There often is a requirement to maintain an enclosure with a controlled gas environment such as to be free of moisture, carbon dioxide and/or oxygen. For example in a precision optical instrument such as a spectrometer, or in certain electronic equipment, it is necessary to purge and maintain the enclosure with a gas such as nitrogen. However such an enclosure is susceptible to changes in atmospheric pressure which can result in leakage. A well sealed enclosure either may buckle under pressure changes or must be designed with thick walls and specially sealed joints and is, therefore, relatively expensive.
Desiccant materials are commonly used in enclosures for adsorbing water vapor, carbon dioxide and/or oxygen that may leak in. However, such detrimental gases may reach a critical component in the instrument before being adsorbed and removed by the desiccant. Therefore such systems have been less than satisfactory.
There also is known to be a Fick's law whereby a tube, with a length to internal diameter ratio of at least ten, minimizes diffusion of gas such as water vapor from one end of the tube to the other at constant pressure. Functioning of such a tube, being statistical, does not ensure blockage of moisture from an enclosure and generally has not replaced desiccants.