Dave, go to the FAQ at www.wkpp.org, and read the "Baker's Dozen" post.
 This pretty much sums up the 80/20 thing.  As for the O2 breaks, these
need to be done whenever you approach or exceed the standard CNS
"clock", and are not a bad idea in any case.  Something like 5 minutes
every 25, and they need to be to the LOWEST breathable PPO2 gas, ie.
bottom mix.  Do not add time to your stops to accomodate these breaks. 
You also will notice an improvement if you use trimixes as deco gases,
such as 35/30 and 50/25, in addition to the oxygen.  You could probably
go higher on the helium on the bottom gas.  The closer you can get to
heliox, the better.  I just settle for the lower He mixes because I
don't have a booster pump.  As for the ascent, at minimum, the deep
stops should start at 4/5ths of the bottom depth, or 160 in that case. 
Not sure how you are deriving your deco profiles, but if you are using
a neo-Haldanean model, such as the Buhlmann ZLH-16 used in the popular
software programs, this model actually calls for a 33 foot per minute
ascent rate to be accurate (which it isn't anyway, but for the purpose
of discussion...)  This equates to a 20 second stop every ten feet
during ascent, at the very minimum.  The time spent on one of these
"deep" stops is proportional to the BT, on a scale from that minimum 20
seconds for a bounce, to around 5 minutes or so at saturation. 
Continue this pattern of deep stops until you reach the stops predicted
by whatever model you are using.  Taking these stops into account, your
actual ascent rate between stops should still be reasonable, so shoot
for a max of 33 fpm, and cut the rate to almost zip on the last 20 feet
to surface (this is the largest pressure drop and highest potential for
damage).  Also, at the gas switches, you need to remain at that depth
for a couple of minutes to allow that gas (your blood) to circulate,
before continuing the ascent.

-Sean


On Mon, 6 Dec 1999 01:30:46 EST, Deep1dave@ao*.co* wrote:

>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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