Decompression

Comment

Comment by kevrumbo on July 16, 2010 at 6:36pm

(Powell, Deco For Divers p.35): the difference between the pressures of inert gases is known as the inert gas gradient. If the gradient is such that the inert gas pressure in the lungs is higher thant the pressure in the blood and tissues then we will be on-gassing. If the inert gas gradient is such that the pressure in the tissues is higher than that in the lungs then we will be off-gassing. Due to the fact that various parts of the body are perfused to a greater or lesser degree, they will receive varying amounts of dissolved gasses from the blood. In addition, different tissues will absorb and release gasses at differing rates due to tissue composition.

Now within the above fundamental framework Nick, can you elucidate better what you mean by "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."

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 kevrumbo on July 16, 2010 at 2:53pm

Alright Nick --fair enough & good sport!

"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" --here's what you're not understanding Nick. . .


Go to page 4 of this link, under the heading of Minimizing the Use of N2 During Deco: http://www.subpacific.cl/icd.pdf

It shows an elementary calculation for inspired PPN2 & PPHe in Imperial Units (Note: they're dividing depth by 34 for fresh water).

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 Rainer on July 16, 2010 at 2:22pm

Unsubscribe.

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 kevrumbo on July 16, 2010 at 2:16pm

I believe that Leigh and the guys out in Sharm limit any gas switch to a half bar change in pressure between the inerts (Vplanner has a setting for this) --Thanks Howard Payne, for the above quantitative tidbit; it jibes with an earlier PPN2 calculation showing a on-gassing inspired nitrogen gradient slam upon a deco gas switch:

Now for 12/65/23 bottom mix switching to deco gas 35/25/40 at 36m/120', your inspired PPN2 is (4.6)(0.23) equals 1.06 coming off bottom mix; but then the PPN2 is (4.6)(0.40) equals 1.84 upon switching to 35/25/40 at 36m/120' [a delta bar of more than 0.5 change of inert nitrogen by Howard's rule of thumb above]. By Powell's account in Deco For Divers, this could be significant for ICD especially if you were at bottom max depths of 80m or deeper.

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" :-)

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 !!!

Comment by Nick Ambrose on July 16, 2010 at 9:23am

Hey Kev: Have you ever tried to communicate with someone that doesn't speak the same language as you very well, and attempted to get something across by just repeating the same thing over and over and louder and louder ?? :)

Howard: Nice post. I definitely agree there is something to vestibular bends/inner ear (esecially given the vertigo I get when I ascent sometimes ... I find it hard to believe something isn't going on there)

However, I believe that's more to do with PFO than anything else.

it's also my understanding (second hand) that WKPP divers back before the "mo He" days used to switch to air for the 190 stops ...

I do think the GUE/UTD standard gases could be playing into our hands here (although I also know of certain people claiming to do 150 foot dives on 10/70 then switch to 50% :)

I dont think we need to change our deco gases for ICD, but I do respect anecdotal reports of people feeling better on deeper dives by using say 50/25

What i dont want to see is stuff like everyone getting all bent out of shape (pun intended) about something like this and ending up diving 26/17 and 100% O2 to 150 just to avoid "spiking" the N2 by uisng 50% (then adding 50/25 as a next gas for similar reasons)

I'd also submit that as experienced as Ellyat may be, he is less than helpful as a reputable data source

Comment by Howard Payne on July 16, 2010 at 9:00am

Kev you need to take a step back from this a bit I would suggest.

Mark Powell is a superb Technical Diving Instructor who has collected together everything he has discovered about decompression over the years and put it in one book. He'll be the first to tell you that he didn't invent any of the stuff and he's not an expert on decompression in the scientific sense either.

He simply understood the frustration a lot of divers had in collecting information from obscure, rare and often expensive sources scattered all over the place, and choose to bring everything he'd discovered together in one book.

Of it's type, it's the best there is available IMO - but I know he's be deeply uncomfortable with you quoting it like some decompression Bible and would point out, as he always does - there is still too much we don't know to be able to state anything with any degree of certainty.

I would say this is particularly the case with IBCD and Vestibular Bends. Simon Mitchell is probably one of the world's experts on the subject and having heard him speak about it - he displays a similar reticence to committing himself to any certainties when it comes to this area of decompression

You might want to question your own certainties and conviction in this area?

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