There are a number of ways to tell when your carbonating cylinder is empty: 1. When you press the carbonating button you don't see any gas going into your bottle of water. 2. Even after repeated presses your water is not carbonated. 3. You hear a sputtering sound when you attempt carbonation. Pro-tip: SodaStream recommends keeping a back up carbonating cylinder handy so that when one cylinder runs out, you simply swap the empty cylinder for a full one. That way, you'll keep enjoying fresh, fizzy drinks without interruption and can exchange the empty carbonator via courier service or in a store near you. Take a look here to learn more!