Jack pointed out that:
> The general relation is PV = MRT, where
>
> P   pressure in lbs/sq.ft.
> V   volume in cubic ft.
> M   mass in lbs, ~ .0807 lbs/cu.ft. for air, IIRC
> R   gas constant, 53.34 for air
> T   absolute temp in degrees Rankine (deg F + 460)

P is absolute pressure, i.e. atmosphere is about 32 psi absolute.

So for your example: from atmosphere to 100 psi gauge (psig) = 32 to
132 psia or a factor of 4.125 times the mass of air in the tank at
constant temp.

If you cool the air from 70F to 40F as well, that's only 530R to 500R
(Rankine, absolute temp) or an additional 1.06 times the mass of air.

Overall, a 4.37 times increase in density (mass of air in a given
volume).

Regards,

Barry.
------------
Barry King
NRG Systems "Measuring the Wind's Energy"
http://www.nrgsystems.com
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