It depends what I am planning... Sometimes I'll do a long single dive in the day.. if I do that a 3hr run time for a single dive i'll run 1.3 for a 100% exposure..
I am not advocating exceeding noaa limits but I have done 5 and 6 hours in a single day with a po2 of 1.3 (usually I'll back it down to a 1.0-1.2 for the shallower stuff) .. In these case I always made sure I had at least 90min surface intervals between dives and dives get shorter as the day goes on...
On a typical 90 minute dive regardless of depth I'm still only going to hit a cns of 50% and my reatio of reall bottom time to dive time is much better than the OC diver.. I think the lack of PO2 spikes allows the body to hanle the elevated PO2 better...
The only negative effect I have ever had from rebreather use was hyperbaric myopia (prolonged elevated oxygen levels alter your vision) it generally returns to normal after a few days.. I experienced this after 3 plus weeks of 4 - 6 hours a days on my rebreather.. (and I probably could have avoided this if I didn;t run the RB as an oxygen RB at my 20ft stop on the deeper stuff)
AN occasional CCR dive probably screwes you up more than anything.. your buoyancy gets all screwed up.. The first thing I tell my CCR students (or potential students) if you really want to dive a CCR and become proficient you MUST give up OC totally for the time being.. If you keep switching back and forth you never begin to thing as a RB diver... its a totally different mind and skill set.
I can vaguely remember my progression... The first thing I remember was... Why the hell did I do this.. I just wasted my money!.. but I gave it a chance.. after about 30 hours on the unit I though this isn't that bad... by 100 hours... I'm never going back..
