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On9/2/00 5:07 AM, Shimell, David (shimell) wrote:
>Bill
>
>I read somewhere, some time [sorry for the appalling reference], that the O2
>carrying capacity of plasma starts at a PO2 of around 1.3.
David -
From what I've read on oxygen transport, there's a small amount of O2
dissolved in plasma even at surface pressure when breathing 100% O2.
There's a good chart at the following link that shows how tissue
oxygenation rises with increasing PO2: http://www.oxytank.com/how.htm
[It's a link to a company selling medical hyperbaric chambers; it's easy
to forget that divers are not the only profession interested in
hyperbaric O2.]
> So, at 6m, 80% is
>only delivering a PO2 of around 1.28, whereas O2 is around 1.6 - making a
>considerably greater difference to deco effectiveness than would at first
>appear to be the case. IIRC, the plasma O2 capacity was around 15% of that
>carried by haemoglobin.
Exactly -- and the downstream relationship between inspired P02 and
tissue P02 levels is even more significant at 2 - 2.5 ATA, which is why
commercial and exploration divers use chambers and dry habitats to
deliver 100% 02 at 30'/10m to wash out inert gas and shorten their 10'/3m
and 20'/6m stops. [The key being *dry* habitat in order to prevent ox tox
fatalities, so don't try this at home]
And that's the primary reason why 100% deco is more efficient than 80/20:
on 80/20 only a small amount of your deco is spent at a high P02 -- the
brief 30'/10m stop -- and the 20% nitrogen you're also breathing
undermines the effectiveness of the 10' and 20' stops even more. It's
not rocket science. This isn't to say that 80/20 is ineffective or
harmful; it's just not the best combination of gasses, so why use it?
The 80/20 zealots out there never want to answer that question -- why am
I using 80/20 and not 100% O2? What we've seen in this and every other
thread on the subject is that they've been taught 80/20 in their TDI and
IANTD courses and therefore assume unthinkingly it's the best.
I can't figure out the closed mindedness we're seeing on the subject.
Even Tom Mount concedes that 80/20 is a compromise he adopted for
*training* dives. Here's a quote from the archives in a message to Bill
Mee on the subject:
From: "Tom Mount" <TOM.MOUNT@wo*.at*.ne*>
Subject: Re: Baker's Dozen Revisited
Date: Thu, 14 May 1998 15:10:35 -0400
* * *
No one has stated that EAN 80 is a better choice for a deco gas, it
is one
we think is more suitable for divers in a training program where
they are
involved with learning a lot of new skills and both gas and equipment
management techniques
>Now, the Buhlmann algorithm does not take into account such physical
>mechanisms and so such a benefit will not be demonstrated by detailed
>analysis of tables. In fact, whilst I have learnt from some of the detailed
>recent posts on this subject, I feel that many are placing too much emphasis
>on the Buhlmann algorithm.
The Buhlmann model is the best we have, but it's not perfect. It doesn't
take into account blood plasma transport of O2 -- among other things. In
that regard, it more closely models deco using 80/20 than 100% O2 which
means that I get out of the water with an extra margin of safety using
100% 02 or, alternatively, have less DCS risk if I have to leave my
10'/3m stop early because of deteriorating weather conditions or an
emergency --> which is exactly what's in the Baker's Dozen posts and what
Bruce Weinke latest RGBM work is finding.
Of course, even though this is entirely quantifiable using blood gas
measurements, the 80/20 crown still wants to claim that the oxygen window
is either a myth, theory, or applies for some unknown reason only to
chamber rides and not diving -- all without proof or even critical
reasoning.
>Of course, I base my deco on the output of this
>algorithm but modify it considerably.
So do I -- though my mods are still pretty conservative since so many
other physiological factors also affect deco [cold, hydration, fatigue,
conditioning, stress, etc.] Deco is far from an exact science, and no
one should try modifications they're not entirely comfortable with, but
like yourself, everyone diving deep should at least understand what
they're doing and why.
Best regards --
Bill
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