As I have observed, and @scubadada nicely confirms, your GF99 drops about 2% per minute at the safety stop.

So if you ascend and see a SurGF of, say, 84% when you really want to surface with 70% or less,
plan on a safety stop of
84-70 = 14% required change
14%/2 = 7 min stop

(But since you set your toy for GF 90/90, you didn't exceed NDL, so there's no "required" deco stop. All you have to do is make the nosey Divemaster think you're looking at fishies instead of doing a 7 min "lite deco" stop.)