Comment Re:Don't forget weight (mass) (Score 1) 239
Besides the "ideal gas law, of PV =nRT", the act of checking the pressure leaks some air out of the ball.. Those valves aren't perfect, same with the gauges..
This happens to me all the time with my mountain bike tires. Each time you measure, some air leaks out and it's a couple PSI less the next time you remeasure a few seconds later.
Lastly, The temperature of the air filling the ball might have been substantially elevated above room temp by the compressor. I.E. Heat of compression, combined with waste heat from compressor motor + friction inside the pump itself is dumped into air tank. Especially if the compressor had just been fired it up(or had been running for some time), and it had to run for a couple of additional minutes rebuilding the reservoir tanks to cutoff pressure.(~150-200psi).
This phenomenon is well known in the diving industry, tanks are routinely filled to 3600-3700psi, only to have it settle down to ~3200-3300psi once the previous hot tank/air inside cools down to ambient temperature.
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