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

[stuartv]

@Jay I think you have some good thoughts mixed into those last couple of posts.

IMO about GTR and SurGF - requiring any more accuracy by adding in time spent at SS or doubling the ascent rate is really just pissing in the wind. Lets be realistic here

Being realistic means recognizing that the gas consumed at a safety stop is a non-trivial amount. I figure about 6 cu-ft, for me. Or, around 200 psi from an AL80.

What you are arguing in favor of is a solution that is both less accurate AND more aggressive (less conservative). And for no good reason.

Other manufacturers have had this down for years. There is no technical reason that Shearwater can't make the GTR calculation both more accurate and, at the same time, more conservative. There is also no good procedural reason, either. When giving a diver info, being both more accurate AND more conservative is always better.

Jay, regarding your thoughts on OC Rec, gases, and Sidemount, here is what I think:

- I can't see any downside to OC Rec being limited to 1 gas.

- I can't see any reason to have GTR behave differently depending on what mode the computers is in - EXCEPT Sidemount mode (see next point).

- The current AI modes offered are: None, T1, T2, and T1&T2. And for GTR, you set it to Off, T1, or T2. I think they need to add one more mode to GTR: Sidemount. The GTR mode "Sidemount" would work with the AI mode setting. If AI mode is None, T1, or T2, GTR would work exactly as it does now. But, if AI mode is T1&T2, and GTR mode is Sidemount, the computer would look for T1 and T2 and would assume they are both on tanks of the same size, containing the same gas. The primary functional difference between other GTR modes vs Sidemount is that GTR would reflect the available gas in both tanks, when in Sidemount mode. This mode would be available and work in OC Rec and OC Tec. There are other things that could also be done with Sidemount mode. For example, they could use some color-coding on the tank pressure display. If one tank is more than 1000 psi different than the other tank, they could display that tank's pressure in Yellow. That would be a warning to the diver that they have forgotten to switch regs like you're supposed to (in Sidemount).

- I would have no issue with it if Shearwater would "fix" GTR (to be accurate AND to display when in deco), if they limited it to when you only have one available gas configured. I.e. if the diver has multiple gases configured (e.g. back gas plus one or more deco gases), then GTR does not display once you enter into deco. Or maybe even doesn't display at all. But, when the diver only has one gas configured as available, then GTR could and should display at all times - in deco or not - and should reflect the expected ascent, including any and all stops the computer will prompt for (mandatory deco and optional safety stops).

The VAST majority of dives that ever happen in the world are happening with the diver only carrying one gas. The VAST MAJORITY. Having a bit of "special" behavior in the AI functionality of a Perdix or Teric to reflect that seems entirely reasonable, to me. As does making GTR both more accurate and more conservative.

 

[biggpa]

My last one was at 45% (see image below - highlighted in yellow).

Is the "End SurfGF" information given in the Android App or Cloud Desktop as an actual number? Is click-and-drag in the graph or export to PDF the only way to get the number currently?

 

[NAM001]

Cool. This is a value that the computer will be displaying real time or is it something calculated before the dive?

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"

Some computers call it GF99

 

[EFX]

The NDL is based on your GF Hi setting. If NDL is 0, your GF99 could be most anything, depending on your depth - but will be less than GF Hi. When NDL is 0, SurfGF will be pretty close to GF Hi, depending on your depth. Slightly above GF Hi, but pretty close. The difference between the 2 represents how much you will off-gas during your ascent (if you were to make a direct ascent at 33'/10m per min).

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.

 

[dmaziuk]

Some computers call it GF99

My understanding, it is gas loading in the leading compartment relative to the compartment's M-value. Considering it's only the leading compartment and that the M-value is, as Wienke put it, "a binary proposition" -- they should call it "meh".

 

[mafi]

Currently soloing wrecks on malta (no penetration). ‘Deep’ 30-35m. Solo -> strictly within NDLs @ GFhi 85. When i ascend, I change to 30/70 for very conservarive simulated deco and watch fish on the way up

 

[stuartv]

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.

That is completely, 100% incorrect.

If you descend and stay at a constant depth until NDL = 0, GF99 will be 0 until you start to ascend.

GF99 is the gradient factor of your leading compartment. The gradient factor is a ratio between your tissue tension and ambient pressure. When you are still at max depth, you are not off-gassing at all, so you have no gradient between your tissue tension and ambient. When you are at max depth, all of your compartments are either on-gassing or saturated. None are off-gassing. None are higher than ambient.

When your NDL reaches 0 (and you are still at your max depth), that means that your GF99 is 0 and, if you ascend to the surface immediately, at 30 ft/min, your GF99 will rise steadily, reaching GF Hi at the same time you reach the surface.

 

[stuartv]

The SurfGF will be much higher as it represents an instantaneous surfacing. So, SurfGF does not take into account offgassing at predicted ascent rates.

At NDL=0, the SurfGF should be slightly higher than GF Hi. The difference is only the amount that you will off-gas during a straight ascent to the surface (at 30 ft/min), which isn't all that much. The difference will be bigger, the deeper you are.

If you are at max recreational depth of 40m, it should take you 4 minutes to ascend directly to the surface. You don't off-gas that much during a 4 minute ascent. My experience is that at a safety stop, I offgas (very roughly) 5 GF "points" every 5 minutes. VERY roughly. A 4 minute ascent from 40m means off-gassing less than you would at 4 minutes shallow (at a safety stop).

So, I would not expect SurfGF to ever be more than 2 or 3 (ish) points above GF Hi (at NDL==0), if that much.

 

[EFX]

That is completely, 100% incorrect.

If you descend and stay at a constant depth until NDL = 0, GF99 will be 0 until you start to ascend.

GF99 is the gradient factor of your leading compartment. The gradient factor is a ratio between your tissue tension and ambient pressure. When you are still at max depth, you are not off-gassing at all, so you have no gradient between your tissue tension and ambient. When you are at max depth, all of your compartments are either on-gassing or saturated. None are off-gassing. None are higher than ambient.

When your NDL reaches 0 (and you are still at your max depth), that means that your GF99 is 0 and, if you ascend to the surface immediately, at 30 ft/min, your GF99 will rise steadily, reaching GF Hi at the same time you reach the surface.

I looked at the manual and you are correct regarding GF99; It is a % of the CTC's supersaturation between ambient and GFHi. I'm not sure you are correct regarding SurfGF. The change manual says surface immediately. I believe that means instantly without any ascent offgassing. Your GF99 will show the current GF taking into account the ascent offgassing.

Shearwater: can you clarify what the SurfGF does? Thanks.

 

[Jay]

I looked at the manual and you are correct regarding GF99; It is a % of the CTC's supersaturation between ambient and GFHi. I'm not sure you are correct regarding SurfGF. The change manual says surface immediately. I believe that means instantly without any ascent offgassing. Your GF99 will show the current GF taking into account the ascent offgassing.

Shearwater: can you clarify what the SurfGF does? Thanks.

Please see post22. SurfGF does assume teleportation to the surface (no off-gassing)