You know Ronald, there are endless scenarios where the end result is... 
"you are screwed". You prepare the best you can and hope that the worse 
thing that will happen is a chamber ride.

You attempt to have sufficient air for most emergencies, but you have to 
do things like make sure you can find your deco bottles if you decide to 
clip them off somewhere. If you can't and thats your final option, you 
are screwed.

If you think that you may loose your deco bottles then you have to 
curtail your dive to the point where you have sufficient back gas 
avaialable for complete deco. If you don't, then you are screwed.

If you think that you can't have enough back gas then you would have 
surface support and a safety diver who can bring gas down from the boat, 
and of course there would be gas available for the safety diver to bring 
down. If there wasn't, then, a chorus now, you are screwed.

The rule of thirds is just one of the many safety measures which a diver 
can choose from, it is not the be-all and end-all. You must give yourself 
other options. And of course you are nuts if you dive with a limited 
number of options, Or, class??? You Are Screwed.

 Jim

Sender: Ronald D Thompson  Date: 9/29/98 7:06 PM

>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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