The pp02 is determined by the fraction of O2 in the mix and the diving depth. The length of the dive has nothing to do with it. I think y'all might be talking about oxygen exposure.
For recreational diving the maximum operating depth is your primary concern. It's good to track the exposure but it's not usually much of a concern for recrational diving, and in practice most recreational divers do not even bother tracking it. I probably would track it if diving nitrox, many dives a day from a liveaboard. Every nitrox computer I have seen will track exposure for you.
Oxygen exposure tracking becomes critical when doing repetitive decompression diving. When in cave country I sometimes fill my doubles to 4000psi of EAN32 and carry another AL80 stage of EAN32 and make very long dives in the 100 ft range. This gives me *much* less decompression time than air would have. I then accelerate what decompression I do have to do with 100% O2. (See my avatar for a recent decompression stop). The downside of this is that the oxygen exposure can get quite high when doing a lot of repetitive dives. If I then takes a DCS hit (certainly very possible when doing decompression diving) the treatment options are a lot more limited since further O2 exposure in the chamber might not be a good idea due to whole body toxicity concerns.
Others may disagree, but IMO, nitrox is only becomes beneficial for recreational dives when you become limited by your NDL time and not by your air supply. For my breathing rate this started to happen in the 80-100 ft range, or even shallower with a lot of repetitive dives. So, recreational diving, I use nitrox when doing many repetitive dives over a short period of time or when diving in the 80-130 ft range.
Edited by grim reefer, 04 January 2012 - 05:38 PM.
The sea, once it casts its spell, holds one in its net of forever - Jacques Cousteau
