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

[Fastmarc]

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.

Having access to just this info alone is good incentive to get such a computer. I currently lengthen my safety stop some, but have no info on how much it may be helping.

 

[Jay]

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 that Stuart. w.r.t. the ascent rate of SurfGF, in my OP I had referenced 9m/min. This is incorrect. "instantaneously" should be read literally. It has the effect of building in some conservatism during an actual ascent.

Here's a SW post on SurfGF Surface GF and Other Teric Musings - Shearwater Research

RevC of the Teric's manual hasn't yet been updated to reflect FW16's changes, but the colouring of SurfGF has changed to:

 

[Fastmarc]

Ok, question:

With the Genius, is the GF NOW the same as Teric's GF99?

"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"

Does GF normally refer to the supersaturation of all tissue compartments and if so, would their GF @ SURF being "the value of supersaturation that at least one tissue ..." be a form of conservatism?

 

[Jay]

Ok, question:

With the Genius, is the GF NOW the same as Teric's GF99?

"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"

Does GF normally refer to the supersaturation of all tissue compartments and if so, would their GF @ SURF being "the value of supersaturation that at least one tissue ..." be a form of conservatism?

It's the current leading tissue. I assume it's just slightly sloppy wording on Mares' part but they intend both to mean the leading tissue. Which would mean GF99=NF NOW.

 

[dmaziuk]

 

[Jay]

If anyone's interested in viewing SurfGF in excel @EFX 's latest creation does that in the %AoM column: Dive Calculation Spreadsheet

One interesting this is that if you slow the ascent rate down the %AoM barely changes (it's an linear change/application).

 

[Jay]

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

If you dive ‘tec’, I guess/ is your GFHi~=75?

I have a GFHi of 90 but know that I’ve got ~5mins (at 30m say) up my sleeve before I really hit deco (according to Buhlmann) . I think your way (if I’m reading between the lines correctly) is smarter; treating deco as unmodified deco, the 5 min yellow warning is a real NDL-low earning, and surfacing with a what seems to be a sensible GF (<75%) - which also happens to approx be the ‘wise average consensus’ of this thread. Have I read between the lines correctly?

 

[RayfromTX]

Look carefully at these two charts. Adding five minutes may not change your gradient factor as much as we might have guessed.

 

[ChuckP]

Look carefully at these two charts. Adding five minutes may not change your gradient factor as much as we might have guessed.

View attachment 535384
View attachment 535383

Ray - where'd those charts come from?? Interesting, wondering if there's any chart showing a certain depth over time comparing those computers....

 

[RayfromTX]

The late Cameron Donaldson (northernone) sent them to me during a discussion we were having about diving high gf settings and padding with additional time. He was making me aware that choosing an appropriate gf and diving it was preferable to diving a higher gf and managing my safety factor on the fly since I might not use correct assumptions in making the decision on the spot.