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Ronald writes;
> 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.
>
-----------------------------------------------------
Ronald,
Redefine the turn around point. It is not necessarily 3rds. It is the
point at which you have enough gas in one kit to get "both" of you back as the
deco obligation and SAC rates require. 3rds is not always sufficient for
cave diving either if current or certain other factors come into play.
First calculate how much gas is required to get the two of you back from
the dive you want to do adding a safety factor or contingency (next time /
depth). This is the turn pressure (or volume). Then calculate how much
will be required to get you into this situation and add it to the turn
pressure or volume. This is how much total gas you will need - if it is
more than you can carry then modify the plan accordingly. Don't forget to
add gas for buoyancy control and to compensate for different SAC's.
I'm assuming you are diving air and not using stage bottles.
Calculating this on the fly is difficult and risky because every minute you
stay adds disproportionately to deco time. You need a solid plan from the
start and you need to stick to it.
It almost sounds like you are using a dive computer and running short of
gas.
If so, get rid of the dive computer for deliberate deco diving ! Or don't
dive the computer, dive a plan ! ! !
Hope this helps,
Chuck Boone
--
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