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Thanks to all who responded to this thread.

My original query which began this thread stemmed from my thinking about
the following scenario:  Each of a pair of air divers descends with two
deco mixes, and removes both deco bottles and leaves them on a wreck at
the point the buoy line ties into the wreck.  The pair penetrates the
wreck via an access point nearby.  At the turn-around point deep inside
the wreck, one buddy loses *all* of his/her backgas.  Upon exiting the
wreck, the now air-sharing buddies cannot locate their four deco bottles
(or the buoy line).  The buddies must now make a free ascent, blow a bag,
and deco on one diver's remaining air.

My original question was:  How might the Rule of Thirds have been altered
to allow both of these unfortunate divers to reach the surface safely?

My own naive view was (is?) that the Rule of Thirds was never intended to
be used for dives where the above scenario is a possibility, and that even
if one is intending to complete an air dive using accelerated deco, he
should plan his *air* requirements so that both he and his buddy can
safely complete the dive even if the extreme scenario described above does
in fact unfold.  Planning this way is laborious and time-consuming (albeit
not difficult), and I was hoping that someone on the list could provide a
quick(er) way of getting minimum air requirements, etc.

Most of you who responded seem to agree that the Rule of Thirds is
inadequate for this scenario, and many of you offered suggestions for how
planning might be done in this case.

I found the following example enlightening:

Jim Cobbs Sept 28th post to this list (Subject:  Re: Trimix question)
included schedules for 40 min air dives to 130 fsw.  The first schedule is
for an accelerated air dive, and the second is for an air dive with air
deco.  The schedules were generated using Decom.  Both are reproduced now:

		Depth	Time		Depth	Time

	Air	130	40	Air	130	40
	EAN50	40	2	Air	40	2
	EAN50	30	4	Air	30	6
	O2	20	6	Air	20	13
	O2	10	9	Air	10	22


Consider, now, two divers who intend to complete the accelerated deco air
dive.  Lets agree to work under the following constraints:

	1.  Descent rate is 60 fsw/min
	2.  Ascent rate is 30 fsw/min
	3.  The switch from air to EAN50 occurs on the fly, at 70 fsw
	4.  Diver A has a working RMV of 0.7 cu ft/min, and
	    Diver B has a working RMV of 0.5 cu ft/min
	5.  Resting RMVs are ignored when dive planning


According to my calculations, Diver A will consume 141.0 cu ft of *air*
and Diver B will consume 100.7 cu ft of *air* if they complete the
accelerated deco air dive.  On the other hand, Diver B will consume 188.6
cu ft of *air* and Diver B will consume 134.7 cu ft of *air* if they must
complete the dive entirely on air.

Case I.  According to the Rule of Thirds (turn the dive when 1/3 of the
bottom gas is depleted, so that 2/3 remain for exit and emergencies), the
minimum *air* required for Diver A is 209.8 cu ft.  And, therefore, gas
matching dictates that the min *air* required for Diver B is 169.8 cu ft.

Case II.  According to a more conservative planning rule which states that
each diver must be able to perform self-rescue and buddy-rescue (though
not simultaneously), on air, back to the first deco stop, the min *air*
required for Diver A is 212.0 cu ft, and for Diver B is 171.8 cu ft.

Case III.  According to an even more conservative planning rule which
states that each diver must be able to perform self-rescue and
buddy-rescue (though not simultaneously), on air, back to the surface, the
min *air* required for Diver A is 307.3 cu ft, and for Diver B is 253.4 cu
ft.

Some observations:

1.  The Cases above suggest that min air requirements determined using the
    Rule of Thirds are not sufficient.  This air cannot get buddies back
    to the surface, perhaps not even back to the first deco stop.

2.  Many (most?) of you who responded to this thread use some variant of
    the planning rule used in Case II when planning such dives.

3.  In the past, I have been inclined to use a variant of the planning
    rule used in Case III to plan my dives.  This has necessarily severely
    curtailed my bottom times.  Maybe I have been too conservative...


Thanks again for all your input.

Ronald


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