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Terry,
In response to your questions on midrange (130-180fsw) gas management.
1. The thirds rule was developed with the presumption that the
second third is needed to return to the start of your decompression/exit
from the overhead environment. This is usually not the case in most
wreck dives. The method you choose is dependent upon your relative
comfort within the depth of the planned dive. (comfort can be equated
with gas consumtion rates during dives at this depth under both normal
and stressful conditions - a good indicator of your relative comfort is
knowing whether your time-depth-gas values (relationships) are in sync).
If you are not "comfortable" in the depth range I always recommend that a
diver follow the thirds rule for this type of diving.
An alternative method of gas management for a diver "comfortable" a the
given depth is to reserve 3-4 times the amount of gas needed for
decompression. (Note: Do not assume that any alternative gas is
available for the decompression). This amount will allow for the safe
decmpression of two divers with an emergency bailout ascent.
2. Equally importnat is to properly manage the way you use two
indepent tanks. Proper management is two use 1/3 of the first, then 2/3
of the second, then switch back to the first. If you use 1/3 the 1/3 you
are increasing your task loading without enhancing your overall safety.
3. Always ignore a pony tank when calculating gas management. It is
an air source that is not actively managed (monitored) during the dive
and should not be presumed usable. To many acidents have occured from
divers who assume they have air in pony's. I recommend mounting pony's
so the valve is readily available and turning off the tank until it is
needed.
4. One final point the, the lift bag bailout system is not the best
for high stress emergency. Time and air needed to deploy it may be
critical. I'll try to post a description of a better system in the next
couple of days.
Curt
===========================================================================
Curt W. Smitka, Administrative Manager Phone: (313) 936-0702
Department of Environmental Fax: (313) 764-9424
and Industrial Health E-mail: CWSMITKA@SP*.UM*.ED*
School of Public Health
University of Michigan
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