Residual moisture within the internal cavity or enclosure of an optoelectronic system can be a ticking time bomb of sorts, providing the potential for disruptive condensation that can fog optic surfaces and blind the equipment in potentially critical situations. Condensation also causes corrosion, which affects performance and shortens the life span of a system.
Optoelectronic systems are used in both commercial and military applications, often mounted on aircraft and transported at high elevations. The extremely low temperatures and the low atmospheric pressure at high altitude can cause condensation when minimal moisture is present.
With so much at stake, manufacturers of laser, imaging, camera, and other optical-based systems are increasingly mandating a nitrogen purge to figuratively wring the moisture out of enclosures and cavities.
In a nitrogen purge, ultra-dry nitrogen with a dew point of -94°F is introduced under pressure into an enclosure or cavity to remove moisture and water vapor, creating a much drier internal environment than standard desiccant can achieve. Nitrogen purging is accomplished through commercially available purging systems or custom-built systems created by the 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 to keep the internal cavity moisture-free during its operational life,” said Paul Ave of Advanced Laser Systems Technology, a division of L3 Communications.
Advanced Laser designs and manufactures lasers and laser range-finding systems for the government and military. Its products are found in a variety of systems, including handheld man-portable thermal imagers, long-range surveillance systems, and gimbal systems on manned and unmanned aircraft, major combatant naval vessels, small attack craft, attack-class submarine periscopes, and on spacecraft.
The stringent dryness requirements for these products mandate that Advanced Laser evacuate moisture from the laser cavity using a nitrogen purge. The company purges approximately 100 units each month.
According to Ave, 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.
One major differentiator in commercially available nitrogen purge systems is the choice between single- 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 just run nitrogen through the system, usually at an elevated temperature, where you’re essentially trying to bake out any moisture,” said Ave.
However, many dual-point 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 hygroscopic materials within the enclosure released additional moisture after the purge.
“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 6 to 8 h, I had to assume it had thoroughly dried out the cavity,” Ave said.
Another potential problem with a dual-point system is that as the gas flows through from the entrance to exit, it creates a “channel” of gas. Weaker eddies of gas must be relied upon to penetrate beyond the main flow path to reach into the cracks, crevices, and other pockets. Therefore, the moisture in the internal air or the hygroscopic internal components may not be completely removed.
A single-point nitrogen purging system 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.
While at Advanced Laser, Ave discovered a single-point purge system from AGM Container Controls of Arizona. 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, rack-mounted) of nitrogen-enhanced purging systems (NEPS).
With an NEPS unit, dry gas—typically nitrogen—enters the cavity or enclosure through a single port and is brought to a pre-determined pressure 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.
“On a typical unit using the old-school method, a nitrogen purge would take six to eight hours,” said Ave. “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 was a major time-saver.”