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Hi Craig & others, (and sorry for the waste of bandwidth for putting this on the list as well, but I got a good many questions on this and decided to get them all with one go) I'm not really familiar with the american system of expressing the tank sizes in cuft at working pressures, that are usually _not_ mentioned. You actually got a good answer on the list already, but I'll try to lay out some basics here. For that, I'll have to explain the bar/litre system briefly. Here the unit of pressure, as regards to diving, is the bar. 1bar is practically equal to 1 ATA (14,7 psi=1 bar). So, in a tank with 200 bar working pressure, you have 200 ATA gas (well, 200+1). The tank size is expressed in how many liters of water it can hold. So, a 200 bar 12 litre tank holds 2400 litres of air when filled to wp., as 12 litres * 200 = 2400. A cuft is 28.3168 litres, which means that this said tank is close to american 85 cuft tank, but as we know the working pressure, we immediately have a good picture of what kind of a tank we're talking about, hp or lp. In our system, there are no "overfills", the 200 bar tank is pressure-proofed at 300 bar, and they're rated and filled to 200 bar, and a 300 bar tank in pressure- proofed at 450 bar, and filled to 300 bar, period. I hope I don't sound too "schoolmasterish", but I just wanted to explain why it's difficult for me to hang on to these umpteen different wp's and overfills that american tanks have. A tank that is bigger in cuft can be smaller in external size due to higher wp, so it's hard to follow unless one is familiar with these tanks. Anyway, as I stated in the previous mail, the advantage of high pressure tanks is a bit dubious due to gas compressibility. This comes into the play between 200 (2940 psi) and 300 (4410 psi) bars. The theoretical gas volume (Volume of tank * pressure) no longer holds. True, the high pressure tanks are often less bulkier, but when calculating the gas requirements for a given dive, this has to be accounted for. In 300 bar tanks, there actually is only 92 % of the theoretical volume. E.g. double 7 litre tanks in 300 bar, theoretical volume 2*7*300=4200 litres. But compressibility accounted for, we have .92*4200=3864 litres left. Now, take double 10 litre 200 bar tanks: 2*10*200=4000 litres. Compressibility factor at 200 bar = .98, 4000*.98=3920 litres, which is *more* than in double 7's/300 bar, although they're theoretically "bigger" tanks. Not a big difference, but still. BTW, all these calculations are for AIR. And more: the hp tanks of course heat more during filling (more gas compressed into tank=more heat=>higher pressure), and when they cool, the pressure drops dramatically due to compressibility diffrences. Actually, if you want to have 300 bars in your tanks at the start of a dive, you'll have to fill them two times: first to 300-something, and after cooling, top them off to full 300. About the hp-systems being more prone to malfunctions, I have no "hard" evidence, just a gut feeling. I used 300 bar system for 10 years, but switched back to 200 bar a few years back for this reason. I'm not sure this is what you asked for, but I hope this helps anyway. Good dives, Juha > > I am new to the techdiver@ and have read many of you interesting comments. > As a recrational diver who enjoys maxamum bottom time and occassinal dives > to about 180 ' with short deco time, the desision between HP and LP tanks. I > currently use 95 lp dolphin (brand)(too bulky-heavy) and want to change to > HP 100's or LP 85's. Both being same diameter and weight. I am told if I > overfill the LP to 3000lb they would both have about the same cu. of air. > Your opinion of. > > Thanks, > > Craig > > > >Send mail for the `techdiver' mailing list to `techdiver@terra.net'. > >Send subscription/archive requests to `techdiver-request@terra.net'. > > > > > >
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