Contamination-Free Plastic Storage Cabinets

Desiccator Cabinet Frequently Asked Questions

What is a desiccator cabinet and how is it used?

The term “desiccator” commonly refers to two types of equipment and ranges of applications.

A desiccator cabinet is typically a larger unit, often with multiple chambers, used to prevent moisture exposure during storage of sensitive parts. Because storage applications must allow periodic parts access, a desiccator cabinet typically includes one or more doors and relies either on a desiccant material (regenerating silica gel or molecular sieve) or the use of inert Nitrogen gas to maintain dry conditions. Terra offers desiccator cabinet options for every application.

A desiccator, on the other hand, is a sealable drying enclosure that uses vacuum or desiccant, sometimes in conjunction with a fan, to remove moisture adsorbed into the surface layer of a sensitive sample. These sealable enclosures are typically fairly small cylindrical or round glass vessels and are commonly used in laboratories to dry biological, pharmaceutical, or other chemical samples. Terra offers a wide selection of vacuum and non-vacuum lab desiccator options.

What are the advantages of a Nitrogen purged desiccator cabinet?

Purging a desiccator cabinet involves flushing moisture and oxygen laden air out of the cabinet using a steady flow of dry inert gas, such as Nitrogen. A purge of purified 99%+ Nitrogen gas is generally preferred in a large desiccator cabinet because such systems achieve a uniformly low-humidity storage environment more quickly than desiccant-based systems, particularly where ultra-low RH levels are required. Nitrogen can be fed into multiple locations in a large cabinet, often using a rear plenum as a distribution chamber. A mixing fan can be added to increase humidity uniformity inside the cabinet and promote moisture dilution. Nitrogen purged desiccators also prevent contact with oxygen, making them ideal for anaerobic applications.

A desiccant based dry cabinet is often prefered in facilities that lack a Nitrogen source. However, because desiccant requires a periodic regeneration (heating) cycle, these desiccators cannot provide continuous moisture protection unless equipped with a redundant desiccant module. They are also typically unable to provide ultra-low humidity levels and require a longer RH recovery time when the cabinet is exposed to moisture.

What is RH recovery time?

RH recovery time is the time required to recover a low relative humidity set point when an access door is opened to part retrieval.

The acceptable RH set point depends on industry and application. Many semiconductor and optical sensing devices feature electronics that require a storage environment of less than 10%RH and are degraded by exposure of just a few minutes to moisture substantially above this level. Unfortunately, depending on size of the cabinet and drying system, some desiccators can require up to an hour to retain a 10%RH set point, particularly if ambient humidity is very high. If a door is opened again before set point is attained, parts may be exposed to unacceptable moisture levels for hours!

For critical applications, Terra provides a number of high-efficiency Nitrogen purged desiccator cabinets, including the IsoDry desiccator cabinet, which ensures RH uniformity and fast recovery time. In large, multi-chamber desiccators, Terra’s NitroPlex system provides automatic humidity control in each chamber independently.

Improved low-RH recovery also reduces Nitrogen costs.

How much Nitrogen does a desiccator use?

Equipped with an automated gas control system, such as Terra’s Dual Purge and NitroWatch controllers, desiccator cabinets consume on average less than five cubic feet of Nitrogen per hour, allowing a single 5’ pressurized liquid Nitrogen tank to operate a large cabinet continuously for 2-3 days, depending on frequency of access and recovery time.

To eliminate dependence on Nitrogen tanks, Terra offers Nitrogen generator options to produce up to 99.5% pure medical grade Nitrogen from compressed air.

How can a desiccator cabinet prevent static-related problems like electro-static discharge?

Even the slightest static discharge can spell disaster for delicate electronic or semiconductor components. Furthermore, dangers of ESD (electrostatic discharge) are amplified in a low humidity environment. Below are a few options to consider to help create an ESD safe storage environment:

1. Static-Dissipative PVC: Acrylic, a low cost plastic commonly used in storage enclosures, is a prolific static generator. Wiping an acrylic desiccator generates surface charges that can discharge onto ESD-sensitive components. Static-dissipative PVC prevents this danger. This material not only dissipates static charges safely but also eliminates the particle attraction that static charges create. Surfaces stay clean, inside and out, making this material perfect for use in cleanrooms. Because static-dissipative PVC is transparent and highly durable, it can be used in place of acrylic in nearly any application, desiccators included. It features a surface resistivity of approximately 10^7 ohms per square and is completely non contaminating—with no measurable outgassing—and it resists a wide range of chemicals.
2. Static-Neutralizing Ionizing Nozzles: By ionizing Nitrogen molecules as they pass through the desiccator plenum wall, ionizing nozzles neutralize electrostatic charges on all interior surfaces. They provide practical ESD control for charged nonconductive materials, which cannot dissipate charges to ground. And because they eliminate static attractive forces, they help control contamination: in a statically neutral environment, any airborne particles that enter a desiccator are more likely to remain airborne and to be purged out of the cabinet.
3. Stainless Steel Conductive Shelves, Shields and Trays: Terra's grounded stainless steel shelving provide a 100% ESD-safe enclosure. Grounded conductive plates (such as shelves) create a Faraday cage that prevents penetration of static charges. Because these conductive units are removable, they also protect your sensitive components from ESD as you transport them from one location to another. Terra’s ESD safe desiccator cabinet utilizes this Faraday design to provide complete ESD shielding for sensitive electronics storage.

What are the disadvantages of a plastic desiccator?

Creating a low-humidity environment requires careful material choice. An acrylic desiccator cabinet for example can be a lower cost option but bears a weakness known as hygroscopy. Plastic surfaces naturally accumulate liquid molecules through adsorption, collecting moisture from the atmosphere that clings to its surface in a thin coat. While this might not present an issue for simple operations, it can be a challenge in moisture-sensitive applications.

A potential solution to the issue of humidity in small acrylic enclosures, such as glove boxes and desiccators, is the use of nitrogen gas. Nitrogen is a dry inert gas commonly pumped into a chamber to lower the relative humidity and purge moisture to protect the materials inside. But constantly using a high flow of Nitrogen gas can be expensive, and for critical applications requiring minimal humidity, compensating for an inherently moisture-attractive material can expend a ghastly amount of N2.

Storage of large or heavy materials exceeding 100+lbs can sometimes be a challenge with plastic desiccator cabinets. The individually partioned chambers in the cabinet can make it difficult to fit very wide or tall items into the cabinet. Plastic desiccator cabinets also have a lower overall weight capacity, so they are generally less suitable for storage of materials exceeding 100lbs. Stainless steel desiccator cabinets offer solutions to both size and weight capacity constraints of plastic desiccator cabinets.

What are the advantages of a stainless steel desiccator cabinet?

Regardless of the type of gas blowing across acrylic, air cannot dislodge moisture particles from its surface or prevent air and liquid molecules from permeating the glued edges of an acrylic chamber. For applications requiring the strictest low-humidity environments, stainless steel provides moisture-impermeable protection as well as corrosion and chemical resistance. And while stainless steel will protect the atmospheric conditions of the environment within the chamber, optional properties for steel types can provide further benefit.

The best resistance to humidity comes from electropolishing, a procedure that removes the top layer of the stainless steel surface. The process entails an electrochemical removal of metal impurities, including carbon, silica and free iron, and it leaves a highly reflective nickel and chromium surface. Electropolishing eliminates the grainy texture of standard stainless steel, which can retain trace amounts of moisture or pathogens that escape cleaning. As a result, the surface has a smooth texture at a microscopic level, and liquid molecules have nowhere to hide.

Stainless steel desiccator cabinets are also ideal for heavy duty storage applications of large, bulky materials. For example, large pieces of equipment or heavy drums of raw materials. Desiccator cabinets with double doors provide extra-wide openings for large items. The shelving inside can also be reinforced and fitted with welded heavy duty shelves for increased weight capacity of 300lbs or more.

What applications use desiccators?