A high side gauge on a regulator depicts the amount of CO2 gas left in the cylinder. CO2 is in liquid form in the cylinder with a gas head-space which exerts a certain amount of pressure based on the temperature environment the cylinder resides. This gauge tells you this head-space pressure. This pressure will read the same amount until all of the liquid is depleted whereas the pressure reading will then begin to drop. This signifies that you are about to run out of gas. This gauge can also be used to determine if your system has leaks. When you turn the cylinder wheel off and the gauge reading drops, you have a leak.
Scott Zuhse, Instructor
Micro Matic Dispense Institute