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Jan: Using standard mixes simplifies things. As someone else mentioned, you begin to know the deco for the dive with that mix because you use it often. It also helps your team of divers to have the same mix for a dive. That way you are all staying a team and doing the same deco. The lower PO2 (e.g. 1.2) is to avoid both Central Nervous System Oxygen Toxicity, but more importantly, to avoid seeing the respiratory degradation which occurs with whole body oxygen toxicity. The respiratory effects will cause inefficient off-gassing on long exposures. This is also the reason to incorporate "back-gas" breaks during the deco. This is where you'd switch to the lowest PO2 mix breathable at a given depth during deco. An example of this for oxygen deco at 20' (6M) would be to breathe O2 for 12 minutes and then the back gas for 6 and then O2 for 12, etc. This helps prevent the respiratory effects. Most decompression tables and programs are based on Buhlmann algorithms for decompression modeling. Buhlmann assumed, without testing, that helium would lengthen the decompression schedule. Actual experience with helium mixtures proves otherwise. Helium on-gasses and off-gasses quickly, and can be off-gassed very cleanly when doing "deep stops" and using slow ascent rates. Not only will you be coherent on dives with helium, but you will feel much better, less tired (and less other symptoms "decompression stress"), etc. after the dive. You need at least 35% helium in the mix to make any real difference. Less than that is a token amount and not effective enough. When I plan a dive using software, such as DecoPlanner, I use the table provided by the program, add deep stops, add time at gas switches to have at least 5 minutes at a switch, and then I trim a few minutes from the shallower stops, following the shape of the decompression curve. I suggest you spend some time reading the articles on the www.WKPP.org web site, as well as the www.GUE.com site. I have some of these articles captured on my own web site on the following page: http:\\members.aol.com\jsuw3\wkpp_articles.html As far as solo diving, I encourage you to develop a group of safety minded divers that are qualified to do the dives you wish to do. Begin diving as a team. By team, I do not mean independent divers in the water at the same time. Plan your dives together. Execute the dives together. Support each other during the dive. Watch each other perform gas switches, etc. It's safer and more fun. Jan Sitchin -- Send mail for the `techdiver' mailing list to `techdiver@aquanaut.com'. Send subscribe/unsubscribe requests to `techdiver-request@aquanaut.com'.
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