Shifnoid CO2 bottle backup air fillers use your regulator to maintain the preset pressures, and enable you to use compressed air directly from your air tank or air compressor to fill your CO2 bottle. Putting a standard valve on the high-pressure side can be dangerous, and if you try to fill it from the regulated side, it won't fill completely. By having these backup air fillers on your tank, you can quickly, easily, and safely fill your CO2 bottles and have them ready for immediate use.