Any residual moisture within the internal cavity or enclosure of an optoelectronic system put into field service can serve as a ticking time bomb of sorts, providing the potential for disruptive condensation that can fog mirrors and lenses, effectively blinding the equipment in potentially critical situations. Condensation also causes corrosion, which is just as destructive as fogging, and affects performance and shortens the lifespan of a system.
Optoelectronic systems are used in both commercial and military applications, where they are often mounted on aircraft, helicopters or even missiles and transported at high elevations. The extremely low temperatures along with the low atmospheric pressure at the high altitudes to which these systems are subjected can cause condensation when minimal moisture is present. Even the minimal amount of moisture contained in circuit boards can lead to problems.
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 before these systems are put into the field.
In a nitrogen purge, ultra dry nitrogen with a dew point of -94°F (-70°C) is introduced under pressure into an enclosure or cavity in order 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 very engineers designing the product itself.
“The concept of a nitrogen purge is essentially to squeeze the internal components (see fig. 2) 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,” says Paul Ave of Advanced Laser Systems Technology, a division of L3 Communications.
Advanced Laser Systems Technology designs and manufactures lasers and laser range finding systems for the government and military. The stringent dryness requirements for these products mandate the evacuation of moisture from the laser cavity utilizing a nitrogen purge. The company purges approximately 100 units each month.
According to Ave, corrosion would be a primary concern in terms of long term reliability, but internal moisture can also affect the performance of a laser. “Most lasers—particularly high powered lasers—require nitrogen purging,” says Ave. “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 performance.”
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.
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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,” explains Ave. “If you don’t take care of that moisture with a nitrogen purge, we would see it manifest as humidity and moisture inside the unit during temp cycle testing.”
Commercially Available Purging Systems – Single versus Dual Purge
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 just run nitrogen through the system, usually at an elevated temperature, where you’re essentially trying to bake out any moisture,” says Ave.
Unfortunately, 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 out flowing 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.
As per Ave, “With the dual purge method, I have 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.”
Another potential problem with a dual point purge 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 or 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, 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.
While at Advanced Laser Systems Technology, 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 the NEPS unit (see fig. 3), 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,” says Ave. “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.”
With proper quantification and the automatic shut off feature when the desired dew point is achieved, Ave reports 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 6 to 8 hours,” explains 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 timesaver.” For more info, visit www.agmcontainer.com and/or e-mail [email protected]. AGM Container Controls Inc., Tucson, AZ. (800) 995-5590.