Keep one of these protective covers on the top of your cylinder/tank valve to keep the sand and dirt off your O-ring. It is also used as the universal sign of a full tank. Cap on - full. Cap off - needs filling.
Intended for use with Yoke (A-clamp) valves. While it can also be used with DIN valves, for these a screw in male DIN valve protector would be a better choice.