Ronald, the rule of thirds gas management practice applies only to the bottom
gas.  In the event of 
a catastrophic loss of gas by one diver at the turn-point of the dive, this
will allow both divers 
to get to the first deco stop.  Having double the needed amount of deco gas
would ensure that both 
divers could complete the decompression according to the original plan.  Proper
deco gas selection 
is critical here.  In the event of a loss of your deco gas, using a gas such as
80/20 or any gas 
with an insufficient oxygen pp is a liability if you have to interrupt or blow
your deco.

-Sean


On Sat, 26 Sep 1998 19:35:04 -0500 (CDT), Ronald D Thompson wrote:

>
>Has anyone resolved the fact that the Rule of Thirds is often (or seems to
>be) inadequate for decompression diving?  I specifically refer to the
>observation that in decompression diving, "going" from the starting point
>to the turn-around point of a dive often (usually?) requires far less time
>and gas than "returning" from the turn-around point to the starting point
>of the dive (because of the decompression obligation).  This means, for
>example, that if one of two identically breathing and kitted dive buddies
>has a total gas failure at the turn-around point, then not both divers can
>return safely to the starting point of the dive if they are diving the
>Rule-of-Thirds!
>
>Does anyone have a standard "fix" or "modification" for the Rule of Thirds
>(in the context of decompression diving) that he/she uses?
>
>Thanks in advance for your responses, and apologies if this has been
>discussed earlier (I couldn't find a reference) in the Archives).
>
>Ronald
>
>P.S.  The only obvious way around this seeming inadequacy with the Rule of
>Thirds that I can see is for me to complete a formal (and time-consuming)
>gas-matching optimization computation using a set of obvious constraints.
>I do this as a matter of course.  However, I was wondering if someone had
>an approach that was more ad hoc (and much quicker) and less brute force
>than mine.
>
>P.P.S.  This problem plagues me whenever I begin planning for the
>loss-of-deco-gas scenario while planning an accelerated deco air wreck
>dive.
> 
>
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