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Kent Lind <klind@Al*.NE*> wrote: >What strikes me immediately when viewing this chart is that the change in >buoyancy for low pressure tanks is far less than for high pressure tanks. > ... >I was under the impression that breathing a set volume of air....say 100 cf >will add the same amount of buoyancy to your tank regardless of the tank ... A rule of thumb is that 13 cubic feet of air at standard temperature and 14.7psi weights almost exactly one pound. Take 13cf out of any tank and that tank becomes 1lb lighter regardless of pressures or tank materials. What I think they're trying to tell you is that they tested the LP tanks with a fairly lean trimix. After all, you wouldn't want to offer a good techdiving tank with any sort of suggestion that users should be Good Deep On Air (tm). It's also possible they only ran the LP's to thirds or something. Meanwhile, they must have been testing the HP tanks as argon bottles or using really really humid air to see if they would rust or overfilling them or something to get such high numbers. Here's your table with another column, based on rated cf divided by 13: >------------------------------------ > Buoyancy change from Should >TANK full to empty Be >------------------------------------ >LP95 3 7.3 >LP104 3.17 8.0 >LP120 3.8 9.2 >HP80 7.0 6.2 >HP102 8.7 7.8 >HP120 11.0 9.2 >AL80 6.2 6.2 (bingo!) >----------------------------------- Meanwhile, the desirability of LP tanks is based on three factors: - being steel, they're heavier both before and after the dive so you don't need to wear a weight belt to keep them down as with AL tanks. - the lower working pressure causes less wear to regulators, increasing reliability. - the lower working pressure makes it easier to get a good fill, especially with a weak compressor or a cascade or an O2 or He bottle that's getting low. I do sort of wonder, though, if these LP tanks come from the same factory (Pressed Steel Tank) as the HP ones, if the HP reputation for early rusting might extend to these LP ones as well. I understand that OMS LP tanks use a different and better galvanizing process. -- Anthony DeBoer http://www.onramp.ca/~adb/ adb@he*.re*.or* (here) adb@ge*.co* (work) #include "std.disclaimer" -- Send mail for the `techdiver' mailing list to `techdiver@aquanaut.com'. Send list subscription requests to `techdiver-request@aquanaut.com'.
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