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Wow what a day!  I surfed some of my best waves in years, here in Santa Cruz! 
 A 13ft. swell with a period of 20 sec., and a sunny beautiful day, but this 
is another subject. 

First, I want to show my appreciation for all the people that have 
contributed to this list, and Jim Cobb's website.  I have learned more about 
technical diving in the past 2 weeks of reading these resources, than in all 
of my diving classes past NAUI open water.  I can't believe all the crap that 
I bought into in dive shops over the past years.  I am in the process of 
switching gear to be more DIR, and my bottom mix to be Trimix rather than air 
on my dives beyond 100fsw.

I have two questions:

1) in regard to the 80/20 I have been using:  I was taught that this was the 
safer gas in our diving environment because of a fear of O2 toxicity.  My 
plan wasn't to breath it @ 30fsw; I was making my switch at 25fsw, after 
clearing my 30 ft. stop.  Rather, the idea was to avoid pure O2.  I addition, 
the surface conditions can change rapidly in the north Pacific of Carmel, CA, 
and a 10-15fsw deco can be difficult.  I do run all my profiles through 
calculations to watch for my CNS clock, which usually is between 23-37%.  My 
hangs above 30fsw are usually 15-25mins.

Is this really an issue?  If it is an issue, at what point do I need to 
schedule a break from breathing O2?  Should I breath my 50/50 during the O2 
break and for how long? 

2) Concerning an ascent from 200fsw after a 20 min. bottom time on 18/40 
Trimix.  Would an appropriate plan be to do short deep stops starting at 150 
fsw, up to 60fsw where the switch to 50/50 is made, until 20fsw, where O2 is 
started?  How is the ascent made from the first deep stop to the switch to 
50/50? 

Thanks,

David Horne DVM  
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