On double cylinder assemblies, one must ensure?

When working with double cylinder assemblies, it is crucial to ensure that the cylinder tray is securely attached to the unit's frame. This provides stability and safety to the assembly, preventing the cylinders from falling or becoming dislodged during operation. A properly secured tray facilitates the correct positioning of the cylinders, which is essential for maintaining the integrity of the gas supply system.

Other options, while potentially relevant in specific contexts, do not address the primary concern of stability and safety in the use of double cylinder assemblies. For instance, while the installation of the joiner pipe is necessary for connecting the cylinders, it is the attachment of the cylinder tray to the frame that ensures a secure setup. The size and equal dimension of the cylinders and whether the valves are open could vary based on the specific application but are not inherently critical to the core requirement of securing the assembly properly.