Evaporation control units contain check valves that reduce the flow of vapor in and out of a solvent supply bottle. During a system run, as liquid solvent travels out of the supply bottle to the system, air must enter the bottle to displace the solvent removed. DV-115 and DV-201allow air to enter the bottle, but not to escape. Part numbers DV-118 and DV-202 operate in the reverse, allowing pressure relief. DV-201 and DV-202 are constructed of PEEK and perfluoroelastomer and are our most chemically resistant option, while DV-115 and DV-118 are constructed from PTFE with Viton seals.