Decompression

Comment

Comment by kevrumbo on June 19, 2011 at 3:25pm

Re-posting & quoting Joel Silverstein's anecdote post on extreme PO2 Exposure:

. , ,We removed the Exceptional Exposure Oxygen tables from the NOAA diving manual 4th editon because there was fear that if the general public saw them printed that they might take it as an endorsement to use them.

The NOAA exceptional exposure limits are set for extreme emergencies only and are not for routine use. IE: should be used for life saving only.

These are for a working dive meaning with light exertion. Remember that there are a variety of factors that come into oxygen toxicity, and crossing the 1.6 atm 45min line does not guarantee convulsion, it also does not guarantee it won't.

NOAA OXYGEN
EXCEPTIONAL EXPOSURE LIMITS
PO2 Minutes

2.8 5
2.4 10
2.0 30
1.9 45
1.8 60
1.7 75
1.6 120
1.5 150
1.4 160
1.3 240


As you can see the exceptional times allow you a fairly large margin to use this method for an "escape." The table is NOT linear. Note that exceptional exposures are DANGEROUS and can only be done once in a day. . .

http://socaltecdivers.com/index.php?PHPSESSID=6gsngutj0n57l1g6df4r5...

Comment by kevrumbo on May 25, 2011 at 4:13pm

Here's an excerpt of a deco gas planning write-up for some of the deeper longer cave dives of the WKPP (utilizing a dry habitat for Oxygen deco), and an example of considerations regarding ICD:

A trimix of 10.5 percent oxygen/ 80 percent helium was selected owing to the average bottom depth of 280'/85m. Considerations in this selection were:Since many tissue compartments will reach saturation and decompression will take longer than a few hours, the high helium content has advantages for off-gassing effficiently later in the dive. The amount of time helium takes to reduce its partial pressures in tissues by one-half are about 2.7 times faster than the half-times for nitrogen. . .As decompressions times lengthen to two and a half hours or more, counterdiffusion of excessive amounts of nitrogen can become a real problem. It can have the effect of doing a deep air dive in the middle of decompression. As shallower stops are made near the end of deco, the diver's body can be loaded with enough nitrogen that it offsets any advantages gained in eliminating helium. Because of nitrogen's greater molecular weight, greater solubility in body tissues and slower half-times, it can take longer and be more difficult to eliminate than helium. This is a special concern at the final deco stop where oxygen is used to remove inert gas from the slowest tissue compartments. . .[Non-standard, intermediate] decompression mixes that achieve an acceptable balance of these factors are a trimix of 19 percent oxygen / 50 percent helium at 240'/73m; trimix 25 / 35 at 190'/58m; trimix 35 / 25 at 120'/36m; trimix 50 / 15 at 70'/21m; 100 percent oxygen at 28'/8.6m [in a dry habitat], with periodic breaks using trimix 15 / 45.This selection allows the fraction of helium to gradually taper off while the fraction of oxygen gradually increases and the fraction of nitrogen remains nearly constant. Helium off-gases efficiently with the reduction in pressure and the increasing oxygen fractions. Nitrogen loading during deco is kept below target limits upon arrival at the [oxygen] dry habitat stop. . .From Erik C. Baker, Decompression Strategies Enable Deep, Long Explorations of Wakulla Springs, Immersed Magazine p.30, Fall 1999.See also Erik Baker and the Varying Permeability Model: Technical VPM Publications

Comment by kevrumbo on January 26, 2011 at 5:23pm

From Ross Hemingway, developer of V-Planner program:

At my web site, I have some basic diagrams of the IBCD problem, and how it relates to deco times. Decompression myths and mistakes In V-Planner we suggest limiting the increase of inert pp to 0.5 between mixes. This is measured between the swap point depth - the inspired pp of each. . .

Quote:

If you're talking strictly about breathing gas, it's fine (and sometimes even beneficial) to switch from a helium-based bottom mix to an N2O2 decompression mix. Nitrogen and helium have different solubility coefficients, so the helium will diffuse out faster than the nitrogen can diffuse in, especially in the 70 foot range where the diver would be switching over to 50/50 nitrox.

This is the standard argument listed in medical journals. In normal deco dives with modest deco times we can all "get away with it" and turn the effect into a reduction of deco time benefit.

But its actually the root cause of IBCD in deep dives. For very deep dives with longer decompression, then we must avoid any spikes of N2 pp in deco. Allowing a big N2 spike mid way through deco, will bring on a gas reversal in the tissues. Helium will diffuses out at an uncontrolled rate, whiles N2 diffuses in at an uncontrolled rate. This happens mid way through your "controlled" decompression. The end result is that the body can not fully perform the reversal, deco is compromised from the mid point onwards. The diver has created an IBCD injury half up way through the deco schedule, which now requires a lot of extra deco time to counter and correct for. . .

You get a warning in [ V-planner] 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.

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

Comment by kevrumbo on January 17, 2011 at 5:11pm

From Mark Powell, p.195 Deco for Divers: Quote: . . .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. . .

Comment by kevrumbo on December 22, 2010 at 6:31pm

. Decompression from an N2-based dive is longer with N2 containing deco mixes because some N2 is continuously diffusing into tissue during deco. Decompression from a He-based dive can be longer with N2 containing deco mixes because N2 is diffusing into tissue as He is diffusing out of tissue. The decompression obligation of a tissue compartment is based on the sum of gas partial pressures in the compartment. This means that if a tissue is loaded with N2 as He is being removed, its tissue has a greater decompression obligation than when no N2 is added to tissue during He off-gassing. . .
p.11-12, Gas Exchange, Partial Pressure Gradients, and the Oxygen Window, Jo...

Comment by kevrumbo on October 29, 2010 at 2:10am

Wkpp Strategy for Minimizing Inspired N2 On Deco Gas Switches and avoiding potential Isobaric Counter-Diffusion (ICD) [Immersed Magazine Article Fall 1999]
Deco_Strategies_Wkpp.pdf

Conceptual article explaining the phenomena of ICD:
icd.pdf

Comment by Nick Ambrose on July 16, 2010 at 3:34pm

Yes, I agree. Inspired N2 is what you are breathing in, and is at 4.6ATA for 120

but I am only ongassing a lot of N2 if the inspired PN2 from the deco gas is significantly higher than the PN2 that is "inside" me, and that is ONLY at the same PN2 as my backgas at 120 feet if I am in total equilibrium, which I am pretty sure I wont be so I wont be ongassing N2 in the ratio you suggest.

Also, I dont see (again) any significant facts behind this PDF, just a tour of PADI tech and some theoretical gas calculations....

I DO believe inner ear/vestibular bends are problematic but I think PFO and other concerns are more relevant there than the boogeyman of ICD especially given the UTD gases and profiles.

I dont even know where to begin on OC strategy #3 in that paper .... I guess it was there for "illustrative" purposes.

I would suggest that if you want to reduce your chance of DCS, you would be better served by ceasing your rather creative uses of "air ratio deco" and reduce your encounters with rogue swell at 20 feet than start decoing on heliox

Comment by Richard on July 16, 2010 at 2:52pm

Actually I'm surprised to hear Kevin talking about this newfangled helium stuff at all :p

Comment by Nick Ambrose on July 16, 2010 at 2:20pm

"Nick, you're communicating a dismissive hijack opinion backed-up only by innuendo and rhetoric; I'm trying to re-establish the theme of strategically tapering deco gas inerts, and the possible relationship to ICD based on plausible hypotheses that have yet to be scientifically tested. Repeat this paragraph over and over to yourself, louder and louder as needed so that you may begin to understand. . . Yours truly, "MR Bendy" :-) "

Now THATS more like the Kevin I know!

I actually was referring to Richard as Mr Bendy :) (Holding my tongue here a little ....)

Comment by Nick Ambrose on July 16, 2010 at 2:11pm

Sorry Kevin, I was trying to by syllogistically helpful with my symbolic logic, rather than being a rude malingerer

I dont see what your 4.6 * 0.23 has to do with anything though since the N2 that is inside me, started off at 9.1 * 0.23 not 4.6 * 0.23

so while i am "spiking" the N2, and yes will be ongassing something I ....ouch ouch my ears, dammit I knew I shouldnt have descended to 272 last weekend without some 35/60/5 to switch too .... oh god I am losing my disorientation....i can't SEEEEEEEEEE. I think I can feel my arm diffusing right now. Back in a bit !!!

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