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Hi Steve, Regarding our earlier discussion on deco gasses, cylinder pressures, and nitrox: I fear I have confused the issue by mixing apples and oranges (steel & aluminum) -- my appologies. The cylinders I typically dive with are: Cylinder Working Pressure Volume @ WP Gas Purpose ------------- ---------------- ----------- --------- --------------------- Steel 72 2475psi (168bar) 72cu.ft. 100% O2 Deco 20-10ft Dbl Steel 95s 2475psi (168bar) 95cu.ft. ea Air/EANx Travel Gas/Bottom Mix Aluminum 80 3000psi (204bar) 80cu.ft. EANx40 Deco 100-30 ft Aluminum 80 3000psi (204bar) 80cu.ft. TriMix Bottom Mix (>200ft) Since it is my steel 72+ that I fill with O2 I normally think of the pressure as being the 2250psi (153bar) stamped on the neck. However being a steel "plus" tank it doesn't contain the advertised volume until it is 10% overpressurized to 2475psi (168 bar). But I never actually have this much pressure in this tank due to the filling method. Using these more accurate numbers: My steel 72, when filled from a 300cu.ft. supply cylinder at 2250psi (153bar), will contain approximately 1800psi (120bar) or 52.4 cu. ft. of O2. (52.4/72 = 73% full). This still gives most divers about 20 minutes at the 20 ft. (6 m) stop and 40 minutes at the 10 ft. (3 m) stop with a comfortable margin. The 232 bar (3400psi) tanks that you dive with are not very common here, although I have started seeing more of them around. Since your convention is to express cylinder volume in a 1 bar state (i.e., 10 liter = 80-85 cu. ft.) how do you calculate air consumption? Don't you calculate gas volume used per unit time? Whether or not I stage my cylinders depends on the type of dive I am doing. In cave diving it is common to stage your tanks (if you don't come out to get them you won't need them anyway). In wreck diving it is more common to carry all gas throughout the dive. Wearing quads takes some getting used to. Also due to the weight, redundant BCDs (Wings) are a must. I once punctured a set of wings and had to practically crawl back to shore. Yes, Haskel pumps are being used in some locations to boost O2 pressure. However the most common approach, at least in Florida, is the partial pressure filling method. And yes, the second O2 cylinder would get only about 1450psi (99bar) or 60% filled. We typically fill two O2 deco cylinders from a large O2 supply cylinder, and then use the remainder of the supply cylinder for mixing Nitrox. When the O2 supply cylinder gets below 400psi (27 bar) it's pretty much useless for filling purposes. Using this method also requires every piece of equipment in the fill system to be O2 clean (see article about Nitrox by Dick Rutkowski in our first issue). As for the Nitrox mixtures available commercially you will probably only be able to buy EANx32 or EANx36 over here unless the shop owner knows you personally. If he has trust in your diving skill/knowledge, you can get just about anything you want. They worry about liability, rightly so. You will typically pay around 6-8 US dollars for a 32% or 36% fill. Pure O2 fills run around 25 dollars US. The "Home-Brew" solution is very common among cave divers in Florida. The problem is finding an air fill station to top off your cylinders that is sufficiently filtered (clean). "Topping-Off" from an air fill station that is not well filtered can contaminate any O2 clean cylinders you may own. On the other hand, if you can find a Nitrox fill station that stocks banks of NitroxI or Nitrox II supply cylinders, O2 clenaing of your equipment is not necessary due to the relatively low percentages of O2 involved. (This statement will probably create some discussion) Decompressing on O2 is not mandatory of course. Any Nitrox mixture or air can be used instead. The trade off is deco time and gas volume required. As for using Nitrox mixtures above 50%, I don't like to do it because you have to place too much reliance on the cleanliness of the air fill station when topping-off the tanks with air. However, some divers use mixtures up to 80%. Let me know if I have confused things further. Win Remley Co-Publisher, DeepTech Journal deeptek@ao*.co*
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