A container designed as a refillable cylinder used to hold a compressed mixture of 79% helium (He) and 21% oxygen (O2) under safe conditions at high pressure (e.g., 50 - 150 Bar). It is typically filled with He/O2 when delivered from the gas supplier and includes a valve stem, an opening/closing valve, and will be graded according to size (capacity) and colour-coded to denote He/O2 content. The cylinder is typically made of steel, aluminium (Al) or other non-ferrous materials and must be used together with a pressure regulator in order to release the He/O2 at the correct working pressure. He/O2 is used as a breathing gas; being three times less dense than air, it is easier to breathe.