BY Paul Ave

For electro-optics systems, any residual moisture within the internal cavity or enclosure that makes its way into the field is a ticking time bomb because it could produce disruptive condensation that fogs mirrors and lenses, which effectively could blind the equipment in critical situations. The other concern with condensation is corrosion, which is just as destructive because it can degrade performance and shorten product lifespan.

Often used in commercial and military applications, electro-optics systems often are mounted on aircraft, helicopters, missiles, or transported at high elevations where extremely low temperatures and air pressure can cause condensation even with minimal moisture present, such as that found in the hygroscopic plastics used to create internal circuit boards.

With so much at stake, manufacturers of laser, imaging, camera, and other optical systems are increasingly mandating a nitrogen purge to wring the moisture out of enclosures and cavities before these systems make it out to the field.

In a nitrogen purge, ultra dry nitrogen with a dew point of -70 degrees Celsius is introduced under pressure into an enclosure or cavity to remove moisture and create a much drier internal environment than standard desiccant can achieve. Nitrogen purging is accomplished through commercially available purging systems or jury-rigged systems created by the very engineers designing the product itself.

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 up to keep the internal cavity moisture-free during its operational life.

L-3 Advanced Laser Systems Technology Inc. in Orlando, Fla., designs and manufactures lasers and laser range finding systems for military handheld man-portable thermal imagers, long-range surveillance systems, gimbal systems on manned and unmanned aircraft, major combatant naval vessels, small attack craft, spacecraft, and attack submarine periscopes.

These products must evacuate moisture from the laser cavity with a nitrogen purge. The company purges approximately 100 units each month. Internal moisture can cause corrosion and affect the performance of a laser.

Most lasers-particularly high-powered lasers-require nitrogen purging. If the coatings on the optics or the prescription of the optics are altered in any way as a result of either contamination or moisture, it can dramatically affect its performance.

Lance Leclair is a senior research scientist at Catch the Wind Inc., a company in Chantilly, Va., that develops laser wind sensors that measure wind speed and direction for the wind power industry. "There are moisture-sensitive components in these systems," he says. "Any condensation can create fogging or corrosion problems that can lead to premature failure. The reliability of the component is extended if we keep it operating in a dry environment."

Hygroscopic Components

It is a common misconception that the majority of the moisture in a sealed cavity or enclosure is contained in the empty volume of air. In fact, the majority of the moisture is contained in the hygroscopic materials, such as common circuit boards or other plastic components within the enclosure.

Hygroscopic plastics readily absorb moisture from the atmosphere and can release that moisture under temperature cycling and other environmental factors.

We found the internal electronics were the main culprit for a lot of residual moisture. If you don't take care of that moisture with a nitrogen purge, during temp cycle testing we would see it manifest as humidity and moisture inside the unit.

One major differentiator in commercially available nitrogen purge systems is the choice between single-point and dual-point purge systems. Traditional purge systems are typically dual-point, with ultra-dry nitrogen entering through one port and exiting out another.

The old school method is to run nitrogen through the system, usually at an elevated temperature, where you're essentially trying to bake out any moisture.

Unfortunately, many dual purging systems lack any real means of quantification in relation to internal dew point. Even if a dew point sensor is used to quantify the outflowing gas, it is possible to return later and discover a dew point spike because those hygroscopic materials within the enclosure released additional moisture.

With the dual-purge method, I had no quantitative way to know what my acceptable dew point was or when I had achieved it. If I purged a unit for six to eight hours, I had to assume it had thoroughly dried out the cavity.

With a dual-point purge system another potential issue is that as the gas flows through from the entrance to exit, it creates a "channel" of gas. Weaker eddies of gas must penetrate beyond the main flow path to reach into the cracks, crevices, or other pockets and, therefore, may not completely remove the moisture in the internal air or the hydroscopic internal components.

A single-point nitrogen purging system, on the other hand, offers several advantages. Because it involves only one exit/entry port, a single-point system creates a pressurized environment for the ultra-dry gas and allows for quantifiable measurement of the dew point on exit. This type of system appears to eliminate the concern over completely drying out the whole system.

Advanced Laser Systems Technology discovered a single-point purge system from AGM Container Controls in Tucson, Ariz. AGM offers a variety of products for the control and monitoring of moisture, including desiccators and humidity indicators. The company also offers several models (portable, rackmounted) of nitrogen enhanced purging systems (NEPS).

With the NEPS unit, dry gas (typically nitrogen) enters the cavity or enclosure through a single port and is pressurized to a pre-determined PSI before a valve opens and the gas backflows back into the unit. There it passes a dew point monitor and displays the current dew-point temperature. The nitrogen is then vented to the atmosphere and a new cycle commences. This cycling continues until the equipment reaches the required dew-point level, at which point it automatically shuts off.

The NEPS system is easy to use. You hook it up to a single port, select a pressure setting and desired final dew point, and when the purge is completed, the system turns off automatically. For us, it streamlined operations.

For Leclair of Catch the Wind Inc., the single-point purge NEPS had an added value. "There's only one filler valve that needs to be installed, instead of two. You save a little on engineering and parts costs. It is not a tremendous saving, but you also eliminate another potential point of failure, which is always good."

With proper quantification and the automatic shut-off feature when the desired dew point is achieved, both Ave and Leclair report a significant saving in nitrogen costs, as well as a significant saving in time of purge.

On a typical unit using the old school method, a nitrogen purge would take six to eight hours. With the NEPS system, we could do the same purge in 45 minutes so significantly less volumes of nitrogen are used and we know for sure when we are done. It not only saved money, but also was a major time saver.

FOR MORE INFORMATION visit AGM Container Controls online at www.agmcontainer.com.

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Paul Ave is president of Advanced Laser Systems Technology, a division of L-3 Communications.

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