Recreational Ascent Rate in the last 15 feet

[FreeFlyFreak]

I get all the values I posted

I see now.
There is a glitch in the website.

If you run Air times first (which is what I did) then switch to 32x and run those numbers it gives a bum result.

Try this, run your Air time @ 70' you get the result you posted
Then run your 32x time @ 70' you get 0% pDCS

If I refresh the page to reset it and run the Nitrox first I get your results.

User beware.
I would advise starting from a "clean" refresh of the page each time before running a calculation through it.

At least on my Android phone using Chrome.

 

[scubadada]

My reductions in SurfGF are more consistent with Subsurface as opposed to EFX. I generally expect 1+/- % decrease in SurfGF for each min spent at the SS

 

[atdotde]

I don't know what the spreadsheet is calculating, but here is what the "Surface GF" is that recent versions of Subsurface show in the info box (the code is here:
Subsurface-divelog/subsurface ). Every model I am aware of is calculating tissue inert gas loadings. The natural units for those are partial pressures, something like 1.53bar for the partial pressure of N2, say. The problem is that this number alone is not very telling (is it high? or low?). So, for the user to judge it is worthwhile to translate that to other units or to set it in some relation to another pressure.

To me, it appears to be most understandable to translate this to a ceiling depth, i.e. to the minimal depth you could ascent to with this tissue loading. And in Subsurface, we show this as the green ceiling areas and numerically in the info box. But if you insist on showing it as a pressure rather than a depth, a good reference pressure to compare to is the m-value, the (according to a model) maximal allowed tissue pressure at a given depth. It has become conventional not to compare the absolute pressures but rather the excess over the ambient pressure, i.e. compute the ratio

(p_tissue - p_ambien) / (m-value - p_ambient)

If you use plain vanilla (GF 100/100) Bühlmann to compute the m-value and express the ratio as a percentage, this is "the gradient factor". Note that since this involves the ambient pressure which is increasing with depth and also the m-value in itself is depth dependent, the numerical value of the gradient factor depends on depth. But we were after an absolute measure of the amount of gas in the tissue and since the amount is not depth dependant, we should better use a value that does not change when you change depth (without changing the amount of gas in the tissue). So the idea off Shearwater was to use the "surface GF", i.e. the gradient factor that you would have if you used depth=0 and ambient pressure = surface pressure in the above calculation.

So naturally, if you are at the surface, the gradient factor and the surface gradient factor agree, but at depth they don't. My guess is that this is the difference (the spreadsheet is computing momentary GF). To repeat: the (momentary) gradient factor tells you how much of the allowed (according to 100/100 Bühlmann) excess pressure over ambient pressure you are currently using, it is a measure of the momentary deco stress if you like. On the other hand, surface GF is a measure for the absolute amount of gas in the tissue, independent of depth.

Subsurface does not numerically display momentary GF (as I think the ceiling depth gives a better representation of the tissue loadings), but it is shown graphically in the tissue loadings bar graph.

So, independent of maxdepth of your dive, at the end of your NDL time, the surface GF will approach GFhigh. This is by definition (of "NDL"), as the NDL is determined by the fact that you don't occur any mandatory stops and you would need to do a stop if without the stop you had a GF at the surface exceeding GFhigh.

 

[rsingler]

So, to make sure I'm following you..

the (momentary) gradient factor tells you how much of the allowed (according to 100/100 Bühlmann) excess pressure over ambient pressure you are currently using,

Is what you are describing here, what Shearwater calls GF99?
If so, there is a recent discussion in a deco-related thread that suggests that having an option to include momentary gradient factor in the data box might be useful.
Are we seeing a graphical representation of momentary gradient factor in the tiny graphic next to the data box? That is, the ratio of the little "mountain" of tissue compartments compared to the red area of GF100? Again, seeing that number might be educational. I gather that you are partial to the ceiling, and that makes sense, too. But we're exploring new territory here, so looking at both GF99 and SurGF simultaneously is quite educational.

Thanks for your input! This is invaluable!

 

[EFX]

My guess is that this is the difference (the spreadsheet is computing momentary GF).

The spreadsheet (ss) calculates both the current (choose Current under %AoM) and the surface (choose Surface under %AoM) GF. Baker gives the surfacing m-values (Mo) in his paper "Understanding M-values" as well as formulas to calculate GF99 and the a/b coefficients. I use the Mo values in the ss to calculate surface GF and to derive the a and b coefficients. You can view my code by pressing Alt-F8 and selecting calc_press and then edit. The Mo values, one for each tissue compartment, are the maximum pressures values when reaching the surface. My ss, fortunately, displays TC pressures. To get SurfGF I simply take the CTC (controlling tissue compartment) pressure and divide it by the Mo pressure and then calculate a percentage. Simply put, the SurfGF is the percentage of the Mo value at any depth or segment of the dive if you could instantaneously surface at that point in the dive.

Now, whether you believe my implementation of SurfGF or GF99 is correct or not we still need to explain Baker's results. You may not have much trust in my ss but I do trust Baker's results on this. Note: Baker computes current GF or GF99. To reiterate, Baker's programmed surfacing GF is 75% but his calculated GF99 is 92% upon surfacing. Why?

I have the code from SubSurface and will study it to see how they calculate surface GF.

 

[scubadada]

@rsingler

It makes intuitive sense to me that the SubSurface GFs are lower than your GF high after a SS with or without a slow ascent. I have trouble understanding why the EFX GFs are all greater than your GF high. This may be entirely my problem. What do others think?

 

[rsingler]

@rsingler

It makes intuitive sense to me that the SubSurface GFs are lower than your GF high after a SS with or without a slow ascent. I have trouble understanding why the EFX GFs are all greater than your GF high. This may be entirely my problem. What do others think?

That's just the issue I am currently discussing with @EFX at this time. He is comparing code with SubSurface. I gather that a lot rests on the fact that he is implementing Baker's formulas. I don't know if current deco computer programming has moved away from that.
More to follow shortly, I hope.

 

[FreeFlyFreak]

@rsingler

It makes intuitive sense to me that the SubSurface GFs are lower than your GF high after a SS with or without a slow ascent. I have trouble understanding why the EFX GFs are all greater than your GF high. This may be entirely my problem. What do others think?

The way I understand it, as explained, EFX's spread sheet uses the same formulas as published by Baker (I believe in deco lessons?)
In the paper the surfacing GF was greater than GFhi.
Therefore the surfacing GF is also greater than GFhi in the spreadsheet.

Whether this is by design by Baker, (in which case I dont understand why), or by copied error in his paper is unknown it this point.

As far as I am concerned, the only way to clear this up is to ask Baker directly.
Point out the results of his calculation formula and that a theoretical diver will surface with a tissue pressure greater than GFhi when calculated using it. Is there a mistake in the formula or why would that be?

 

[dmaziuk]

That's just the issue I am currently discussing with @EFX at this time. He is comparing code with SubSurface. I gather that a lot rests on the fact that he is implementing Baker's formulas. I don't know if current deco computer programming has moved away from that.

It has not. However a sensible way of showing "SurfGF" is as a fraction of Workman-style M0 and the actual programming for Buhlmann's model is in terms of "a and b" coefficients, and depending on where and when you convert from one to the other, and how you translate from Buhlmann's absolute pressure to ambient pressure at sea level, and from msw that M0/dM are typically given in, to bar that Herr Buhlmann tends to use, and so on and so forth... E.g. you could run the whole thing in fsw, which IIRC Baker's Fortran code does but then if you use 3 feet to 0.1 atmosphere, you end up with infinite fraction that you have to round-off somewhere.

Computers make very fast very accurate misteaks

 

[atdotde]

@EFX don't divide the absolute pressures to get gradient factors you first need to subtract the ambient pressure from both. That is what Baker does. It's about the excess pressure above the ambient pressure.

@dmaziuk If done properly, these calculations are independent of the unit system used (if done properly, of course).

@rsingler you are right that the momentary gradient factors are in "the tiny graphic". They are in the heat map as well. Please see the Subsurface user manual for an explanation: Search for "The Gas Pressure Bar Graph" and "heat map" in Subsurface Version 4 User Manual | Subsurface