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John Crea wrote:

> >As to the O2 clock, there's no problem.  The O2PP for the first dive
> >1.107ATA with a single exposure allowed of 3.5hrs.  This was just a 20 min
> >dive.  The deco was less than 10 minutes on 80% O2 at 10fsw for an O2PP of
> >1.3ATA with a total of 3hrs. max allowable.  The dives were then seperated
> >by over 2 hrs on air at the surface.  The last dive was an O2PP of 1.51ATA
> >for 20 minutes with 2hrs allowed.  So each segment was well within limits
> >and in total the limits were not exceeded. But not a bad question to ask.
> 
> I am not sure what CNS O2 limits you are using, but if your ppO2 for
> the second dive was 1.5+ ATA, then you should use 1.6 as your limiting
> value, and this (according to NOAA) has a single exposure limit max of
> 45 minutes, with the maximum of 2.5 hours allowed in any 24 hour period.
> As I read the NOAA manual, their single exposure limits apply to
> each anmd every single exposure made during a 24 hour period, and the sum
> of those exposures are not to exceed the 24 hour limit.
> 
> So, how are you calculating your CNS clock?  Based on the single exposure
> limits, or only on the 24 hour limits?  Are you calculating recovery between
> dives?  If so, how?

I was about to send this when John's reply arrived, but being a trouble
maker I'll send it anyway :-) I'm with John.

The first dive was PO2max = 1.3  => NOAA limit 3 hours. No problem.

Second dive is limited by either max 24 hour exposure or max single
exposure. Whichever is smaller :-(

Thus on the second dive (at PO2 = 1.51) you have 45 mins.  

Your max 24 hour exposure is 2.5 hours, which given your profiles isn't
a problem.

IMO I don't think you should be calculating any recovery between dives.
The NOAA table gives a 24 value so it should already account for that.

I also think that the NOAA max exposure table is confusing. It shows
both CNS max and pulmonary max. If you look towards the top of the
table (0.6 ATA) your are looking at limits to avoid CNS toxicity. At
the bottom of the table (1.6 ATA) you are looking at the limit to
avoid pulmonary toxicity.

Thus whereas one might want to push the pulmonary limit at 0.6 ATA
(flame proof jacket on) it is not sensible to push it at the CNS side
(1.6 ATA) as was suggested by Hal's post, where he picked the 1.5 ATA
limit for 1.51 ATA.

However, because it is difficult to know where the cross-over occurs
between CNS and pulmonary toxicity in that table you are better not
pushing it at all ((flame proof jacket off - phew). I could tell you
where I think the cross-over is but then I'd have to kill you in case
of a law suit :-)

Alan
awright@gs*.bt*.co*.uk*

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