Depending on your cylinder size - 5, 10, 20 lb - determines how much gas liquid CO2 they refill into the cylinder. These sizes represent the actual liquid weight of the gas. The high side gauge represents the internal pressure of the head space of gas in the cylinder. Since gases expand and contract with temperature, the environment you store the cylinder in will determine the pressure.
At 70 degrees F, the gauge should read about 850 PSI until all of the liquid gas is gone. At this time the gauge will begin to drop and there is not much left. If you store your cylinder in the kegerator, the cylinder pressure will read at or in the red zone due to the cold environment. No need to worry. The same amount yield exist. You need to keep a closer eye on when the needle begins to drop signifying an empty cylinder.
Scott Zuhse, Instructor
Micro Matic Dispense Institute