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.
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.
Traditional purge systems are typically dual point, with ultra-dry nitrogen entering through one port and exiting out another. 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.
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.
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.
On a typical unit using the old school method, a nitrogen purge would take 6 to 8 hours, but with the NEPS system, the same purge can be completed in 45 minutes, thereby using significantly less volumes of nitrogen. The NEPS not only saves money, but is also a major timesaver.
