Mailing List Archive Mailing List Archive
Intro. Hi My name is Rich Chapski. I've been reading this list for some
time now but have not responded to anything yet. So hello to all my
friends I've seen send mail on this list.
I'm a PADI Master Instructor, IANTD Nirtrox Instructor, Full Cave
trained through the NACD and I'm just completing my IANTD Trimix course
I've been diving since 1986 and actively teaching Scuba since 1988.
Wrolf asks about O2 tox calcuations on a typical Doria dive. I ran through
some as a lunchtime exercise.
First, one has to define "typical Doria dive." I am speaking second hand
here, but I understand that since the Doria has such a high profile (254 fsw
max depth, 170 fsw to the side) that a "typical" dive can be anywhere from
195 fsw max (touring the Promenade deck) to 225 fsw (hunting for second
class
china).
Second, one has to compute cumulative O2 tox for the whole dive. Randy
Bohrer has developed an approximation for this which will be used in the
Dive-Rite nitrox computer (the Bridge). The procedure is to divide a dive
into time increments, determine the PO2 a diver is breathing in each
increment
based on FO2 in breathing mix and depth, and then compute the fraction of
the
NOAA CNS tox limit for that PO2 which the increment constitutes. E.g., if
you
are breathing air at 218 fsw, then your PO2 is 1.6 ATAs with a NOAA limit
of
45 min, and 10 min at this depth would be 0.22 of the CNS limit. One then
sums the fractional limits over time increments to get the total CNS
exposure.
This should be a conservative model, since going from high PO2 to low PO2
should
actually reduce cumulative O2 effects, whereas in Randy's model the
exposure
function does not decrease with time regardless of PO2.
Calculation:
NOAA limits for PO2 can be found in your nitrox manual. I happen to have
one
here, but don't have the Navy tables handy (the ones in the manual only go
to
150 fsw), so the following calculations are based on a guess at the actual
hang time required:
200 fsw / 20 min bottom time
3 minutes to ascend to 50 fsw
50 fsw 1 min
40 fse 3 min
30 fsw 8 min
20 fsw 15 min (on O2)
10 fsw 30 min (on O2)
Your PO2s, NOAA CNS limits, and fractional limits are as follows:
Run
Time PO2 NOAA Fract. limit Cum. Limit
0-23 1.47 120 .19 .19
24 .53 720+ .001 .19
27 .46 720+ .004 .20
35 .40 720+ .01 .21
50 1.6 45 .33 .54
80 1.3 180 .17 .71
Here I have included ascent in the bottom time, and have rounded PO2s up
when
looking up the NOAA limit, for conservativism. I should emphasize that the
deco stops I have listed are purely hypothetical, but shouldn't be too far
off.
The total cumulative CNS exposure on this dive is thus 0.71. Dives which
do
not exceed 1.00 are assumed not to exceed NOAA limits, hence this
hypothetical
dive does not exceed NOAA limits.
I agree with John calculations above. The only tweek I would make is
the ascent from 200 feet to the first deco stop(50 ft). I would find the
average depth between 200 ft and 50 ft. In this case 125 ft. then add this
in to the calculation for the 3 min. The 200 foot dive would then only be
calculated as 20 min. If you add this additional step in you would
actually come up with a number slightly less than the .71 John came up
with.
These calculations are great but there are some other factor which need
to be mentioned. Exercise greatly reduces a divers tolerance level to
Oxygen, Thermal stress is another factor, CO2 build also reduces ones
level. All of the item can be present on this type of dive. From a numbers
point of view O2 toxicity should not be a problem. From a practical
point of view one must take all of these factors into consideration.
One other point I would like to make is the incident happened on or
inside the Doria. It didn't happen at the end of the dive when ones
O2 clock would have run up towards the limit.
Disclaimer: the procedure I have outlined is as Randy briefed it at the
1993
Boston Sea Rovers show, and as I have discussed it with him subsequently.
It
should be a conservative approach to O2 safety, but I (and I assume Randy
also)
cannot be responsible for any consequences of its use.
Richard Chapski
Chapski@Ri*.SC*.Sy*.CO*
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]