Enough talk! Let's science!!

Here we see my compressor at 0 psi weighing 52.8lb:

Here we see it at 120psi weighing in at 54.3lb:

Discuss!

Simple. Just apply PV = nRT, since Volume,Temperature and R ( assuming ideal gas laws and no phase changes) are constant only n and P change and they change in linear fashion, so converting from psig to psia, the pressure has risen from 14.7 to 134.7 or 9.16 times, so the amount of air in the volume V is 9.16 times. The density of air is 0.0765 lbs/ft

^{3} so Weight change = V*0.0765*9.16 - V*0.0765 = V*0.624 lbs

Since crutonius didn't tell us the volume V, (jroa derailed that discussion) but he did tell us the weight change, so we can backtrack for an estimate of his compressor volume deltaW = 54.2 - 52.8 = 1.4lbs = V*0.624, V = 1.4/0.624 = 2.2 cu ft

So what is the actual compressor tank volume? How close is close enough for a proof?

There will be a temperature change caused by compression, but given the thermal mass of the steel tank I'd expect it to only contribute a few extra psi to the pressure gage.

PS. When is America going to join the rest of the world and adopt the metric system!!!