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again, UTTER BULLSHIT - you are repeating the dumbest fairy tale in diving.
The fact is you should stay ON helium for as long as you have it. Helium is
the most suitable medium for diving and deco. Where do you guys get this
bullshit?
Some of our divers do their entire dives on mixes with little or no nitrogen
all the way to the oxygen steps.
Nothing any of you is citing explains anything that has anything to do with
reality and practice.
-----Original Message-----
From: Clifford Beshers [mailto:beshers@cs*.co*.ed*]
Sent: Thursday, April 19, 2001 9:28 PM
To: scottk@nw*.co*
Cc: techdiver@aquanaut.com; trey@ne*.co*
Subject: Re: Counter diffusion
>From: "Scott" <scottk@nw*.co*>
>Cc: "Trey" <trey@ne*.co*>
>Date: Thu, 19 Apr 2001 09:38:21 -0700
>
>George,
>
>Can you put the issues of "counter diffusion" into language a Jarhead can
>understand?
>
>I was accosted this past week by a "Master Diver" who warned us that "If
you
>have to take a chamber ride after breathing that shit (helium) it will
kill
>you."
>
>I was so stunned by his position and attitude that I just nodded and
>listened.
>
>Plus, I have no idea what counter diffusion is, or if or why it is an
issue
>to SCUBA divers.
>
>Scott
>
I have no idea. So I'll answer the question anyway.<g>
SBH on rec.scuba, another of your faves, was babbling on about counter
diffusion and spikes with the backgas breaks between O2 and mix.
While I don't consider him a good source of safe diving practices, he
seemed to know his medicine. Here's what he said:
When you switch to a new inert gas at the same pressure ("isobaric
switch") you undergo isobaric "counter-diffusion" where your new
inert gas goes from lung IN to tissues, and the old one starts to
come OUT of tissues into lung. If the new gas is lighter, it goes
in faster than the old one comes out, for a while. This is bad
because if the tissues are already oversaturated, the total inert
gas partial pressure in them goes up for a bit (this includes in
the blood, where the effect has been directly measured) and this
contributes to bubble formation.
There is no question of whether or not you should switch from
light inert gases to heavy ones on deco-- the only question is
when, and how many stops to do on helium first. You've heard
varying views, and I don't know the answer. In theory, you should
get off helium as soon as you can, so it can diffuse out and not
in (there is a lot of it loaded, so a lot has to come out). There
is also the question of whether to do Pyle stops, which are deep
stops which the Buehlmann partial pressure gradients models really
do not demand, but which some divers have said made them feel
better, and have suggested are due to bubble kinetics. No
empirical evidence for them. They were and are advocated by an
ichthyologist who noticed that when he stopped to mess with his
speciments by stopping a third of the way up from very deep dives,
he felt better. Hmmmm. It's not clear to me if he was doing this
on helium or nitrogen. Helium offgasses so fast that your normal
ascent rate might be fast enough to avoid such things, and they
may work (if they work) only on deep air divers. Speculation on my
part, but I really dunno.
SBH
I think this is a question for Quest. In the vein of improving lists
by asking questions, I think I will go ask...
--
Clifford Beshers
--
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