Dry Storage Desiccators

WHAT IS A DESICCATOR?

The desiccator or dry storage cabinet is used to store dried samples in a dry atmosphere. It should not be used to dry an object, but to maintain an already dried object indefinitely in a dry condition. Desiccators (also known as dehydrators or dehumidifiers) are self-contained moisture control systems used in a wide variety of aerospace and electronic applications. There are two basic types: dynamic and static. Dynamic systems are self-regenerating, usually large and expensive, and require a degree of control and an external power source. Static-type desiccators on the other hand can vary in size from a small in line cartridge to a large cabinet that is either purged with nitrogen or contains a desiccant tray.

HOW DOES A DESICCATOR WORK?

A desiccator or dry storage cabinet would either contain a desiccant such as ‘silica gel’ or can be closed and purged with nitrogen. A desiccator cabinet is usually an acrylic case with individual mini environments that are closed and purged with Nitrogen, therefore moisture control with the cabinet is achieved preventing oxidization of moisture sensitive devices. (MSD). A good example of this would be during the manufacture of silicon wafers where desiccators work exceptionally well for dry and dust free storage. Often the cost of the cabinet and the cleanroom floor space is more than outweighed by the cost of improved wafer yield. This improvement in yield would mostly apply to any moisture sensitive devices that also require dust free controlled storage. Some prefer to purchase a low cost desiccator / dry storage cabinet without a nitrogen purge system and use a desiccant tray.

DRY STORAGE WITH DESICCANT OR NITROGEN PURGE CABINET?

If your application has extreme humidity or if nitrogen costs are high then perhaps a dry storage cabinet using a desiccant would be preferable. Of course the performance of the cabinet would greatly depend on the quality and quantity of the desiccant used.

DESICCANT

Substances of a hygroscopic nature, capable of absorbing moisture and therefore used as drying agents. e.g.. Anhydrous sodium sulphate, anhydrous calcium chloride, phosphorous pentoxide, or the mostly popular SILICA GEL and ZEOLITES.

SILICA GEL was developed just prior to WWI and was used in Allied gas masks; then in WWII it was used extensively to keep penicillin dry. Not a "gel" as commonly thought of, and not to be confused with "silicon gel", silica gel is a porous, granular form of silica, synthetically manufactured from sodium silicate.
The internal structure of each small silica gel granule is composed of a vast network of inter-connecting microscopic pores, which attract and hold moisture by a phenomena known as physical adsorption and capillary condensation. (It sounds ridiculous, but a single teaspoon of Hydrosorbent Silica Gel has an internal adsorptive area equivalent to a football field, including the two end zones.) Hydrosorbent Silica Gel is inert, non-toxic and safe to use to protect foods, medicines, sensitive materials, electronics, films, etc. Even when saturated with adsorbed moisture, silica gel looks and feels dry to the touch.

Once saturated with moisture, silica gel can be easily reactivated in a conventional oven for lifetime protection.
Since World War II, silica gel has been the desiccant of choice by government and industry. At 25°C , 5%RH silica gel can hold 5.5% of its weight in water. (Fer. And for more information see Hydrosorbent Dehumidifiers)

ZEOLITES. Zeolites are microporous crystalline solids with well-defined structures. Generally they contain silicon, aluminium and oxygen in their framework and cations, water and/or other molecules within their pores. Many occur naturally as minerals, and are extensively mined in many parts of the world. Others are synthetic, and are made commercially for specific uses, or produced by research scientists trying to understand more about their chemistry. Cation-containing zeolites are extensively used as desiccants due to their high affinity for water and also find application in gas separation, where molecules are differentiated on the basis of their electrostatic interactions with the metal ions. Conversely, hydrophobic silica zeolites preferentially absorb organic solvents. Zeolites can thus separate molecules based on differences of size, shape and polarity. At 25°C , 5%RH zeolite can hold 20% of its weight in water. (Ref. And for more information British Zeolite Association)

The information on desiccants is provided for your information, we do not supply desiccants. The information here is only intended to provide an introduction to desiccants and their uses in a desiccator cabinet. Further information and a supplier of desiccants can be obtained by contacting us or following one of the desiccant links above.

NITROGEN Nˆ2 (N2) CABINETS.

Nitrogen purge cabinets use a plenum chamber design to provide a continuous laminar flow of nitrogen over the moisture senstive devices contained within the dry storage desiccator cabinet. The volume of nitrogen used would depend on the volume of the cabinet, the RH of the external environment, the moisture contained within the contents of the cabinet and how often the cabinet is opened. In some areas of high humidity (e.g. Florida), nitrogen gas flow could be high to retain a desired set point of RH within the cabinet, desiccator. To minimise the flow and therefore the volume of nitrogen used we recommend the use of a properly calibrated RH meter to monitor the RH so that the flow of nitrogen can optimised. See INSTRUMENTATION for further details. Also some hygroscopic plastics such as plexiglass® will absorb moisture from a high humidity external environment and pass it through into the cabinet. If your application does require a barrier from a very humid environment, then perhaps Stainless Steel should be considered for you desiccator cabinet. However we find that in most applications acrylic dry storage cabinets and desiccators perform well and of course provide a 'visual' advantage over a metal walled cabinet and thus are likely to be opened less for content inspection, comprimising the environment within. The choice however is yours!

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