Question about using SurfGF...

[UWSojourner]

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.

You can still use Baker's formula I think if you just look for conditions where you're already in deco (i.e. NDL=0) or where you can never reach deco (i.e NDL=999). For example,

 

[dmaziuk]

There is a formula in Baker's "Calculating the no-stop time" but ... 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.

Interestingly, in Baker's code example he has Pi .GE. M0 which could result in ln(0) which is undefined.

 

[dmaziuk]

You can still use Baker's formula I think if you just look for conditions where you're already in deco (i.e. NDL=0) or where you can never reach deco (i.e NDL=999).

E,g, ZH-L16C 12.5 min TC has surfacing M-value of 3.3 bar so no point in checking it at depths < 23-ish metres. For 635-minute compartment it's 2.3 bar but since you're never going to hit the NDL in this one in practice, it's hardly worth bothering with at all.

The other noop condition is where P0 < M0 and Pi < P0: i.e. if you're ascending and the TC is already not in deco, it'll never be. Of course if it's a no-stop dive, all TCs are not in deco, so this reduces to Pi < P0.

 

[UWSojourner]

The other noop condition is where P0 < M0 and Pi < P0: i.e. if you're ascending and the TC is already not in deco, it'll never be. Of course if it's a no-stop dive, all TCs are not in deco, so this reduces to Pi < P0.

P0<M0 means condition 1 passes. In that case Pi<P0 implies Pi<=M0 so the table provides NDL =999.

 

[dmaziuk]

P0<M0 means condition 1 passes. In that case Pi<P0 implies Pi<=M0 so the table provides NDL =999.

It needs to be Pi < M0 then: in this case Pi stands for partial pressure of inert gas in alveolae, and P0 is current tissue loading. I.e. P0 < M0 means you're not in deco now, Pi < M0 means you'll never be.

 

[UWSojourner]

It needs to be Pi < M0 then: in this case Pi stands for partial pressure of inert gas in alveolae, and P0 is current tissue loading. I.e. P0 < M0 means you're not in deco now, Pi < M0 means you'll never be.

I guess it depends on whether exceeding your NDL means 1) you exceed a certain max value (then use Pi<=M0), or 2) you equal or exceed the max value (then use Pi<M0).

 

[Mike1967]

 

[NAM001]

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.

I l;am inclined to disagree. Until you have actual data to compute the surface GF it is just an estimate. What do you use for actual remaining assent rate to the surface when you have not done it yet. Sitting at a Safety stop how can a computer calculate what the surf GF is before you hit the surface if it can not use assumed ascent rate values. If you deviate from assumed values the result is no longer a valid guess. I am not suggesting that if you are at 60 ft and surf GF says 1.8 THAT IT WILL ALSO SAY 1.8 WHEN YOU GET TO 20 FT. IF YOU DID depth change from 60-20 AT 10 FPM IT WILL calculate the current gf and form there,,, calculate the updated surf gf. Current GF is based on facts. surf GF is an estimation. since current GF changes and is dynamic. the surf is some what dynamic up to the point of having real data. when you don't have actual data you have to use a default. The same issue exists when you use a dive planner and it calculates NDL it uses assumed ascent rate to compute the estimated amount of off gassing during ascent. TTS calculations used assumed ascent rates for the upcoming ascent also. Your manual should tell you what the assumed rates are. Mine says that assusmed rate is 10 MpM or 33 fpm when making calculations.


I think your premis of instantanious surface is some what correct but if so,,,, if you were at deep depths it would be bogus. I dont think that the purpose of surf GF would be data you would use at depth. Surf GF is not a usable metric at 80 ft with out some assumption of remaining off gassing to be done. I say this because if you looked at (I AM GUESSING) surf GF at 80 ft it would say equivilantly a 3 or more to 1 ratio or GF. Credibility to your premis would be that ;surf GF was intended to be used only immiediatly prior to dong the final few feet of ascent to the surface from say SS whether 20 or 10 ft.. it would have to assume that no effective off gassing will take place from your current calculated depth to surface. under those assumptions such a value would be given for instant arrival to the surface. That makes use of surf GF only usable at very shallow depths such as SS.

In addition to those thoughts I have seen documented over and over again that surf GF is not the guaranteed value of gf current you will break the surface with only an estimation and that GF current should normally be less than calculated surf GF form say 20 ft.. That makes sense since many will do much slower ascents in the last 20 ft than 33 per minute resulting in a lower current GF than calculated in the surf GF. That is even supported in videos dealing with the GF surf topic.

 

[EFX]

I disagree. until you have actual data to compute the surface GF it is just an estimate. What do you use for actual remaining assent rate to the surface when you have not done it yet.

SubSurface calculates surface GF this way:

surface_mvalue = surface_pressure / b + a
surface_gf = (tissue_pressure - surface_pressure) / (surface_mvalue - surface_pressure)

I'm using this approach in my spreadsheet because it provides better numbers than the method I outlined in my post you quoted. All of the variables on the right side of the two equations above are produced from real data -- there are no estimates or fudges. In addition, there is no term for ascent tissue pressures. The definition I use in my spreadsheet is: It is the GF that would be produced based on the current real data, at the present point in the dive, IF you could instantly arrive at the surface without any additional off gassing. Shearwater, in their definition of SurfGF state it is the GF if you could instantly surface. So, there is no ascent off gassing needed. It does not represent your GF upon surfacing after ascent off gassing. That would be indicated by the current GF or GF99 to use Shearwater's nomenclature.

What I'm seeing in my spreadsheet is that, in general, surface GF is higher than current GF. That is exactly what we expect to see if ascent off gassing is not included in the calculations for surface GF. The surface GF eventually coincides with the current GF on the ascent to surface segment, both coming in under GFHi. Now, everything is coming together and making perfect sense. There is no confusion for me. It just took a better understanding, helped along by others comments, and the right equations.

 

[NAM001]

SubSurface calculates surface GF this way:

surface_mvalue = surface_pressure / b + a
surface_gf = (tissue_pressure - surface_pressure) / (surface_mvalue - surface_pressure)

I'm using this approach in my spreadsheet because it provides better numbers than the method I outlined in my post you quoted. All of the variables on the right side of the two equations above are produced from real data -- there are no estimates or fudges. In addition, there is no term for ascent tissue pressures. The definition I use in my spreadsheet is: It is the GF that would be produced based on the current real data, at the present point in the dive, IF you could instantly arrive at the surface without any additional off gassing. Shearwater, in their definition of SurfGF state it is the GF if you could instantly surface. So, there is no ascent off gassing needed. It does not represent your GF upon surfacing after ascent off gassing. That would be indicated by the current GF or GF99 to use Shearwater's nomenclature.

What I'm seeing in my spreadsheet is that, in general, surface GF is higher than current GF. That is exactly what we expect to see if ascent off gassing is not included in the calculations for surface GF. The surface GF eventually coincides with the current GF on the ascent to surface segment, both coming in under GFHi. Now, everything is coming together and making perfect sense. There is no confusion for me. It just took a better understanding, helped along by others comments, and the right equations.

Ive been trying to deal with this topic same thing for some time. Expecially for those that have GF current or GF99 and not the extra ;surf GF It is either a useful tool only at near surface or a tool that gives valid info form deeper depths. ;I did not find an exception to the use of assumed ascent rate in the book regarding surf GF. but like I said if it assumes instant surface you would never access this data for thepurpose of using form a depth other than near surface. That too makes sense. At that point the time for additional off gassing would be minimal , if not moot, and the reaultant surf GF would be the worse case value if you suddenly got sucked to the surface.