Why plan decompression with a Gf (lo)?

[Dr Simon Mitchell]

Hello Tursiops,

??
Why do you say this? What relevance does the positioning of the scale have? Is it not just a way to have the GF line intersect something and give it a numerical value?

No, the GF is a proportion of the Buhlmann M value at a specific depth. Therefore the scale needs to be vertical to correspond to a particular depth.

Maybe, although the graph also says, "Divers began using Gradient Factors to force gas content models to impose deeper stops." This is incorrect,. of course. The reason for the GFs was to stay away from the M-values. That some choices of GFs do in fact impose deeper stops was a consequence, not a motivation.

The statement on the graph is completely correct in the context of the presentation in which it appeared. It was about deep stops and how divers went about adopting them during the height of the belief in deep stops. They did this by adopting bubble models or "using Gradient Factors to force gas content models to impose deeper stops".

Simon M

 

[stuartv]

Reversing the GF's for low and high will add deco time. You can experiment with this if you download my spreadsheet dive_040318_v24_0.xls. To reverse the values of GFLo and GFHi you need to disable the error checking on the dive sheet. You can do this by opening the VB editor by pressing Alt-F8, in the right window page down to the check_profile() routine, go to the bottom and comment out the if block which checks that GFHi >= GFLo. Then save the code.

No disrespect intended, but you can also download Subsurface, which is free, open source, and runs on Windows, Mac, and Linux, and play with it on there. Reversed GF's work just fine in Subsurface. It does a nice graphical display of the "ceiling" when you do this, too.

Subsurface

For my technical dives, I generally plan them in Subsurface, because the planner is nice, and then when I have it "final", I run the same plan in Multi-Deco just to check it and make sure it yields the same results.

Then write it down in my wet notes and don't look at it again (okay, being slightly hyperbolic here), while I use 2 tech computers during my dive. But, that's a whole other SB battleground...

 

[Jay]

I have an additional question that I'll open in a new post so I don't go too OT, but first could I check I have this (in pic) a/ is the ranking correct, b/ what do we know about the slopes (are there intersections etc I should add). I realise some are black boxes / proprietary. I'm asking this q so I don't propogate bad info.

 

[EFX]

No disrespect intended, but you can also download Subsurface, which is free, open source, and runs on Windows, Mac, and Linux, and play with it on there. Reversed GF's work just fine in Subsurface. It does a nice graphical display of the "ceiling" when you do this, too.

Thank you stuartv. I have both those programs and they are excellent tools for planning dives. Multideco has the ability to select either VPM or Buhlmann with GF's among others. Subsurface provides dive logging in addition to dive planning. I didn't write my spreadsheet for dive planning. It was mainly written for educational purposes showing the tissue compartment pressures for each dive segment and how they change throughout the dive (DiveProMe+, a web-based program also shows TC pressures in graph form with numerical data). There's a CALC sheet which provides calculations for volumes and pressures, conversion of units, and buoyancy of an object. The PLAN sheet will help you calculate SAC and RMV as well as gas usage. The RB sheet will calculate turn pressures for rock bottom and rule of thirds. Check it out.

 

[Jay]

Thank you stuartv. I have both those programs and they are excellent tools for planning dives. Multideco has the ability to select either VPM or Buhlmann with GF's among others. Subsurface provides dive logging in addition to dive planning. I didn't write my spreadsheet for dive planning. It was mainly written for educational purposes showing the tissue compartment pressures for each dive segment and how they change throughout the dive (DiveProMe+, a web-based program also shows TC pressures in graph form with numerical data). There's a CALC sheet which provides calculations for volumes and pressures, conversion of units, and buoyancy of an object. The PLAN sheet will help you calculate SAC and RMV as well as gas usage. The RB sheet will calculate turn pressures for rock bottom and rule of thirds. Check it out.

Very educational and useful. Thanks for sharing.

link from earlier post:

dive_040318_v24_0.xls.

 

[atdotde]

In my understanding, the argument for a lower GFlow was not so much the worry about the fast compartments (although those end up to be affected by it most since the leading tissue goes from faster tissues at the deeper, earlier part of deco to the slower ones later and further up, for normal profiles at least) but about the fact that it was argued one needs to be extra conservative at the deeper part of the deco: As gas that goes out of solution there forms bubbles and thanks Boyle/Mariott they would grow and amplify the problem. So additional conservatism would be extra beneficial down there.

And I think, nobody would argue against extra conservatism (at least to a certain degree). The controversy only came about when it was argued that to compare different strategies one should fairly compare plans with equal total runtime (just as extra conservatism is almost always good and a plan that would spend an extra hour at 3m compared to another one would be pretty sure to be safer from a DCS perspective). With that proviso, adding extra time at depth means cutting that time at the shallower stops and then it is no longer obvious that this is a good idea (and empirical evidence seems to indicate it is in fact not).

Just as in the old days it was argued that this Boyle/Mariott expansion of bubbles was not covered by the more traditional diffusion based models (as there were not bubbles) the more modern argument of why the deeper stops are not as beneficial as originally thought is also outside the strict model: It is argued that it is not only the supersaturation in the leading compartment that matters (as it is both in plain vanilla as well as GF-enriched Bühlmann as well as for example VPM-B) but one has to watch also the total supersaturation in the sub-leading compartments (this is for example what the above mentioned heat maps visualise: you see the warmer colours across the board not only in the leading compartment).

Just a last side remark: From a logical point of view, the GFlow scale should indeed be vertical but thanks to the intercept theorem it does not matter from a mathematical point of view, the lines of constant GF are the same.

 

[rossh]

I have an additional question that I'll open in a new post so I don't go too OT, but first could I check I have this (in pic) a/ is the ranking correct, b/ what do we know about the slopes (are there intersections etc I should add). I realise some are black boxes / proprietary. I'm asking this q so I don't propogate bad info.

View attachment 453496

Excellent diagram. OK its generic, and without scale, but it gets the general message across. All models draw a limit line, some distance above ambient pressure.

Here is some other things to consider.. Tissue microbubble growth, is only possible when supersaturation exists, and that occurs anytime above the ambient pressure line. GF 0/0 is the ambient pressure line (not the on/off gas line). Therefore, the safest ascent, would be one with no supersaturation.... while the new current wisdom says increasing supersaturation is better ??

The on/off gas line is lower than the ambient line. This is where tissue pressure equalizes.


A more detailed diagram for you.

 

[stuartv]

Excellent diagram. OK its generic, and without scale, but it gets the general message across. All models draw a limit line, some distance above ambient pressure.

Here is some other things to consider.. Tissue microbubble growth, is only possible when supersaturation exists

Really? I thought that microbubbles had been measured in people who have never even been scuba diving. If so, how do they get them in the first place if they can't grow without supersaturation? Maybe I'm misremembering reading something that talked about finding microbubbles in people who had not been diving.

 

[rsingler]

Where in the literature might we find another diagram at all similar to this one?

It seems to me that the yellow decompression limit curve is rather exaggerated, and having a single risk arrow is hypothetical. IOW, the risk of exceeding the GF0 line saturation is both compartment- and depth-dependent. Therefore, having a single big arrow that implies "don't exceed ambient pressure" is misleading.

It does seem to portray one philosophical difference well, however: it seems to show that in the ZHL-C model there is less proportionate risk in higher M-values at depth, because you are further away from your curved yellow "Decompression Limit" line at depth. Which is what happens when you decline a deep stop, and do not on-gas into slower compartments that must then offgas when your yellow line is closer to ambient in shallower waters.

Or have I completely misinterpreted what you are portraying?

 

[rossh]

See the message at the bottom of the graph... "Note: not to scale, some details missing / out of place"...

Most other drawings will put all the "M" value line as a straight line. But the model does have the curve I drew, because the first cells encountered - the fast ones, have a higher M value, than the (slower) cells encountered later in the ascent.

The VPM-B model version is typically a relatively constant M value across the ascent. So while it generates the deeper stop, it also has Lower stress through the middle of the ascent too. Bubble models are more than just one deep stop.


The big arrow is the start of the supersaturation region, where tissue micro bubble growth can start. This is basic deco theory, and its written into the preamble of many science papers and texts.

As you can see, we all deco in the supersaturation region, because some SS is tolerable for some amount of time. The exact amount is the outcome and difference between all of the different deco models, and methods.