Pelican pressure equalisation valve
The Pressure Equalisation Valve on Pelican cases is a simple device. It consists of a housing with a hole through it that is covered with a thin Gore-Tex® membrane. According to the Gore™ tech sheet: “The microporous expanded polytetraflouroethylene (ePTFE) membrane continuously allows the free passage of gases and vapors, equalizing the pressure differential between the enclosure and ambient before it builds to the point that a seal is compromised. Water, dust and dirt are repelled by the hydrophobic membrane, thereby protecting expensive and sensitive electronics.”
"Automatic Pressure Purge" valve
The opposite is also worth considering .... water/humidity once in, cannot escape when the case is securely closed. Around surf, rivers and high humidity tropical environments it is almost inevitable that moisture will be encased with your equipment. The solution that Protective Cases suggest for handling this is the inclusion of a reusable desiccant pack to absorb and hold the moisture away from your sensitive equipment.
"Hydrosorbent Silica Gel" - Desiccant