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

[NAM001]

I believe the NDL is based on GFHi. If GF99 is the real-time indication of the CTC's pressure status then at an NDL = 0 the GF99 should be, very close to if not, GFHi. The SurfGF will be much higher as it represents an instantaneous surfacing. So, SurfGF does not take into account offgassing at predicted ascent rates.

the video on shearwater covers it really well. If you comply with the rates etc in the software then gf99 equals gf hi at time of surface. GF high controls the assent protocol through NDL and established ascent rates. gf hi sets a time where those 30 ft per minute will get you to the surface at the designated gf hi. GF99 is the current GF value and it decreases as SI progresses. I think I have that understood correctly. now if you extend your safety stop then the gf99 upon surfacing will be less than the setting gf hi. as it should be.

 

[EFX]

Thanks KWS. My issue is not with GF99. My issue is with SurfGF. What is your understanding of SurfGF?

 

[CandiveOz]

There seems to be two different implementations of surface GF based on manufacturer
Shearwater = SurfGF = instantaneous
Mares = GF @ Surf = ascend now at the allowed ascent rate disregarding any deco and safety stop

Shearwater was first off the blocks but Mares refined it.

In terms of the current leading GF compartment at depth, they are both the same; just different names:
Shearwater = GF99
Mares = GF Now

 

[Jay]

There seems to be two different implementations of surface GF based on manufacturer
Shearwater = SurfGF = instantaneous
Mares = GF @ Surf = ascend now at the allowed ascent rate disregarding any deco and safety stop

Shearwater was first off the blocks but Mares refined it.

In terms of the current leading GF compartment at depth, they are both the same; just different names:
Shearwater = GF99
Mares = GF Now

Correct, and as estimated/calc'd by @ChuckP post 58 and @stuartv post 68 SW’s SurfGF difference by not factoring in ascent is trivial (and on the conservative side).

 

[Fastmarc]

I have to say, with my increasing knowledge regarding GF and just re-reading this thread, from the pespective of this REC diver this thread has been the most helpful in expanding my understanding of the benefits of this model and role GFs play in general.
Good stuff. Keep it coming.

 

[Jay]

The more I dive being aware of my SurfGF the more I love this feature.

I'm now running my GF at 99/99, prior it was 60/90 (I'm a rec only diver). The difference in GFHi's at 30m is ~5min of NDL, so the 5min warning of a 'low NDL' works nicely.

On those (typically) deeper 'aggressive NDL' dives my surfacing SurfGF is typically in the high 50's ... in general lower than what it is for shallower dives with much higher NDLs.

 

[ScubaWithTurk]

I have learned quite a bit reading this thread so far. I can't really speak to Rec mode as I always dive my Perdix in OC Tec mode with a GF of 50/85.

As for GTR, I used to use it just to see if I could beat, much like a previous poster. Now, I never use it. I am only concerned with my cylinder pressures ( I dive mostly Sidemount) and my SAC rate. I know my SAC but it is always interesting to see how it changes based on the dive profile, what I am doing and what the conditions are at the time.

Surface GF is a great tool but like Jay (I believe it was him) said, it thinks you will teleport to the surface. I would much rather it present me with a more accurate reading based on the ascent rate of 10m/min. Now I tend to run 9m/min ascent to the first stop (dives to 30m and below I always do multiple stops even if some are only for a minute) and then run an ascent rate of 3m/min. But none the less, having that Surface GF tell me where I would end up should I run at 10m/min and surface would be quite nice.

 

[FreeFlyFreak]

Excellent thread.

I am a recreational only diver.
I analyzed all my Shearwater Cloud graphs after a recent dive trip and I realized that most of the time I am barely off gassing until i am above my 15ft safety stop then it shoots up from 15 ft to the surface.

Unfortunately in Rec mode, you can't simultaneously show GF99 and GFsurf on the main screen, so I went digging around in the Tech mode. In Tech mode you can show both in the middle line. So I left it in Tech mode so I could see both.
My intention is to use this information to plan my ascent from my safety stop

My thinking is the soda bottle analogy, anything over 1% is off gassing but often at my SS my graphs show GF99 of only 1% then i surface slowly but i reach the surface at 50%, the change is rapid in the last 15ft or so. Like cranking the top off the bottle vs easing it off.
Better to know your GF 99 and your GFsurf and use it to a) choose a safety stop depth appropriately and b) make a staged ascent from the safety stop.
Lets say 10% GF increase per minute, or maybe 1/3rds of your surfacing GF, or whatever makes for easy math.

Hell it might be just as easy to come up to let's say 5% GF which for me is often at or slightly above 15' do the safety stop there then to 1 minute at 10' and one minute at 5' before surfacing.
I dunno but I think I am going to try it and see.

All i know is the GF99 graphs coupled with how I feel, especially after cold water dives have indicated to me that the ascent from a depth where GF begins to ramp up is important to how tired i feel afterwards, some cold water shore dives I have done I swam back along the bottom into 5ft of water, GF slowly increasing. I have always felt better after those dives than the ones where I did 3 mins at 15' showing 1% GF and then gone straight up in 1 minute.

Thoughts?

 

[Jay]

Surface GF is a great tool but like Jay (I believe it was him) said, it thinks you will teleport to the surface. I would much rather it present me with a more accurate reading based on the ascent rate of 10m/min. Now I tend to run 9m/min ascent to the first stop (dives to 30m and below I always do multiple stops even if some are only for a minute) and then run an ascent rate of 3m/min. But none the less, having that Surface GF tell me where I would end up should I run at 10m/min and surface would be quite nice.

I like the concept (and practise) of a variable ascents rates (and super-slowing it from 10m ...). I don't care if my computer goes yellow (or even red) at 30m but sure do care if it's near 10m. Perhaps another wish-list idea for SW

Yeah the 'teleporting diver' didn't seem too neat, but SW did want to reflect a worst case what-if dash to the surface ... and when looking at SurfGF at depth, the error by not factoring in ascent rate (to SS) is only a couple of GFs (posts 58 & 68 estimate that), and it's conservative so it's pretty much in the 'rounding bucket. And likewise from SS to surface ...

 

[Jay]

Unfortunately in Rec mode, you can't simultaneously show GF99 and GFsurf on the main screen, so I went digging around in the Tech mode. In Tech mode you can show both in the middle line. So I left it in Tech mode so I could see both.

In OC Rec mode you can select Big [screen view] (sounds like your current setup) or Standard, which also gives the middle/extra row where you can add up to 3 items. (no need to go into Tec mode). FWIW on mine in Big I have NDL and T1, then in Standard that shifts up a row (so it's consistent viewing if I change display), and I have SurfGF, SAC and GTR in the centre, so deco related stuff is on the left, and tank pressure stuff is on the RHS.

My thinking is the soda bottle analogy, anything over 1% is off gassing but often at my SS my graphs show GF99 of only 1% then i surface slowly but i reach the surface at 50%, the change is rapid in the last 15ft or so. Like cranking the top off the bottle vs easing it off.
Better to know your GF 99 and your GFsurf and use it to a) choose a safety stop depth appropriately and b) make a staged ascent from the safety stop.
Lets say 10% GF increase per minute, or maybe 1/3rds of your surfacing GF, or whatever makes for easy math.

Hell it might be just as easy to come up to let's say 5% GF which for me is often at or slightly above 15' do the safety stop there then to 1 minute at 10' and one minute at 5' before surfacing.
I dunno but I think I am going to try it and see.

All i know is the GF99 graphs coupled with how I feel, especially after cold water dives have indicated to me that the ascent from a depth where GF begins to ramp up is important to how tired i feel afterwards, some cold water shore dives I have done I swam back along the bottom into 5ft of water, GF slowly increasing. I have always felt better after those dives than the ones where I did 3 mins at 15' showing 1% GF and then gone straight up in 1 minute.

Thoughts?

I had both GF99 and SurfGF displayed on that row but now replaced GF99 with SAC as GF99 is too finicky and SurfGF tells you a lot.

Super slow is super nice