use of condensers in chemical procedures—when not performed at fixed, lowered pressure by careful vacuum control—inevitably involves transiently fluctuating pressures within the apparatus, so that isolating the apparatus while allowing it to be an open rather than sealed system becomes a practical issue; this is particularly true, when chemical reactions are performed that are air- or moisture-sensitive. if the reaction or process using condensers cannot be left open to atmosphere, its isolation is accomplished in simplest fashion via drying tubes (an attached tube packed with desiccant) or other specially packed scavenging tubes, that allows gasses to pass, and so pressure equalization, but prevents entry of substances deleterious to the ongoing chemistry; alternatively, the apparatus can be vented through a bubbler that prevents entry of laboratory atmosphere either by allowing the internal volume to push and pull against a volume of resisting liquid (e.g., mineral or silicone oil), or by keeping a positive pressure of an inert blanketing gas (such as nitrogen or argon) that vents through a similar volume of liquid. attachment of such during tubes and bubblers can be direct, or indirect via gas/vacuum lines and manifolds.