What should never be done when filling the barrel of an inflation device?

When using an inflation device, it is crucial to avoid completely filling the barrel. This is because leaving no air space can lead to overpressurization of the balloon or device when it is inflated, which poses significant risks including rupture or improper functioning. An optimal filling typically allows for some air space, which can accommodate expansion and provide safe pressure management during the inflation process.

Using saline or any other fluid (as referenced in the other options) can be essential for specific procedures, but the key concern with fully filling the barrel is its potential to compromise safety and efficacy. Proper practice involves understanding the mechanics of the inflation device and ensuring that filling is done with the appropriate volume in mind, considering both the fluid used and the need for an air cushion.