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First, let me apologize for the length and ask you to read this in its
entirety. I've deleted as much of the irrelevant as I can to make my
points clear without obscuring Julius' points.
Julius Loennechen
>
> reply NITROX DECOMPRESSION
> by the author of the original posting
I think there are two things going on here. First, there is
terminology confusion. "Oxygen window" as Julius' sources define it
is not the most common definition. "Inherent unsaturation" was
brought up by me as an alternate term for the common "oxygen window,"
not as another name for Julius' phenomemon. I would call Julius'
phenomenon "the nitrogen gradient" and not "the oxygen window" since
there are other gases aside from oxygen which contribute to the
offgassing gradient of nitrogen.
Second, I believe Julius has some basic decompression concepts
confused, such as when bubble growth begins and ends, the causes of
bubble formation, and the relationship between tissue loadings and the
Haldanean model parameters of halftime and M-value. See below for specifics.
> When David Story commented on my oxygen windows as shooting way beyond, he+s
> probably referring to the normal, one atmosphere value.
No, I was referring to a very different phenomenon, the metabolic
consumption of O2 in the body, which results in a lower total gas
pressure in the venous circulation than in the arterial circulation.
> [...] to my best knowledge a practical rule is
> that: (sic) `the oxygen window is, slightly simplified, the partial
> pressure of oxygen at a given depth+, or in fact 3.87 feet less for air.
Let me suggest we stop using the term "oxygen window." I suggest that
your phenomenon is the "nitrogen concentration gradient," and that my
phenomenon is the "inherent unsaturation (due to metabolic action.)"
> A non-diver is saturated at a P N2 of
> .79 atmospheres, not 1 atmosphere which is the maximum an ambient pressure
> of one atmosphere would balance perfectly. I think this is the essence of
> +inherent unsaturation+.
How can this be inherent unsaturation? The diver is *saturated* --
she cannot accept any more gas -- she is in equilibrium. I fail to
find any unsaturation in this situation. You misunderstand the
phenomenon I describe.
As an aside, perhaps expressing saturation tensions is most clearly
done when expressed as a ratio. In this situation, with constant
temperature and atmospheric pressure and composition, the non-diver
would have a 1.0 saturation of N2. Conversely, a diver performing a
no-stop dive will, upon surfacing, have a saturation tension ratio
greater than 1.0.
> The higher the O2 percentage, the lower is the inert gas percentage and
> more +slack+ is provided - the inert gas tension will never rise beyond the
> partial pressure of the inert gas.
Unless there is a breathing media change, there can be no "slack" by
the definition of saturation: equilibrium with atmospheric pressure &
composition. If you reduce the ambient pressure, a saturated diver
will immediately become supersaturated.
> Nitrox 32 at 130 feet gives a P N2 of .68( 130 + 33 ) feet absolute = 111
> feet absolute. What pressure does it take to keep a full load of N2 in
> solution? 111 feet absolute, or ( 111 - 33 ) feet = 78 feet
> Ascending from 130 feet to 78 feet takes the nitrogen load up to the
> pressure that balances the tissue tension. From there on we+re dealing with
> supersaturation and the inevitable nucleation of microbubbles. At 78 feet
> we+re dealing with 100 % saturation - Nitrox 32 ( 68 % N2 ) at 130 feet
> corresponds to 100 % N2 at 78 feet.
But neither air nor EANx32 are composed of 100% N2, so your "fully
loaded" (saturated) diver is already supersaturated when he ascends
even the slightest amount. Are you suggesting that a diver who is
saturated ("full load") at 130fsw on EANx32 can ascend to 78fsw on
EANx32 and not be supersaturated?
If so, you are missing an important issue here: the diver is
supersaturated whenever she is breathing media at an atmospheric
pressure which results in the breathing media containing less total
pressure of inert gas than her tissues.
> One might ask, why am I talking saturation - according to m-values SCUBA
> diving will never saturate anything but the fastest tissues? ^^^^^
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Pardon, but how can this be? You must be making some assumptions here
of which we're not cognizant. Please list your assumptions, and then
explain why decompression models all contain "slower" compartments.
> The same
> physical laws apply for any tissue - more than 100 % saturation will
> initiate gas nucleation whether that tissue be blood or cartilage.
Actually, a saturation ratio greater than 1.0 is not the only issue.
I assert that, I can cause a non-diver to form bubbles, and I can take
a diver to saturation and back without ever causing her to bubble.
This has been proven again and again; I assert that although
supersaturation is the primary contributor, supersaturation does not
*always* lead to bubble formation.
> If one +takes in the slack+ of the
> oxygen window, the time he+ll have to spend at the ceiling in order to be
> able to proceed and still avoid nucleation will depend on tissue m - values.
The time required at a decompression stop always depends on all three
of the following: compartment half-times, compartment M-values, and
current tissue loadings. The first two do not differ by breathing
media; only the current tissue loadings are affected. Perhaps that is
what you intended to mention, instead of M-values? Your sentence is
unclear. What does "taking in the slack of the oxygen window" mean?
> In practice we+ll probably break the oxygen window anyway, and accept some
> amount of microbubbles. The oxygen window gets increasingly critical as one
> approaches the surface - it+s practically impossible not to violate it from
> 30 feet and up.
I think your use of the term oxygen window is poor in this context.
You should be saying that "*supersaturation* is difficult to avoid in
staged decompression." The term oxygen window does not apply in this
context: theoretically, no bubble formation has yet occurred!
However, you should also note that while supersaturation is a primary
*cause* of bubble formation, it is not the *only* cause. As I stated
above, I can supersaturate you without forming bubbles, and I can form
bubbles in you without supersaturating you. I'm completely ignoring
individual variation here.
> However, breaking it close to the surface is no big deal
> since the microbubbles formed won+t undergo too much further decompression
> and won+t grow to attain obstructive sizes.
Here I must strongly disagree. Decompression does not stop at 0fsw!
Bubbles grow after surfacing by inward diffusion of supersaturated
nitrogen from tissues.
> But, as I wrote in my previous posting, by switching to an oxygen-richer
> blend and wash out nitrogen at depth rather than undergo further reductions
> in pressure (Nitrox 60 at 50 feet) most, if not all, inert gas nucleation
> can be avoided. I+m actually combining the Haldanian and the
> +thermodynamic+ decompression approach.
I agree, this is a better decompression method than pure air,
especially with the long pulls advocated by the US Navy Tables & derivatives.
> B.A. Hills 1966 A Thermodynamic and Kinetic Approach to Decompression
> Sickness. Library Board of South Australia, Adelaide
> A.R. Behnke 1967 The Isobaric (Oxygen Window) Principle of Decompression.
> Trans. Third Annual Conf. Marine Tech. Soc. 3, 213
I'm glad you posted your sources, this helps me understand what you
are using as the basis for your conclusions.
For a different interpretation of these references, I refer you to
_Diving Medicine_, 2nd Edition, 1990, Bove & Davis, to page 30-32,
chapter 4, by Dr Richard Vann, section title: "Undissolved Inert Gas
Exchange: The Oxygen Window." Here is an appropriate quote:
"When bubbles form ... the speed of diffusion is determined by the
difference between the nitrogen partial pressure in the bubble and the
nitrogen tension in tissue. This difference is the direct result of
the metabolic conversion of oxygen, a relatively insoluble gas, into
carbon dioxide, which is 21 times more soluble."
This is what Dr. Vann and I refer to as the "oxygen window:" the reduction in
venous dissolved gas pressures caused by metabolic consumption of oxygen.
Dr. Vann goes on to state that for treatment of decompression illness,
the diver is commonly placed on pure O2 at 60fsw...
"...where the absolute pressure is 2.82ata in the bubble and lung.
Metabolism soon returns the oxygen and carbon dioxide in the bubble to
their tissue levels. This raises the nitrogen partial pressure in the
bubble, and nitrogen diffuses rapidly out of the bubble because of the
large concentration gradient. Momsen called this gradient the
'partial pressure vacancy,' Hills called it the 'inherent
unsaturation,' and Behnke called it the 'oxygen window.'"
Note that Dr. Vann comes to a somewhat different reading of Hills &
Behnke that you do: the "oxygen window" is across the *bubble
interface*, and is not a general description of decompression procedures.
Cheers,
David Story NAUI AI Z9588, PADI DM 43922, EMT
story@be*.wp*.sg*.co* Every dive is a decompression dive.
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