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PfcAJ
Contributor
We did 60/85 a few weeks ago. 7hrs in the water.What gradient factors has he talked you in to diving?
The Chairman
Chairman of the Board
If that question was directed at me, I dive the default GF programmed into my Shearwaters. Since I also dive with a Garmin Descent, I have matched it to the Shearwater as best as I can.
huwporter
Contributor
You didn't include a quote, so it's hard to tell, but if you were referring to my question in post #111, then no, it was directed to the person that I directly quoted before asking the question.
And it was kind of an "inside joke" following seeing a presentation a while back where a particular NEDU researcher mentioned that he would personally be inclined to use higher GFLo values than he was able to persuade most of his buddies to adopt...
PfcAJ
Contributor
PfcAJ
Contributor
Hello Diving Doc,
Your question is effectively unanswerable because it would be virtually impossible to create your "dumped that supersaturation into a fast compartment" scenario without either a very short (and somewhat meaningless) dive or an explosive and therefore very dangerous decompression from a more meaningful dive. In decompression dives as we know them, a properly conducted decompression according to virtually any of the available models will result in a relatively greater integral supersaturation in slow compartments later in the decompression and at the surface.
However, the degree of deep stopping can markedly affect the distribution of supersaturation across fast and slow tissues. Your line of questioning embodies the quintessential difference between a decompression that distributes the decompression stops excessively deep vs a decompression of the same length that distributes the stops more shallowly. If you have x amount of tissue gas loading and y amount of time to decompress, a deeper distribution of decompression stops over that decompression period (y) will inevitably result in less supersaturation in fast tissues early in the ascent (because the fast tissues are protected by the deep stops) but more supersaturation in slow tissues later in the ascent (because of continued slow tissue gas loading during the deeper stops). This effect will be more important the deeper and longer the dive and decompression.
Once again, I need to thank UWSojourner for the diagrams that appear below.
These heat maps show supersaturation patterns across fast and slow tissues (the barely perceptible horizontal bars labelled from 1 - 16 where 1 equals the fastest and 16 equals the slowest tissues) during and after the equal-length decompressions from the two NEDU study dives (which had identical inert gas loading). The timeline of the diagram (horizontal axis) starts at the end of the 30 minute bottom time. Remember that the tissue gas loading at this point (the start of the decompression) is identical in the two decompression profiles. Hotter colours represent supersaturation. Cooler colours (purple and blue) represent tissue on-gassing. In these diagrams 210 minutes on the horizontal axis is the point of arrival at the surface.
The one hard outcome we have from this study is a higher incidence of DCS in the deep stop profile which, as you can see from the heat map, protected the fast tissues from supersaturation early in the ascent at the expense of greater supersaturation in the slower tissues later in the ascent. In addition, the shallow stops profile was associated with a ~18% lower integral supersaturation (not shown in the diagram).
There has been much debate about the relevance of the NEDU profiles to tech diving. However, the same pattern of supersaturation across tissues that was associated with greater DCS in the NEDU study can be seen in a decompression profile (VPM-B+4) when compared to a GF profile chosen to give the same length of decompression from a typical technical rebreather dive (parameters shown in the diagram). In addition, the GF profile with less emphasis on deep stops (but which is exactly the same length) has ~23% less integral supersaturation (not shown in the diagram).
The lower integral supersaturation in the GF profile together with the pattern of supersaturation that favours protection the slower tissues (shown to be more favourable in the NEDU study) almost certainly implies less risk of DCS.
To your specific question, the integral supersaturation is not equal in either of these pairs of dives. It is lower in the dives with less emphasis on deep stops. In relation to distribution of this supersaturation across tissues, as alluded to above, you could not have two properly executed decompression dives (according to any of the current models) with equal integral supersaturation but in which most of that supersaturation occurs in the faster tissues because fast tissues don't remain supersaturated for long.
Just as a caveat, (and at the risk of stating the obvious) none of this is intended to imply that endlessly shallower stops are better. If the stops are too shallow, then the supersaturation in all tissues will be greater and the integral supersaturation would also increase. Our current challenge is in identifying the optimal depth of the first stop in any decompression dive and I do not pretend to have the answer to this. I am, however, reasonably confident that it is not as deep as predicted by bubble models or UTD-RD.
I would also say that integral supersaturation (and the type of heat maps shown above) are best used for comparing decompressions of equal length that follow identical time / depth profiles (that is, dives with the same inert gas loading).
Simon M
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Simon, could I bother you to explain something to me? It may be a very stupid question, but please bear with me.
You say:
Now, I'm looking at the second heat map graph (83m for 20 min). The fastest compartments (#1 to #4) are black from about 70 minutes run time, which makes be believe that they are neither supersaturated to any noticeable degree, nor are they on-gassing. But from a little more than 105 minutes through 120 minutes, they suddenly turn purple and thus are ongassing quite noticeably. Why do they suddenly start on-gassing when the diver is ascending to lower ambient pressure? Does it have something to do with the fact that this is a CCR dive with constant pPO2?
You say:
Now, I'm looking at the second heat map graph (83m for 20 min). The fastest compartments (#1 to #4) are black from about 70 minutes run time, which makes be believe that they are neither supersaturated to any noticeable degree, nor are they on-gassing. But from a little more than 105 minutes through 120 minutes, they suddenly turn purple and thus are ongassing quite noticeably. Why do they suddenly start on-gassing when the diver is ascending to lower ambient pressure? Does it have something to do with the fact that this is a CCR dive with constant pPO2?
Hi Storker,
Those tissues become undersaturated with inert gas whilst breathing a high PO2 during the shallow stops, but when you arrive at the surface (around 108 minutes on the horizontal axis), come off oxygen and start breathing air again, they actually on-gas transiently to equilibrate with the inspired gas nitrogen pressure.
Simon
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