Mailing List Archive

Mailing List: cavers

Message Display

Scott, I was working on me gear yesterday and found a copy of your
protocol for tox. If you have that loaded into the machine, maybe fire
it out and remove the specific references that we do not want public,
and shut these guys up .


Scott Hunsucker wrote:
> 
> 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

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]