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On Mon, 17 Apr 1995, Steve Millard wrote: > This means that in your example, the 80 cu.ft. deco O2 cylinder is a > 'full-size' cylinder working at approx 1/2 normal working pressure. Is this > what people on your side of the Atlantic are normally using for O2 deco ?? My primary O2 deco gas supply is an industrial cylinder in the boat with a 50-foot HP hose and an O2-clean regulator. This way, no transfilling of O2 is needed, and you have access to all the O2 you need. The rig doubles as an excellent IWR rig if you need it. > This is fine if the cylinder is waiting on a surface bouyed line .... > assuming you make it back to the line *every* time, but a pain to lug aroung on > the whole dive in addition to the main twinset if you don't want to risk missing > the deco line. I take a 20 cuft aluminum medical O2 cylinder rated to 1900psi (sorry about the 'merican units) on the dive with me as an emergency supply in case I can't get back to the surface supply rig. The pony is enough to do most or all of my O2 deco on most dives. If more deco is needed, I just breathe off my nitrox-32 for triple the amount of remaining time of the O2 stop, then surface, get the attention of the boat tender, and go back for additional time on O2 when I get access to the rig. Filling the medical cylinder from a bank of 3 industrial cylinders works fine for many fills. > We normally send up a delayed bouy deco line from the bottom or on the > way up. That way we can do our stops 'hanging free' of a fixed line. It means > that the surface boat has to follow all the dive-pairs separate bouys, but with > our often strong currents the advantage is a stress-free deco stop rather than > all 'flapping like flags together' on a fixed line. There are many other ways > of working this out...but this is our most common practice, so we need to have > all of our deco gas with us on the dive. In my experience, this is by far the best way to do decompression time in a current. If you send the float up from your deeper nitrox stops, then the boat should be on you by the time you get to your O2 stops and they can deploy the surface-supplied O2 rig. If the boat never comes (engine trouble, etc.), then you always have your backup O2 with you (and your reserve of nitrox). > Alternatively we need to get a bank of 3 or 4 O2 storage cylinders. > More costs ! I use a bank of three...not outrageous. I've generally use the lowest pressure as the surface supply and run it near-dry during deco (of course we also have another full cylinder in the boat just in case, and the boat tender pays close attention) > > 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. > > OK... but using a 4 litre/33cu.ft. 'pony' deco cylinder, it may well not > be enough if it's only '1/2 full' or less. Using a decanting system, a smaller pony cylinder will fill to a much-higher-than half pressure, because the equalization pressure for a large capacity scuba tank will be lower than the eq. pres. for a small tank. > As a matter of 'academic interest' however, ...if you had a for example > a Nitrox50 deco set and went onto it at say 50m in order to 'bolt for the > surface' (say you had a manifolded twin set, had a catastophic free flow & you > were unable to shut off the central isolation valve & hence lost all of your > bottom gas), the nitrox50 deco bottle would give a max ppO2 of 3 bar, which > would quickly reduce to 2 bar at 30m and finally to a 'safe' 1.5 bar at 20m. > The max ppO2 is nearly 2 times the accepted 'safe' limit (although there seems > to be differing opinion on just how much is really safe) Any idea of how long > you would get (seconds/minutes ??) before an O2 hit 'got' you ?? In other > words, if all else failed would going onto the Nitrox50 set be totally suicidal > or would you stand a fair chance of making it from 50m ? The probability of an O2 hit at 3 bar (for a few seconds) is a helluva lot lower than the probability of drowning if you have nothing to breathe, or the probability of AGE if you hold your breath. Bottom line: with no other option, breathe first, worry about O2 convulsions second. The question of how much time you have at such high PO2 cannot be answered. Considering the variaty of factors involved, and the range of individual variation for O2 tolerance, that value could be anywhere from a few seconds to many hours (breathing O2 at 3 bar). To reduce this dilema, I plan my mixed-gas dives with deco on nitrox-32 from 130fsw-20fsw (again, sorry for the units), which reduces the probability of O2 problems if I have to use the nitrox as a bailout from 250fsw (or whatever). The decompression advantage of using nitrox-50 instead of nitrox-32 is only a matter of 10-15 minutes even on relatively hard-core exposures (2 or more hours of required deco). Aloha, Rich ******************************************************************* Richard Pyle deepreef@bi*.bi*.ha*.or* "WHATEVER happens to you when you willingly go underwater is COMPLETELY and ENTIRELY your own responsibility! If you cannot accept this responsibility, stay out of the water!" *******************************************************************
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