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Comment by Maciej Arkuszewski on July 15, 2010 at 3:36pm
If anything I would worry about spiking helium, not N2. offgassing/ongassing speeds does matter. I believe there's only one experiment cited in Medicine and Physiology of Diving with ICD demonstrated and it involved a bunch of goats saturated fully with air at 4 ATA. After being moved to 21% O2 79% He atmosphere at 4ATA they developed DCS symptoms. Switching them the other way around didn't do anything. I personally am with Eddie Brian on this - his opinion is that it's not a concern for our depth range.
Comment by Nick Ambrose on July 15, 2010 at 2:07pm
Kev, I think thought that the dissolved N2 was absorbed at depth, so it will still have a gradient as you switch at a shallower depth than you inspired it ...

I think the WKPP is trying not to reduce the He gradient maybe too quickly for those bigger exposures, and the theory that "more He is good for you"?

I am not an expert by any means
Comment by Nick Ambrose on July 15, 2010 at 1:54pm
What I am saying is that we dont seem to see specific signs of this ICD, so how do we know if by changing gas mixes, we are helping or not helping?

I understand people say not to spike the N2 on ascent, but thats the PO2 not the percentage, correct ?

SO if I was at 150 feet on 21/35 then I breathe 0.44PPN2 @ 5.5 ATA

If I then switch to 50% at 70 feet, I have 0.5 @ 3 ATA so even though the percentage has risen, the inspired PPN2 has not, so I dont create a "reverse" gradient ... or is this something else that's presenting a concern ?

Same going from 12/65 @ 270ish to 35/25 at 120, right ? I still have an offgassing gradient for N2

I didnt do the calculations for the other standard gases, but am I missing something here ?

Now, I could agree that maybe you want to make the He gradient more gradual for longer dives, and that can be done by increasing the He in the deco mixes which may not be the worse thing
Comment by Jan de Kock on July 15, 2010 at 1:34pm
who did scientific research anyway...
For example show me scientific papers about something simple like deepstops........
There is a scientific aproach from Mark but there is not much scientific research on decompression at all.
Comment by Nick Ambrose on July 15, 2010 at 10:16am
Kevin....does he offer any analysis of those recommendations ? Point to any real scientific papers, or actual incidents of ICD ? What is his recommendation based on ? Theory or observation or both ?

I bought a copy for Jamie for her birthday so I can't sneak a peek until end of August!
Comment by kevrumbo on July 15, 2010 at 12:28am
Mark Powell in Deco For Divers says on p189 that the issue of ICD (Isobaric Counter Diffusion) is not an issue until you reach depths of around 80m. I guess I'm taking literally his "straightforward rule of thumb" recommendation [for a deco gas] to never let your partial pressure of nitrogen rise significantly. Powell also goes on to quote a less conservative approach by Steve Burton on p195, by "increasing the nitrogen percentage by no more than 1/5th of the reduction in the helium percentage. . ."
Comment by Nick Ambrose on July 14, 2010 at 9:43am
We used 50/25 (2 of us, one on just 50%) on our recent dives on the Fletcher on Clemente. Honestly cant say we noticed much difference than standard 50% and no obvious difference in feeling compared to the 1 buddy on straight 50%

Only measurable difference was the extra cost of the gas :)

During a presentation in Mexico, Messersmith did cover this a little, indicating that in the WKPP, it's standard to dramatically increase the He on deco gases but only for the longer bottom times, not the typically 20-40 min BTs we would usually do in the ocean for these deeper dives.
Comment by kevrumbo on July 13, 2010 at 5:21pm
I've been considering a similar strategy here locally in SoCal, of using "best mix" deco gas progressions of trimix blends for gradual off-gassing of the helium & nitrogen inerts, coming off a hypoxic trimix bottom mix. (This is assuming of course, you have a gas logistics support source that uses partial pressure blending and can afford using higher helim concentrations for the deco gases).

For instance, from bottom hypoxic mix of 12/60/28 used at 90m, instead of a standard deco mix of 21/35/44 and switch at 57m --use a best mix of 21/51/28. The next switch at 36m, instead of standard deco mix of 35/25/40, use 35/37/28. The next deco gas switch at 21m, use 50/25/25 and finally 100% O2 for the 6m.

In essence you're providing a lessening concentration gradient for the helium, and although you still have a constant nitrogen fraction of 28% to 25% until the O2 stop -- it's mitigated by gradual ascent thru the deco stops with successive decreases in ambient absolute pressure (from 6.7ata to 4.6ata and 3.1ata in this example), and you are not significantly ongassing any more amount of nitrogen than you would if you decided to use standard gases as deco mixes instead (i.g. 21/35; 35/25 and 50%). And of course finally by the end of the prescribed 6m O2 stop, all inerts should be at a low level to permit ascent to the surface. . .
Comment by Richard on January 30, 2010 at 9:48am
N2 makea people feel poopy, at 70ft depending on the previous mix you can actually have quite an ongassing gradient, a moderate example:
18/45/37 switching to 50/0/50 leads to ongassing in some tissue compartments. Going from 15/55/30 obviously has a bigger N2 ongassing gradient at a 70ft switch onto EAN while 50/25/25 provides a gas gradient for both He and N2.
Staying saturated (or nearly so) with He you can eliminate that at the 20ft stop. Whereas maximizing N2 gradients and not having opportunities to ongas N2 is making people feel better - cause N2 is slower and more difficult to eliminate even with O2. This is my internet diving version.
Comment by Maciej Arkuszewski on January 29, 2010 at 11:26pm
Whats the reasoning why?
 

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