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Hello,
Well there have been numerous requests for more information on O2
toxicity and how we (WKPP) handle/mitigate the risk. I do not want to go
into great detail as most would get bored and I would wear out my keyboard,
but I will hopefully shed some light on this subject that you guys may or
may not have seen before.
The biggest question I received was for clarification on the oxygen
clock. That will be the focus of this post, and I will discuss others later.
We should all be familiar with the oxygen exposure "limits" as set
forth many years ago of 1.6 PO2 for 45 min., 1.5 for 120 min., etc. The
"oxygen clock" is nothing more than this "table" expressed in terms of a
percentage. Easily done with square profile dives (averaged depths used in
deco planning would be a close comparison) by taking the minutes of the
exposure (dive) and dividing them into the allowed exposure time for that
PO2. For example most of us are now wisely diving a 1.3 PO2 for bottom
mix, this allows 180 min for a single exposure (210 for daily). Lets say
you do a dive that last for 140 min BT using a 1.3 PO2. Take 140 divide it
by 180 and you get .78 (rounded off), multiply this by 100 and you get 78%
of the "oxygen clock". There is of course deco to account for as well,
this is also done in the same way, then added to what you have already
established. The clock concept is a convenient tool for expressing the
exposure you have endured. If you have not done a square profile then you
need to take the exposure for every depth incurred during the dive and
interpolate the values from there.
Back gas breaks also present another problem in calculating the
exposure for the dive, but they dramatically reduce the PO2s and thereby
decrease one of many risks. The last time I inquired into this, those much
brighter than I had not been able to accurately compute them. When you go
onto a back gas break you drop the PO2 that you are exposed to, thus
creating another factor that must be calculated into the total. That is
one of the reasons I gave you the 15-20,000 percent of the CNS clock.
Taking that dive (18000 at 300) for example, one can see that the daily
limit of O2 exposure was encountered before deco, where one encounters
higher PO2s. Using a PO2 of 1.0 for the dive, one has a daily (and single)
limit of 300 min. The dive was 300+ min at depth, then came all of the
deco with PO2 of 1.6. This will rapidly accelerate the "clock". The body
can handle higher PO2s at rest than it can while working. This is one of
the reasons for higher PO2s during deco. Plus if the PO2s for deco were
lower, then the deco time would be longer (but please do not think that
higher PO2s, above 1.6 will work better), and a subsequently increased risk
for pulmonary toxicity would have to be accounted for.
Most dives using a mix appropriate for the dive, not air, and correct
deco gases, no 80/20 (or as I have seen at TELFORD 74%, and it was a
swimming back gas only dive!!) or such, one will usually exceed (or come
close, depends on BT) the O2 clock.
The thought of an automatic CNS hit at 101% is probably in the minds of
many. However, most have disproved that to themselves (whether they are
aware of the fact is another story) on past dives. The real concern comes
in when the exposures are massive, or the PO2s are high ( the 40% at
130-140 PO2 of 1.9 that was recently discussed) even for short
durations. Many people have died from high PO2 spikes. This historically
accurate fact alone should be enough to eradicate deep air and poor bottle
markings, but alas . . .
More later, any specific questions you can either ask now or wait and see
if I answer them in coming post.
Sincerely,
Scott Hunsucker
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Hello,<br>
Well there have been numerous requests for more information
on O2 toxicity and how we (WKPP) handle/mitigate the risk. I do not
want to go into great detail as most would get bored and I would wear out
my keyboard, but I will hopefully shed some light on this subject that
you guys may or may not have seen before.<br>
The biggest question I received was for clarification on the
oxygen clock. That will be the focus of this post, and I will
discuss others later. <br>
We should all be familiar with the oxygen exposure
"limits" as set forth many years ago of 1.6 PO2 for 45 min.,
1.5 for 120 min., etc. The "oxygen clock" is nothing more
than this "table" expressed in terms of a percentage.
Easily done with square profile dives (averaged depths used in deco
planning would be a close comparison) by taking the minutes of the
exposure (dive) and dividing them into the allowed exposure time for that
PO2. For example most of us are now wisely diving a 1.3 PO2 for
bottom mix, this allows 180 min for a single exposure (210 for
daily). Lets say you do a dive that last for 140 min BT using a 1.3
PO2. Take 140 divide it by 180 and you get .78 (rounded off),
multiply this by 100 and you get 78% of the "oxygen
clock". There is of course deco to account for as well, this
is also done in the same way, then added to what you have already
established. The clock concept is a convenient tool for expressing
the exposure you have endured. If you have not done a square
profile then you need to take the exposure for every depth incurred
during the dive and interpolate the values from there.<br>
Back gas breaks also present another problem in calculating
the exposure for the dive, but they dramatically reduce the PO2s and
thereby decrease one of many risks. The last time I inquired into
this, those much brighter than I had not been able to accurately compute
them. When you go onto a back gas break you drop the PO2 that you
are exposed to, thus creating another factor that must be calculated into
the total. That is one of the reasons I gave you the 15-20,000
percent of the CNS clock.<br>
Taking that dive (18000 at 300) for example, one can see
that the daily limit of O2 exposure was encountered <b>before</b> deco,
where one encounters higher PO2s. Using a PO2 of 1.0 for the dive,
one has a daily (and single) limit of 300 min. The dive was 300+
min at depth, then came all of the deco with PO2 of 1.6. This will
rapidly accelerate the "clock". The body can handle
higher PO2s at rest than it can while working. This is one of the
reasons for higher PO2s during deco. Plus if the PO2s for deco were
lower, then the deco time would be longer (but please do not think that
higher PO2s, above 1.6 will work better), and a subsequently increased
risk for pulmonary toxicity would have to be accounted for.<br>
Most dives using a mix appropriate for the dive, not air,
and correct deco gases, no 80/20 (or as I have seen at TELFORD 74%, and
it was a swimming back gas only dive!!) or such, one will usually exceed
(or come close, depends on BT) the O2 clock.<br>
The thought of an automatic CNS hit at 101% is probably in
the minds of many. However, most have disproved that to themselves
(whether they are aware of the fact is another story) on past
dives. The real concern comes in when the exposures are massive, or
the PO2s are high ( the 40% at 130-140 PO2 of 1.9 that was recently
discussed) even for short durations. Many people have died from
high PO2 spikes. This historically accurate fact alone should be
enough to eradicate deep air and poor bottle markings, but alas . .
..<br>
<br>
More later, any specific questions you can either ask now or wait and see
if I answer them in coming post. <br>
Sincerely,<br>
Scott Hunsucker<br>
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