Atmospheric Exchange
Environmental temperature variations drive cyclic pressure changes inside sealed or semi-sealed enclosures, leading to the exchange of gas with the surrounding atmosphere. The process of thermal pumping occurs when heating causes the air inside a housing to expand and escape, followed by cooling that draws external air and moisture back inside. This action acts as a continuous mechanism for transporting ambient humidity into the enclosure.
Sealing Stress
Each temperature cycle generates a differential pressure across the sealing interfaces of the sensor housing. During the hot phase, the internal pressure rises, pushing against the rubber o-rings and adhesive joints. As the temperature falls, a partial vacuum develops inside the housing, pulling external air and pollutants through any microscopic leaks or imperfections.
Over many cycles, this continuous pressure reversal degrades the elasticity of the elastomeric seals, making them more vulnerable to bypassing and eventual structural failure.
Moisture Accumulation
The primary consequence of this atmospheric exchange is the gradual accumulation of liquid water inside the sensitive electronic chamber of the sensor. External air drawn in during the cooling phase contains water vapor that condenses on cold internal surfaces when the temperature drops below the dew point. Because the internal temperature rises slowly during the next cycle, the condensed liquid often does not evaporate completely before the next cooling phase begins.
This cumulative behavior can fill a housing with liquid over several months of outdoor operation. For sensitive sensing instruments, the presence of this liquid water leads to drift, short circuits and total failure.
Prevention Technique
Design engineers prevent this degradation by utilizing hermetic metal-to-glass seals or by integrating breathable vent membranes into the housing. These hydrophobic membranes allow air pressure to equalize between the interior and the exterior of the enclosure while blocking the entry of liquid water. Where ventilation is not permissible, filling the enclosure with dry nitrogen or potting the electronic board entirely removes the internal air volume, eliminating the driving force behind this destructive cycle.
