Recreational Ascent Rate in the last 15 feet

[Jay]

@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

A 2c feature request: Ability to graph GF99(Current) and SurfacingGF, and also to add CurrentGF to the Information Box that appears on the screen.

 

[rsingler]

A 2c feature request: Ability to graph GF99(Current) and SurfacingGF, and also to add CurrentGF to the Information Box that appears on the screen.

I agree! Would make SubSurface much more useful, at least to this interested group!

 

[atdotde]

As I said, the heat map is already a plot of exactly that information. I created a pull request Add current GF to infobox by atdotde · Pull Request #2409 · Subsurface-divelog/subsurface that adds the numerical value to the infobox.

What kind of analysis are you expecting to do with this value? I doubt that it is actually very meaningful. Just one example to illustrate: Plan a 20min dive to 60m on air with GF 99/99 (don't actually dive that, it's just for illustration). Then at the end of a 39min stop at 6m, the GF is down to 8%. Sounds like not much.

Well, then you ascent to the surface. There it is 98%, a value that many consider too high for their diving.

The actual tissue loading has not changed much. It's only that the ambient pressure changed (and the M value by a tiny bit).

So, what have you learned from looking at the current GF?

 

[EFX]

@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.

I don't see where Baker subtracts the ambient (surface?) pressure. Please show me the code where Baker does this. Here is a partial explanation from Baker's paper Decolessons where he describes the subroutine used to calculate the current m-value.

"The M-value (in absolute pressure) is then calculated for each compartment based on the PRESENT ambient pressure (absolute) and the PRESENT "intermediate" Coefficients (AHEN2 and BHEN2)."

 

[atdotde]

Have a look at figure 4 of http://tecvault.t101.ro/Clearing up the confusion about the deepstops.pdf (I consider this the main reference on gradient factors). There you can see the gradient factors being 0 on the ambient pressure line. In figure 3 of https://www.frogkick.nl/files/understanding_m-values.pdf you have both in the inset but "% M-Value Gradient" is what I would say most people call gradient factor (at least this has the word "gradient" in it).

 

[dmaziuk]

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

:shrug: if done properly your birdix will use the reading of its barometer for atmospheric pressure at sea level and its water salinity setting for metres to bar conversion. So its results are guaranteed to be different from my back-of-the-envelope calculations that use 1 and 0.1 resp. instead. (Though granted metres are just eye candy.)

OTGH I've seen enough scientific data tables printed neatly to %6.4f mask with no indication of any error control... and my director tells me what comes from the instruments is only good to two decimal digits. Three if you spend a really long time calibrating and purifying and shimming. So when I see 0.6200 "a" coefficient for the 27-minute TC (C set), my first thought is "really?"

 

[EFX]

@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.

There are two ways to express the amount of tissue supersaturation with respect to the m-value line: (1) as a %m-value which equals the inert gas pressure / m-value at that depth. Both values in the formula are with respect to zero pressure (absolute pressure), (2) as a %m-value gradient which equals the inert gas pressure gradient / m-value gradient. Both values are with respect to the ambient pressure line (gauge pressure). See the graph on page 7 of Baker's paper "Understanding M-values" included below.

 

[EFX]

There are two ways to express the amount of tissue supersaturation with respect to the m-value line: (1) as a %m-value which equals the inert gas pressure / m-value at that depth. Both values in the formula are with respect to zero pressure (absolute pressure), (2) as a %m-value gradient which equals the inert gas pressure gradient / m-value gradient. Both values are with respect to the ambient pressure line (gauge pressure). See the graph on page 7 of Baker's paper "Understanding M-values" included below.

This is the answer to the discrepancy between my ss and Subsurface. Baker and I are calculating % of the m-value pressure (item 1 method above) where subsurface is calculating a percentage of GF which is item 2 above. The item 2 calculation will produce lower GF99 and SurfGF numbers than the calculation with item 1.

From my cursory inspection of Subsurface's code for surface GF they are using the item 2 calculation. The code subtracts out the surface pressure in calculating the surface GF which essentially gives gauge pressures.

I will provide a major revision of my ss that will give the user a choice between these options. So, to be clear, the %AoM on the ss does not equal GF99 or SurfGF as most users here have interpreted it.

 

[Fastmarc]

This is the answer to the discrepancy between my ss and Subsurface. Baker and I are calculating % of the m-value pressure (item 1 method above) where subsurface is calculating a percentage of GF which is item 2 above. The item 2 calculation will produce lower GF99 and SurfGF numbers than the calculation with item 1.

From my cursory inspection of Subsurface's code for surface GF they are using the item 2 calculation. The code subtracts out the surface pressure in calculating the surface GF which essentially gives gauge pressures.

I will provide a major revision of my ss that will give the user a choice between these options. So, to be clear, the %AoM on the ss does not equal GF99 or SurfGF as most users here have interpreted it.

Why should there be two differing ways to calculate this? What reason would a user need to choose one over the other?

 

[atdotde]

What I can say is that decompression schedules computed with subsurface‘s Interpretation how to compute gradient factors pretty much agree with what other programs on the market compute as gradient factors.