Inert gas
Pressure transfers can be slow. Inert gas lines are usually vented out of a gas bubbler placed in-line to prevent overpressure. The vents need to be isolated by capping the bubbler outlet, or stopping the egress of inert gas with a stopcock or pinch clamp, to ensure sufficient pressure to complete the transfer. The use of a mercury bubbler instead of one filled with oil used to be popular, but is out of favor due to the difficulty in dealing with mercury spills.