Question about using SurfGF...

[hammet]

SurfGF tells me my GF if I were to be instantly transported to the surface. It was my original assumption that that value represented a theoretical maximum GF value since because in reality, as I ascend, I would be off-gassing during the ascent so the actual GF that I would have after surfacing could be no higher than the value that I had read at depth.

However, as we know from learning about deep stops, it is possible to still on-gas in some compartments even though you have ascended from your maximum depth. Therefore, I am wondering, is it not possible that you could surface with an actual GF at the surface that is GREATER than the value you read before you started your ascent which would in a sense defeat the purpose of knowing your SurfGF at depth?

 

[Mike1967]

This thread will answer all your questions...


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

 

[dmaziuk]

Assuming a well-calibrated perfectly spherical diver of uniform density... in vacuum... probably not. On a no-stop dive your ascent is controlled by fastest tissue compartment and there isn't enough on-gassing anywhere else during direct ascent to the surface to matter. On a yes-stop dive it's meaningless to begin with because you can't teleport to the surface, you have to make a stop.

The deal with "bad deep stops" is they aren't padding the shallow stops as appropriate, not the slow tissue on-gassing per se. (Well also padding shallow stops as appropriate results in longer ascent time during which you may catch pneumonia or die of boredom. So it's decidedly suboptimal.)

 

[Fastmarc]

SurfGF tells me my GF if I were to be instantly transported to the surface. It was my original assumption that that value represented a theoretical maximum GF value since because in reality, as I ascend, I would be off-gassing during the ascent so the actual GF that I would have after surfacing could be no higher than the value that I had read at depth.

However, as we know from learning about deep stops, it is possible to still on-gas in some compartments even though you have ascended from your maximum depth. Therefore, I am wondering, is it not possible that you could surface with an actual GF at the surface that is GREATER than the value you read before you started your ascent which would in a sense defeat the purpose of knowing your SurfGF at depth?

If you are interested in utilizing SurfGF, trust me, that thread linked above is a great resource. Go through it from beginning to end. You'll learn a lot. I actually requested it become a sticky, that's how good I think it is especially for us Rec divers.

 

[NAM001]

SurfGF tells me my GF if I were to be instantly transported to the surface. It was my original assumption that that value represented a theoretical maximum GF value since because in reality, as I ascend, I would be off-gassing during the ascent so the actual GF that I would have after surfacing could be no higher than the value that I had read at depth.

However, as we know from learning about deep stops, it is possible to still on-gas in some compartments even though you have ascended from your maximum depth. Therefore, I am wondering, is it not possible that you could surface with an actual GF at the surface that is GREATER than the value you read before you started your ascent which would in a sense defeat the purpose of knowing your SurfGF at depth?

Dont qoute me but I think it assumes an ascent rate of 33 fpm for surf GF like all other ascent related metrics. that would be about 40 seconds to the surface from 18-20 ft. whether that 40 seconds are counted in the surf GF calculation I am not sure. I hope it is not so that it will provide buffer for loss of depth control. That the surf GF value is the worst possible case. Since conservatism of low in rec mode is gf hi of 95 on the shearwater that concern may be moot. My thoughts are rooted in the case of the diver being at a Safety stop and not deeper depths.

 

[EFX]

Surf GF should not be calculated on any ascent rate. The definition of an instantaneous surfacing implies this. It is based on the ratio of the current inert gas pressure to the surfacing m-value expressed as a percentage. In my spreadsheet I calculate surface GF as:

(p_gas - p_amb) / p_Mo * 100

where p_gas is the total inert gas (n2 and he) pressure at the current depth, p_amb is the ambient pressure at the current depth, and p_Mo is the proportional surfacing m-value of both gases calculated as:

p_Mo = (Mo_n2 * p_n2 + Mo_he * p_he) / p_gas

where Mo_n2 is the surfacing N2 m-value, p_n2 is the current nitrogen pressure, Mo_he is the surfacing He m-value, and p_he is the current helium pressure.

I calculate current GF as:

(p_gas - p_amb) / (p_tol - p_amb) * 100

where p_tol is the tolerated (maximum) pressure at the current depth. Both current and surface GF's are for the controlling tissue compartment. Since no one can instantaneously surface it provides a worst case indication of GF at the current point in the dive because there could be no off gassing which would give the maximum pressure gradient for the calculation.

Keep in mind that both current and surface GF's are values calculated on the current (present) tissue pressures, at least as I calculate them. Including an ascent rate implies a calculation projecting a future condition. Providing that calculation IMO is pointless because it is not needed. You can get an idea of how the GF's are changing on the ascent.

 

[dmaziuk]

Including an ascent rate implies a calculation projecting a future condition. Providing that calculation IMO is pointless because it is not needed. You can get an idea of how the GF's are changing on the ascent.

... and this is exactly how NDL is calculated because on dives to 30-ish metres and below you're going to have a ceiling and therefore "instant ascent" NDL of 0. And if you don't include off-gassing during ascent, there will be thousands of SB posts screaming and wailing about how impossibly conservative your product is.

Of course in the case of NDL there's the 2nd projection of a future condition as well: the calculation is based on the user staying at their current depth. Having two ass-sumptions in there makes it right, I'm sure.

 

[EFX]

Yes. You have to include off gassing on ascent to make the calculations as realistic as possible. The inert gas pressures are calculated based on the inspired inert gas pressure which is derived from the ascent or decent depth. The NDL by definition is the remaining time at that (constant) depth at that present point in the dive. Once you ascend or descend a new NDL must be calculated. There is no assumption of ascent (or descent) time needed in the calculation. All dives whether NDL or deco have a ceiling. Zero is taken as the surface. Deco dives have a ceiling that is positive showing a virtual ceiling below the surface. NDL dives have a ceiling that is negative depicting a ceiling above the surface of the water.

The process of iterating the dive at the current depth starting at one minute and adding one minute for each iteration is the same for both types of dives. For deco dives the iterated time becomes the time at the current stop before the diver can ascend to the next stop. For each iteration the ceiling lowers toward the depth of the next stop. When the ceiling equals the next stop depth the amount of iterated time becomes the time at the current stop. Once that time has been completed at the current stop the diver can ascend to the next stop depth. This process is repeated all the way to the surface. For NDL dives which start at some negative ceiling the iteration in time increases the ceiling (makes it more positive) toward the surface. When the ceiling hits zero the accumulated time becomes the NDL time.

 

[dmaziuk]

There is a version Schreiner's equation, in Deco for Divers I think, that gives you NDL. (As opposed to Buhlmann's "ceiling".) You could probably pre-compute a fudge coefficient to account for off-gassing on ascent so you don't have to do the iterative dance. It doesn't have to be exact since your diver's depth and Pinsp is going to change by the next recalculation anyway, and so will their NDL. It only needs to be "close enough" so it doesn't show NDL 0 once the user hits 30 msw.

 

[EFX]

There is a calculation in the back of Deco for Divers for NDL but for a specific ascent time it gives you the partial pressure at which you must start ascending. There isn't anything in that equation that represents time spent at the current depth.

There is a formula in Baker's "Calculating the no-stop time" but it can only be used where Pi > Mo > Po (on-gassing) or Pi < Mo < Po (off-gassing). Pi is the inspired inert gas pressure, Mo is the surfacing m-value, and Po is the previous inert gas pressure of the particular tissue compartment being computed. I tried using it in an earlier version of the spreadsheet but I had to ditch it because there were some dive segments that didn't meet the conditions of the formula.