inner ear DCS

[Kevrumbo]

I was under the (apparently mistaken) impression that He was less soluble in both aqueuous and lipid tissues (from Chris Duer's paper) so Duke Docs comment on the higher solubility makes me want to understand the issue. I understand that He diffusion is much faster than N2 (aqueous diffusion coefficients are like 3x) so is that what the issue is?
We use helium for removing dissolved N2/O2 in much of our lab gear, precisely due to its lower solubility.
Bill

From Mark Powell, p.195 Deco for Divers:

. . .The allowable changes [in a deco gas to mitigate ICD] are based on the physical properties of nitrogen and helium. As we have seen, helium is a fast gas and will diffuse in and out of a tissue approximately 2.7 times faster than nitrogen, however nitrogen is approx 4.5 times more soluble in lipid tissues than helium.

. . .When switching from a high helium mix to a high nitrogen mix, the higher solubility factor of nitrogen offsets the decrease in pressure of the helium [and its faster diffusivity]. This results in an increase in the total quantity of dissolved gas. As nitrogen is approx 4.5 times more soluble than helium, Burton proposes that by increasing the nitrogen percentage by no more than 1/5th of the reduction in the helium percentage, then the total quantity of dissolved gas will not increase and hence an ICD event can be avoided. . .

 

[thunderball5]

Thanks to Kevrumbo for the reference. Now that makes perfect sense. thanks to Ross too, for reminding me how much I like Vplanner. however, it sure would be nice if Vplanner had a tool that
1. warned the diver when any proposed deep gas switches from He to N2 rich mixtures were too risky.
2. a more sophisticated tool for selecting gas mixes. specifically, intermediate trimixes that allowed a diver to transition between the He-rich bottom mix and the first N2-rich deco gas (usually eanx50).

safe diving guys.
Dirk Peterson.

 

[rossh]

Thanks to Kevrumbo for the reference. Now that makes perfect sense. thanks to Ross too, for reminding me how much I like Vplanner. however, it sure would be nice if Vplanner had a tool that
1. warned the diver when any proposed deep gas switches from He to N2 rich mixtures were too risky.
2. a more sophisticated tool for selecting gas mixes. specifically, intermediate trimixes that allowed a diver to transition between the He-rich bottom mix and the first N2-rich deco gas (usually eanx50).

safe diving guys.
Dirk Peterson.

V-Planner has most of that already. You get a warning in the plan if the ppN2 or ppHe changes more than 0.5 between mixes in deco. There is also the pp graph that lets you see the pp values throughout the deco phase.

Regards

 

[thunderball5]

I knew about the pp value graph, but not about the warning of 0.5 between mixes. perhaps that's because i have not tripped it yet. What form does the warning take? Does the text change color, is their a message, warning bells and whistles?

-Dirk.

 

[Kevrumbo]

The warning will be a crimson highlight along the row of the gas switch exhibiting a counterdiffusion trend (as shown in V-planner on my old pocket PC).

For example, enter a dive in V-planner to a depth of 90m for 20min with a bottom mix of 12/60, and deco gases of 21/35 at 57m; 35/25 at 36m; Eanx50 at 21m; and Oxygen at 6m. There will be a red warning highlight along the row corresponding to the 21/35 gas switch, signifying a 0.5 or greater change in ppN2 in this instance. Note that you're going from an fN2 of 28 percent from the bottom mix of 12/60, to an fN2 of 44 percent in the intermediate deco mix of 21/35.

Check:
At 60m, your inspired ppN2 on backgas is: 0.28(7.0) = 1.96
Switch at 57m, your ppN2 on 21/35 is now: 0.44(6.7) = 2.94
Your ppN2 change is: 2.94-1.96/1.96 = 0.5 hence an ICD warning.

If you use an intermediate deco gas of 18/45 instead (fN2 of 37 percent):
Switch at 57m, your ppN2 would be: 0.37(6.7) = 2.47
Your ppN2 change: (2.47-1.96/1.96) = 0.26 , and the ICD warning clears. . .

 

[thunderball5]

Now that makes perfect sense.

 

[adelman]

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