Substances that oxidize when exposed to air are typically kept under vacuum in a desiccator to reduce the reaction rate. In laboratories, vacuum desiccators are used for removing moisture from samples, maintaining a dust- and moisture-free environment, vacuum testing, de-foaming, and storing samples under various atmospheric conditions.
For applications with aqueous vapors, Welch recommends standard duty diaphragm pumps or WOB-L piston pumps. In cases involving solvents and harsh acidic or basic fumes, a chemical-resistant diaphragm pump, such as MPC, should be used.
Vacuum desiccators are used frequently in laboratories for the removal of moisture in a sample, storing moisture/ oxygen-sensitive samples under a vacuum or inert atmosphere, vacuum testing, and degassing/de-foaming samples. As a result of this wide range of uses, vacuum pump selection depends on desiccator volume, vapors removed from samples and vacuum level needed to protect the sample.
The volume of the desiccator needs to match the free air displacement of the pump to ensure a satisfactory pump down. Economical Wob-l® pump can be used for drying moist samples. Chemical duty diaphragm pumps are used for removing organic solvents and/or acid/bases from samples. CRVpro pumps are used for high vacuum applications where the desiccator is used for long term storage.
The common plastic and glass desiccators found in most laboratories will hold a maximum vacuum to 29 in Hg (31 mbar) for 24 hours. Some specialty manufacturers of glass and metal desiccators will rate their desiccators for long term high vacuum storage to 1x10-3 torr (1.3 x 10-3 mbar) and are capable of holding a vacuum at 27 in. Hg (100 mbar) for 5 years. Check with desiccator manufacturer on the vacuum rating to aid in using Welch’s selector table.
Oil-free chemical duty diaphragm vacuum pumps are commonly used with samples where organic solvent, base or acid vapors are evolved from the sample during drying and/or degassing. Chemical duty diaphragm pumps use PTFE and other chemical duty materials for protection against damage from these vapors. Where a high vacuum is required, oil-free CHEMSTAR DRY or CRVpro direct drive pumps with in-line cold traps are recommended.