Question about surface gradient factor

[BlueTrin]

This page explains how some people use SurfGF.

Myself, I like to do a one minute per meter ascent from the last stop and see the SurfGF decreases while doing it.

Surface GF and Other Teric Musings - Shearwater Research

Powerful yet Small Imagine having a tool that would give you a numerical percentage value of the decompression stress you would encounter if you were to instantaneously ascend to the surface. Not all dives are […]

www.shearwater.com

 

[dmaziuk]

Almost all my dives never have a surface GF higher than 80% at any point. Does this mean that the risk after a rapid ascent accident is low or this number gives a false sense of security?

Does anyone know exactly how Shearwater calculates "SurfGF"? What it says in e.g. Evolution of Dive Planning - Shearwater Research is

The Surfacing GF displays the GF that you would get if you were to ascend directly to the surface right now, without doing any stops.

That means exactly zip in the context of your question because the risk with rapid ascent is blowing past the M-value in your fast tissues, and taking, for example, the fastest TC with 4 minutes half-time:
- at safe ascent rate of 10m/min it'll take you 4 minutes to ascend from 40 msw, i.e. the compartment's entire half-time.
- In a not-too-rapid ascent I could probably swim that in 1 minute and off-gas 1/3rd to 1/4th of what I would during 4 minutes.

So if SurfGF is calculated based on the safe ascent rate, it is the latter: it is irrelevant to rapid ascent -- for some values of "rapid".

If it is calculated using the "teleport" ascent rate, it is the former -- but if you never have your SurfGF over 80%, then I have to assume all your dives are to max depth of ~7.8msw (i.e. no-limit depth) and/or shorter than ~20-30 minutes (6 half-times of your fastest tissue compartment). AFAIK these are the only two options that give you no "transient ceiling" in your fastest TC and therefore SurfGF < 100% on "instant" ascent.

 

[inquis]

Does anyone know exactly how Shearwater calculates "SurfGF"?

Shearwater's surfGF is the teleport variety.

I don't know how your operating window (depths & times) was calculated, but as a counter example, a 60 minute dive to 15 msw has a surfGF of 80% just prior to ascent. The piece you may be missing is surfGF uses each tissues' M0 value, which are larger for faster tissues.

ETA: for the ith tissue, surfGF_i = (tissue_i - surfacePressure) / (M0_i - surfacePressure). The displayed surfGF is the largest of the 16 per-tissue values.

The 5 minute tissue requires an inert gas pressure of 25.7 msw to yield a surfGF of 80%. If breathing air, the depth required to saturate tissues to that pressure is over 30 msw.

 

[harvalen]

I don't know how Shearwater calculates it, but Subsurface and Garmin seems to calculate it as teleport to surface. Neither shows any deco time until you're quite a bit over your GFHigh, since they take ascent rate into account.

Most of my dives look more or less like this, I briefly hit Surface GF 81 at the 20 min mark, with GFHigh 70 Subsurface switched from no deco to 1 min at 3 m:

 

[dmaziuk]

Actually, on 2nd thought, yes: M0 for ZH-L16C's 4-5 minute compartments are in the 4.5 bar ballpark, so you would have to overstay your NDL to saturate them (6 half-times at the bottom of the "recreational" depth range). I.e. the "teleport" thing should work for many/most no-stop dives, esp. if you drop your conservatism to .85 or less (i.e. shorten your no-stop time).

So that only leaves the bubbles, perfusion, and AGE for rapid ascent risks.

 

[Belzelbub]

I don't know how Shearwater calculates it, but Subsurface and Garmin seems to calculate it as teleport to surface. Neither shows any deco time until you're quite a bit over your GFHigh, since they take ascent rate into account.

Shearwater seems to calculate the same as the Surf GF on my left arm (Garmin) is always very close to the Surf GF on my right arm (Shearwater). I think I've seen only momentary discrepancies of 1%. Nothing more.

Back to the OP, while this is all hypothetical as we don't have teleportation devices. If we did, we could probably scrub the excess N2, but that's another discussion.

All that Surf GF means is that if you were to suddenly find yourself on the surface with a Surf GF of less than 60%, your computer wouldn't see any violations in the tissue saturation levels. That doesn't mean you won't have problems. Surf GF is additional data that can be used to modify your dive.

For example, you may use it to extend a safety stop if Surf GF still feels to high for your personal preferences. Conversely, you could also use it to shorten your safety stop for the same reasons, if it seems to be beneficial.