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1) Assuming temperature stays constant (not a fair assumption maybe): P1 * V1 = P2 * V1 (from the ideal gas law)
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Originally Posted by jee7s
This is a bad assumption. The ideal gas law assumes that the energy in the system in constant. The compressor of course puts energy into the system, so that violates PV=PV.
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The ideas gas law does not require that the energy in the system remain constant. However, on the other side of that equation, in addition to R (constant) and T (assumed constant) is n - which is definitely changing. You need to add the "new" air, which began at a gauge pressure of zero. This serves as a reminder that you need to add about 15psi to every gauge pressure that you're measuring to use it.
If you did not account for the slowing, the time to fill a 35 in^3 tank from zero pressure would be based on the volume of this gas after it were returned to ambient. Since 120 = 15 * 8, we started with 8 x 35 in^3 = 280 in^3 = .162 ft^3 outside the tank, in the atmosphere. For a 1 cfm pump, this would take about 9.7 seconds to compress. I haven't measured it, but it seems that the sound of an air compressor drops about two octaves as it goes from zero to 120 psi. Assuming the same amount of air is compressed in each cycle, this means that at the end, it is pumping about 1/4 cfm, which would take 39 seconds to fill. Your time of 20 seconds to fill from 60 to 120 sounds about right.