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Ken,

I haven't been in a physics class for over two decades, but like you, I
think this cannot be. A given amount of air will retain the same number of
molecules, therefore the same mass, no matter how much you compress it. So
removing a given amount of air from *any* scuba cylinder should result in
the same change in cylinder weight.

I have a reference that gives the density of Nitrogen as 1.1649 grams per
liter, and Oxygen 1.3318 grams per liter. If I did my math right, that works
out to 0.075 pounds per cubic foot (probably at STP). So 100 cubic feet of
air at STP weighs about 7.5 pounds, and it will still weigh 7.5 pounds at
2400 psi.

Since buoyancy is equal to the difference in displaced mass (i.e., something
that displaces 100 pounds of water but weighs 101 pounds will be 1 pound
negative), the change in mass translates directly to a change in buoyancy.

=Art=

At 06:29 PM 2/15/97 -0900, Ken Lind 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.

[ ... balance snipped ... ]

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