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Nitrogen is used for the purge gas for contamination-free storage because it is relatively inert—it neither reacts with stored materials nor carries moisture—and because it is readily available safe and relatively inexpensive
Desiccator Cabinets are set up so that the small flow of nitrogen (controlled by a simple needle valve flowmeter) forces out all moisture and air. Because nitrogen has a lower density than air, it is ported into the upper section of the desiccator; the heavier air is then purged out of the bottom through a non-return valve.
Failure to maintain the nitrogen flow into a desiccator, or to bleed the Cabinet effectively prevents a Cabinet from functioning in it designed manner. The Cabinets are not “pressure vessels” and have a very low crack pressure on the NRV.
Acrylic is a perfect material for the chambers to be made from as it is strong and transparent, enabling the contents to be easily seen from outside however acrylic, static-dissipative PVC and most other plastics are also slightly hygroscopic: they absorb moisture from outside the Cabinet and pass it inside. The greater the difference between the humidity levels outside the unit and inside, and the greater the surface area of the desiccator, the greater the moisture that can enter the desiccator. For this reason, a constant purge is recommended. It is possible to monitor and control the humidity levels and have a controlled rate of purge to reduce the Nitrogen consumption but the costs have to be balanced against the savings in gas.
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