Is 1.58 Npp too much, and 2h+of bottom time on trimix

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Ruzo

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Location
Vienna
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200 - 499
Hello,

I just worked my way through the 2 amazing books of Powell and Salama (Deco for divers, and deep into deco) and had a blast of reading (and trying to understand) them.

There is a part in each book where I can't wrap my head around, and it takes about 3+ more weeks until I can meet up with my tech instructor to poke him with questions, so might as well try and dig for enlightenment here.

1.) First part is on page 20 of Salama's book, the chapter about Workman.
"If a diver on air (79% nitrogen) can stay indefinitely at a 10m depth and not get bent upon surfacing (this assumption later was found to be inaccurate), will this hold true if,..."

My question is about the part in bold. Inaccurate in what regard? Sadly I couldn't find a reference in the book (nor in Powell's book) on why, and when spinning up google or looking through any notes I got, it seems to be "commong knowledge" that one could indeed stay at <10m indefinitely on air and surface without getting bent. (IIrc Powell's (?) book mentions an experiment where divers at one point stayed an extended time at 10m and some got bent, so there is some empirical evidence that 1.58 pp = can surface any time doesn't hold true. But no explanation was given, would need to reread and find that part again.)

My assumption would be that fast tissues can tolerate a higher level of supersaturation and that if (for whatever reason) a person were to stay at 10m until all compartments were fully saturated, 1.58 npp would be considered too much supersaturation for the slower compartments?

2.) Powell's book, the chapter about Trimix decompression.
The general rule of thumb states that for dives <2 hours traditional deco models produce a longer deco time when using helium.
"For dives with a bottom time of less than 2 hours the progression is: Nitrox > Air > Trimix > Heliox"

That part I understand. What I don't understand is the progression of deco times if the bottom time happens to be >2 hours.
"As the bottom time reaches 2 hours the overall decompression time converges and then for dives over 2 hours bottom time the order is reversed with helium based mixtures giving shorter overall decompression time: Heliox > Trimix > Air > Nitrox"

Why would Nitrox lead to a longer decompression time than air? Is that due to a bigger gradient when switching to a rich nitrox deco mix from the bottom gas air, than when switching to a rich nitrox mich from "regular" nitrox?

Hope my questions make sense.

Thx.
 
If a diver on air (79% nitrogen) can stay indefinitely at a 10m depth and not get bent upon surfacing (this assumption later was found to be inaccurate)
A saturated dive to 10 m results in a surface loading that is 121% of the limit of the Buhlmann ZHL16C model, which I think all would characterize as "risky". That model (GF 100/100 in today's parlance and considered risky by most) shows a no-stop time of about 4 hours EDIT: 6.5 hr.

FWIW, the acceptable no-stop, saturated depth value I've heard is 20 ft (about 6 m). Buhlmann agrees with that as the lowest tolerated ppN2 of 12.74 msw is never reached with an inspired ppN2 of 12.31 msw at that depth. This corresponds to a surface loading of about 84% of the limit.
 
One reason why decompression on heliox dives might be seen as 'more efficient' is that the only supersaturated gas in the body is helium, which diffuses faster--despite what the "helium penalty" (trimix penalty?) might construe. During deco you would also typically move to pure nitrox or pure oxygen as a deco gas at the final stops, with no helium left in these final deco gases that might slightly lower the ppHe off-gassing gradient. For that reason some believe that for a heliox (or near-heliox) dive, all of the final deco (6 meters or shallower) could even be done on 50% nitrox or even air, with 100% oxygen not being any better or faster. You are just off-gassing helium, with negligible on-gassing of nitrogen at those shallow stops.

For typical trimix blends, the model is more complicated because of two different gases saturating different "compartments" at different rates, with some disagreement about whether or how the two gases "see" each other for "total inert gas pressure" effects in these compartments.

There is a possibility this is not actually calculated ideally in current models
 
"As the bottom time reaches 2 hours the overall decompression time converges and then for dives over 2 hours bottom time the order is reversed with helium based mixtures giving shorter overall decompression time: Heliox > Trimix > Air > Nitrox"
This is almost certainly a published mistake. Nitrox (with lower nitrogen content) will never require more decompression than air, for a dive with the same depth & bottom time.

I am also curious if they (or you) ran the above statement through any of the popular dive planning programs?
 
This is almost certainly a published mistake.
I suspect the primary intent was to reverse the order to put helium first (for reasons you outlined above) and reversing 21% nitrox (aka air) and 22+% nitrox was never intended.
 
. . .

Why would Nitrox lead to a longer decompression time than air? Is that due to a bigger gradient when switching to a rich nitrox deco mix from the bottom gas air, than when switching to a rich nitrox mich from "regular" nitrox?

Hope my questions make sense.

Thx.
Remember, the book is talking about the implications of the decompression models/algorithms. The models/algorithms are not reality, they are just the most useful approximations we have come up with so far. much of your questions are the reasons researchers are still investigating deco and trying to come up with more consistent models/algorithms.
 
https://www.shearwater.com/products/swift/

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