Mailing List Archive Mailing List Archive
Ronald
IMHO, in your scenario there have been two or three failures. Firstly, the
divers dropping deco stages (dropping *both* could count as 2 failures)
followed by a back gas failure of one of the pair. If you need your deco
gas to complete the dive then you must ensure that you can get back to it.
David Shimell
Project Manager, Sequent Computer Systems Ltd., Sandton, South Africa.
Email: shimell@se*.co* <mailto:shimell@se*.co*>
-----Original Message-----
From: Ronald D Thompson [SMTP:rthomps@ma*.co*.mi*.ed*]
Sent: Wednesday, September 30, 1998 1:06 AM
To: techdiver@aquanaut.com
Subject: Rule of Thirds for Decompression Diving (LONG)
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
--
Send mail for the `techdiver' mailing list to `techdiver@aquanaut.com'.
Send subscribe/unsubscribe requests to `techdiver-request@aquanaut.com'.
--
Send mail for the `techdiver' mailing list to `techdiver@aquanaut.com'.
Send subscribe/unsubscribe requests to `techdiver-request@aquanaut.com'.
Navigate by Author:
[Previous]
[Next]
[Author Search Index]
Navigate by Subject:
[Previous]
[Next]
[Subject Search Index]
[Send Reply] [Send Message with New Topic]
[Search Selection] [Mailing List Home] [Home]