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For Sensitive Internal Components, Moisture Is the Enemy

Photonics Spectra
Mar 2011
Del Williams

Moisture control technology – critical to the defense, aerospace, electro-optical and many other industries – moves from inside containers to inside the equipment.

For high-tech equipment ranging from missiles and munitions to engines, transmissions, cameras, imaging systems and optical guidance systems, moisture has long been the enemy during storage, transportation and field usage. Sensitive electro-optical equipment is particularly susceptible to moisture damage. Moisture can corrode wiring and metal parts, cause condensation, and fog lenses and mirrors, thereby compromising mission-critical performance.


“The concept of a nitrogen purge is essentially to ‘squeeze’ the internal components like a sponge to remove any residual humidity or moisture out of the system and then seal it to keep the internal cavity moisture-free during its operational life,” said Paul Ave of L3 Communications. Images courtesy of AGM Container Controls Inc.


Historically, to prevent the moisture that exists naturally in the air from getting into equipment during transport and storage, military and civilian engineers have designed shipping containers that were airtight and could withstand large pressure, temperature and altitude differences. However, as these heavy, bulky steel containers were costly to build and ship, so-called controlled breathing systems were introduced to allow for lighter, smaller containers. Because of the cost and logistical advantages of lighter containers, many modern shipping containers employ some, if not all, elements of a controlled breathing system.


Because of the cost and logistical advantages of lighter containers, many modern shipping containers employ some, if not all, elements of a controlled breathing system: a breather valve to control pressure, vacuum and moisture entry; a desiccator to remove moisture; and a humidity indicator to show when to replace a saturated desiccator.


“The key elements of a controlled breathing system are a breather valve that controls pressure, vacuum and moisture entry; a desiccator that removes moisture; and a humidity indicator that indicates when to replace a desiccator when it’s saturated and humidity levels begin to rise,” said Jim McCorry, director of business development at AGM Container Controls Inc. of Tucson, Ariz., which specializes in the design and fabrication of moisture control and monitoring products. “The breather valve basically allows a sealed enclosure to ‘burp’ a little air in and out, to equalize internal and external air pressure.”


In a nitrogen purge, ultradry nitrogen with a dew point of –94 °F (–70 °C) is introduced under pressure into an enclosure or cavity to remove moisture and water vapor, creating a much drier internal environment than a standard desiccant can achieve.


The humidity problem in shipping has long been solved with controlled breathing systems in containers. In the field of battle, however, moisture control has evolved to a greater level of precision. The continual pressure to improve the mission-critical functionality, reliability and longevity of electro-optical devices for military use has resulted in controlled breathing systems for moisture control that are installed inside the equipment itself. After all, what is a camera housing or missile casing other than a container for the sensitive components inside

“Reducing moisture in the presence of electronics and optical imaging systems is critical for performance and longevity,” said James Weaver, a mechanical engineering manager at Flir Systems. “With superior moisture control in the shipping container and even the equipment enclosure itself, you can operate longer and more reliably in harsher and harsher environments.


With the Neps unit, nitrogen enters the cavity through a single port and is pressurized to a predetermined “pounds per square inch” before a valve opens and the gas backflows into the unit. This cycling continues until the equipment reaches the required dew point level, at which point it automatically shuts off. “Everything is demanding higher definition, more functionality and more processing capability in a smaller, more rugged package.”


While a traditional shipping container has plenty of room for a separate breathing valve, desiccant and humidity indicator, space was at a premium for Flir’s Star Safire HD system, which features megapixel thermal, daylight and low-light cameras with high-magnification optics, laser payloads and a fully integrated inertial measurement unit to precisely locate targets.

“For us, every inch and every ounce must provide functionality,” Weaver said. “We didn’t have the luxury of using a large panel to put three separate controlled breathing system modules in our device. To free up space for enhanced capabilities on the Star Safire HD system, we sought to combine the moisture control modules.”

Flir and AGM jointly designed and developed a controlled breathing system, called the TA340, with a combined breather valve, desiccator and humidity indicator for space-efficient moisture control.

Incorporating a humidity indicator and two-way breather valve all in the same aluminum housing not only saved space but also helped to avoid drilling an additional hole in the case. With only one mounting hole in the equipment, potential leak paths in the equipment were reduced, making its moisture control capabilities even more reliable.

Because the TA340’s desiccant cartridge is mounted on the back end of the valve, when the device breathes air in, the air goes directly over the desiccant bed, so moisture is efficiently captured upon entry. For ease of desiccant replacement, the system is also designed to accept an externally accessible replacement desiccant cartridge.

“It’s critical for anyone servicing a controlled breathing system to have an externally accessible replacement desiccant cartridge,” Weaver said. “Besides being easier and quicker, it minimizes moisture entry from outside air since the entire enclosure does not have to open. I’m glad we were able to include this in its compact design.”

The controlled breathing system per-mitted Flir to pack even more functionality into the compact enclosure.

“Since the TA340’s 3-in-1 controlled breathing system reduced its footprint by two-thirds compared to separate moisture control systems, we had more space to enhance our thermal imaging, visible imaging and laser elimination capabilities,” Weaver said.

On a more recent project, the two companies did themselves one better: They added RFI/EMI [radio-frequency/electromagnetic interference] shielding to their compact controlled breathing system. “RFI/EMI shielding enables an electronically tighter enclosure with even less susceptibility,” Weaver noted. “It should allow [the controlled breathing system’s] use on installations with higher emissions, and should shield customers from noise made on more sensitive equipment.”

Weaver plans to collaborate with AGM on future projects where moisture control must be customized for specific purposes. “By jointly developing the compact, controlled breathing systems with us for use in the actual equipment, not just the storage container, they helped us deliver more value to our customers, while they delivered more value to us,” he said.

 Meet the author

Del Williams is a technical writer based in Torrance, Calif. For information on AGM Container Controls Inc.’s products, e-mail: controlledbreathing@agmcontainer.com.


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