Why must there be residual pressure in acetylene cylinders and oxygen cylinders?

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Keeping a few kilograms of pressure inside the bottle, so that the pressure inside is greater than that outside, prevents other gases from entering and ensures safe use. Because acetylene has a very low explosive limit, even a small amount of air mixed in can cause an explosion when the temperature reaches a certain level. Therefore, an exhaust valve must be installed on the outlet of the acetylene cylinder to prevent air from entering the cylinder; otherwise, there is a risk of explosion during the next use. The pressure relief valve is used to prevent the pressure inside the cylinder from falling below that of the surrounding air, thus avoiding backflow of air into the acetylene cylinder; oxygen cylinders should retain a residual pressure of at least 0.098–0.196 MPa gauge pressure. Acetylene cylinders should maintain a residual pressure of 49Kpa to 98KPa in winter, and 196KPa in summer.