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>Could somebody replay the "Baker's Dozen" reasons why we do not use
>80/20.
Bill Mee then posted this response:
--------------------
George,
Thank you for exhaustively laying the reasons why we or anyone else
should not use 80/20. The only thing missing from this discussion is
the Q.E.D. at the end.
Reason #8, reiterated here for discussion purposes is perhaps the
soundest reason, among many very cogent ones, as to why this practice
should be avoided:
" Any perceived decompression benefit of using a higher ppo2 at 30 feet
with 80/20 is then given back by the lowered ppo2 at 20 feet, not to
mention the fact that the presence of the inert gas in the breathing
mixture defeats the purpose of using oxygen in the first place ( see
"The Physiology and Medicine of Diving") . The ppo2 of 80/20 at 20
feet is 1.28, not much of an oxygen window, and at 10 feet it is 1.04
useless for deco. To make matters worse, you can not get out from your
30 foot stop in an emergency ( not doing the other stops) on the 80/20
mix without really risking a type 2 hit. "
The rush to embrace this practice, recommended by technical diving
diving opinion leaders, was widespread and in retrospect, irrational and
poorly thought out, like so many of the "trial balloons" in this field
of endeavor. It seemed to many, at first glance, to be a simple means of
increasing one's supply of deco gas while eliminating its bothersome
volume and mass. In fact, the perceived benefits tranform into
liabilities when subjected to a thoughtful analysis. When you view
decompression as a two pronged challenge: to progrssively widen the
oxygen window and increase the diffusion gradient to maximize passive
transport of dissolved inert gas, it becomes clear that the 80/20
solution falls short on both requirements at a critical point in the
decompression profile.
Section 11 emphasizes a very compelling argument for those who are
concerned with managing dive related crises. When diving in the open
ocean divers and boat operators should always be prepared to "scram" the
deco at any time. This could be for any number of reasons, not the
least of which might be a sudden change in the sea conditions or
unscheduled events such as dive accidents or impending ship collisions.
Just follow the Whitefish Point thread for an excellent example of why a
deco may require being aborted (or never started in the first place).
Most unfortunately the "80/20 problem" bears a strikingly resemblance to
several other ad hoc technical contrivances mentioned in this same
article (section 7) i.e. dual bcs, colored regulators, bilateral stage
bottle postioning, poodle jacketed second stages and harness quick
releases. All of these ideas, while seemingly reasonable, become
tainted when subjected to thoughtful review.
Good show Director Irvine.
Bill Mee
---------
"C'mon, you apes,
you wanna live forever?"
-unknown Platoon Sergeant, 1918
---------
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