Consider these variables when selecting a desiccator system:
• Humidity requirement. The humidity requirement is the single most important consideration in selecting a desiccator and the associated controls. Extremely low-humidity applications (under 10% RH) generally call for a “smart controller.”
• Access. As discussed above, if you access stored materials frequently, you will need a variable-purge system that can block inrushing air when a door is opened. Access once every few hours probably isn’t a problem, but every 20 to 30 min probably is.
• Nitrogen expense. A smart control system conserves nitrogen and reduces costs by up to 78%.
• Ambient humidity. A desiccator in a humid environment requires a more efficient humidity controller than one in an arid environment. In a dry region, a setpoint of 10% RH probably isn’t a major challenge, but where the humidity levels are much higher, it could be.
• Desiccator material. Plastic is hygrosopic, so moisture will penetrate the wall material, especially with a high gap between the ambient RH and the setpoint RH.
• Desiccator size. Large plastic desiccators typically require more nitrogen, and a more efficient delivery system, than small ones to compensate for the absorption of moisture by the plastic walls. This is particularly true of multi-chamber cabinets, in which humidity levels will vary considerably as doors are opened and closed.
• Static attraction and/or discharge. Any problems associated with static surface charges (that includes cleanliness, since fine dust is hard to control once it acquires a static charge) become much worse at low humidity.
This cleanroom tip was taken from “Desiccators Keep Products in Spec,” which appeared in the February 2013 issue of Controlled Environments.
