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Hi Steve, It's really not necessary to fill your O2 deco cylinders all the way because you are only using them at the 20ft (6m) and 10ft (3m) stops where your air consumption is very near to that of the surface. (ppO2 at 20ft (6 m) is 1.6 ATA.) A 300 cu ft. (8500 liter) O2 supply cylinder pressurized to 2250psi (153 bar) will fill an empty 80cu.ft (2265 liter) deco cylinder to 1775 psi (120 bar) or 47 cu. ft (1330 liters). More if the deco cylinder is not completely empty to begin with. An average diver with surface air consumption of around .5 cu. ft. per minute will use O2 at the 20 ft. (6 m) stop at the rate of .8 cu. ft. (22.6 liter) per minute, and .65 cu. ft. (18.4 liter) per minute at the 10 ft. (3 m) stop. This is enough gas to last 20 minutes at the 20 ft (6 m) stop plus 40 minutes at the 10 ft. (3 m) stop with a margin of about 5 cu. ft. (140 liters). This amount of gas is typically more than enough for even a heavy exposure dive. Remember to specify pure O2 at the 20 ft. (6 m) and 10 ft. (3 m) stops in your dive software when calculating your dive tables. I typically find it useful to deco on Nitrox 40 from 100 ft. (30 m) to 30 ft. (10 m), and then switch to O2 for the last two stops. This gets me out of the water fairly quickly. Don't forget to watch the OTU's also. As for using Nitrox 50 as an emergency bail-out gas, this is definitely NOT a good idea if you are below 72 ft. (22 m). The dynamic physiology of oxygen toxicity makes breathing any gas with a ppO2 greater than 1.6 ATA very dangerous, even for a very short period. Even maintaining a maximum ppO2 of less than 1.6 ATA is no guarantee that you will not take an O2 hit. A better approach in this emergency scenario, in my opinion, is to share air, buddy breathe, or perform a controlled, emergency swimming ascent until you reach a depth appropriate for the Nitrox. An ESA won't work if you are in an overhead environment though--obviously. I hope the math wasn't too detailed. It should be clearer after your advanced Nitrox class this weekend. Good Luck & Safe Diving. Win Remley Co-Publisher, DeepTech Journal deeptek@ao*.co* p.s. Glad to have you as a subscriber.
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