 Open circuit scuba divers by definition are independent of surface supply, and must take any gas mixture with them that is to be used on the dive.However, if they are confident of returning by a specific route, the decompression gas may be stored at appropriate places on that route.The cylinders used for this purpose are called stage cylinders, and they are usually provided with a standard regulator and a submersible pressure gauge, and are usually left at the stop with the regulator pressurized, but the cylinder valve turned off to minimize the risk of gas loss.