[quote=Paul Copioli]If the pressure of the tank is only 60psi, then when I fire the cylinder once I expel all of the air in the tanks. If the pressure in the tanks is 120psi, then I have double the available air to use at 60psi./QUOTE]
If we assume the volume used by the cylinder equals that of the tank, then the amount of air you can get from the tank is one half of what is there - the system will equalize at 30 PSI. That is why you use 120 PSI, so your cylinder can be sure to see the full 60 PSI, just as you stated.
You see, while the compressor can keep up pressure, it is limited in volume (Cubic Feet per Minute). The tanks act like 'pressure inductors', allowing brief periods of high flow volume despite a lower volume compressor.
Besides, isn't more better?
Don