I just read this today and I was surprised to see that there was no
additional comment on that.

You say you switch to air from your pony when coming back from 200 for a
buffer?  For what?  The amount of gas you absorb during ascent is small
(this won't affect your deco by 20 minutes!).

Deep stops are a good idea.  Many people have some difficulties
understanding decompression theories.  It should be noted that most models
(haldanean and pseudo-haldanean) consider only nitrogen absorbtion and
elimination.  These models assume (or do not even consider) that no bubbles
are formed and they certainly do not consider the effect of bubble
formation on gas elimination.

Nitrogen absorbtion-elimination based theories will dictate a faster
ascent, and shallower decompression stops.  The reason is that the lower
the ambient pressure, the greater is the gradient between PPig and PPtg
(inspired partial pressure of inert gas and tissue partial pressure of
inert gas).  Thus, with a greater gradient, elimination is achieved faster.
 These models use some form of allowable supersatation ratio (M values,
coefficients, etc) to determine how much reduction in inspired partial
pressure of inert gas we can take before bubbles are formed.

In practice, bubbles get formed.  Often, no symptoms are felt.  These
bubbles are filtered through the capillaries in the lungs.  However, when
this happens, inert gas elimination efficiency is affected negatively.  The
underlying nitrogen absorbtion-elimination decompression model becomes
flawed.  The risk of obtaining symptoms will increase on successive dives
or it will also be greater for susceptible individuals (the ones with PFO).

To address that, some decompression theories consider gas phase separation
and bubble mechanics in their models.  This said, they consider bubble
formation and growth.  It may be logically deducted that bubble prevention
is achieved by 'not ascending'.  Back in 1854 (I think), it was known that
it is not the dive that cause the problem, it is the ascent. Sure, we MUST
ascent.

So, a solution is to reduce the speed of ascent and to stop at certain
levels during our ascent.  The objective is to allow for a reduction in
inspired inert gas partial pressure, but to an extend where we are sure
that bubble formation is better controlled.  Theories that consider bubble
formation and growth will often require deeper decompression stops and
slower ascent.

While most people believe that deep stop penalize the diver on
decompression time, it should be known that by avoiding bubble formation,
we increase the decompression efficiency.  So, decompression *should* be
shorter.

Some people know about that; however, the widely used decompression models
(Buelmann) are not developed like that.  

J.P. Imbert (Comex) wrote an interesting article last year.  What he
explained is that there are two types of bends:
1. the friendly bends : pain only (type I DCS)
2. the serious bends : neurological problems (type II DCS)

Type II DCS will be better avoid by taking into consideration bubble
formation during ascent and by including deeper stops in the profile.

For type I DCS, Imbert suggested that the present decompression models are
way too much sophisticated.  There is no purpose to have 32 tissue
compartments (how many times did you get bent in the belly? ;-).  He
suggested that a single tissue model would take care of that problem in his
article.  Sadly, I can't find it back (too many magazines in my library and
I don't remember which one it was).

Michel Therrien


At 03:07 PM 4/14/98 -0600, David Morrison wrote:
>Hello,
>
>On a recent dive trip visiting divers were making deep stops when using
>Trimix.  Until now I have always followed the deco schedule provided to
>me by my diving software.  Depending on the dive, I carry a pony bottle
>between my doubles with Air. As I pass the 200 foot mark I'll switch to
>air just as a buffer - this switch is not in the dive plan and I don't
>include it in the deco profile. Generally, I don't waist any time
>getting to my first stop. Depending on the dive, I'll travel at 100 fpm
>to around the 120 foot mark.
>
>The divers on this trip were halving their bottom depths and doing 2
>minutes before proceeding to their first required stop. For example, on
>a dive to 250 feet for 20 minutes (using a 17/35, 40% and 80%) they
>would stop at 150 feet for 2 minutes. 
>
>When I plan a dive to include these deep stops there doesn't seem to be
>much difference in the overall dive time, CNS% or OTU values. Also, I
>can't find much literature to support the additional safety factor that
>this practice is supposed to provide.
>
>I assume that the deep stop allows better (slower or more consistent?)
>off-gassing of the fast tissues and therefore limiting the potential of
>a hit involving CNS - a serious problem.
>
>Any thoughts...
>
>
>
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