What does the presence of a plus sign (+) on a cylinder's marking allow?

The presence of a plus sign (+) on a cylinder's marking indicates that the cylinder has been tested and approved for a higher fill pressure than what is typically allowed for standard cylinders. This marking signifies compliance with specific safety and testing protocols, often linked to hydrostatic testing, which evaluates the integrity and strength of the cylinder under pressure.

When a cylinder is marked with a plus sign, it is assessed to handle additional pressures safely, which expands the amount of gas it can hold compared to non-marked cylinders. Therefore, this increased allowed fill pressure means that the cylinder can be filled to a greater capacity, which is essential for applications requiring more gas without needing a larger or additional cylinder.

The other options illustrate important concepts about gas storage and usage but do not directly relate to the meaning of the plus sign. For instance, increased storage time and duration of use are influenced by factors such as gas consumption and cylinder size rather than the cylinder’s pressure rating. Replacement concerns relate to the cylinder’s condition and not the marking itself. Thus, the correct interpretation of the plus sign primarily centers around the ability to safely hold a higher fill pressure.