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The simple way to look at it is that each molecule has a mass and therefore a weight in this gravity well called earth. the high pressure tanks hold more molecules in the same volume, and this volume of gasses weighs more at the higher pressure (when full) because of the much higher number of molecules in the same volume. This higher density produces more change in weight from full to empty, and it results in more of a change in bouyancy for the high pressure tanks. > > >At 06:29 PM 2/15/97 -0900, you wrote: >>Hello folks: >> >>I have sitting in front of me the 1997 US Divers catalog and I am looking >>at the tank specifications chart. US Divers now appears to be carrying the >>entire Pressed Steel line of low pressure and high pressure tanks as well >>as aluminum 80s. For your reference, I have duplicated parts of the chart >>as an ASCII table below (Sorry you metric folks, this probably won't mean >>too much) >> >>---------------------------------------------------------------------- >> >>Tank/Size LP95 LP104 LP120 HP80 HP102 HP120 AL80 >>---------------------------------------------------------------------- >>PSI 2400 2400 2400 3500 3500 3500 3000 >>Diameter (in) 8.0 8.0 8.0 7.25 7.25 7.25 7.25 >>Wt. empty w/o >> valve (lbs) 41 46 52 27 33 38 33 >>Height (in) >> (w/valve) 27.3 29.5 33.0 23.0 27.25 30.38 29.0 >>Buoyancy >> full -4.74 -4.17 -3.8 -8.0 -8.2 -10.0 -1.9 >>Buoyancy >> empty -1.75 -1.0 0.0 -1.0 +0.5 +1.0 +4.1 >>---------------------------------------------------------------------- >> >>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. >>According to the chart, each tank changes in buoyancy as follows: >> >>----------------------------------- >> Buoyancy change from >>TANK full to empty >>----------------------------------- >>LP95 3 >>LP104 3.17 >>LP120 3.8 >>HP80 7.0 >>HP102 8.7 >>HP120 11.0 >>AL80 6.2 >>----------------------------------- >> >>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 >>you are using, but I haven't had a physics class for over a decade. So >>unless US Divers blew their own tank specs it appears that the wet volume >>of the tank and/or the starting pressure are also factors. So, I pose the >>following questions >> >>1. Can anyone out there explain the relationship between tank volume, and >>buoyancy at various pressures and why low pressure tanks do not change >>nearly as much as high pressure tanks when releasing the same volume of air? >> >>2. Is this one of the reasons that low pressure tanks are so preferred in >>cave and tech diving? The tradeoff seems to be a heavier tank on the >>surface for less change in buoyancy. >> >>3. And finally, does anyone happen to know the exact buoyancy of aluminum, >>steel, and lead in sea water? In other words, if you are 1 lb positive in >>sea water, how many lbs of aluminum, steel, and lead, respectively, would >>be required to bring you back to neutral buoyancy. >> >>Regards, >>---------------- >>Kent Lind >>klind@al*.ne* >> >> >>-- >>Send mail for the `techdiver' mailing list to `techdiver@aquanaut.com'. >>Send list subscription requests to `techdiver-request@aquanaut.com'. >> >> > -- The real Handicapped people are the ones that refuse to see their own. -- -- Send mail for the `techdiver' mailing list to `techdiver@aquanaut.com'. Send list subscription requests to `techdiver-request@aquanaut.com'.
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