Humidity and Moisture in Submerged Die-Cast Aluminum Enclosures
Your 2-minute 10 PSI pressure test is laughable in terms of your actual requirements. Let's work the numbers.



Your 4" &times; 5" &times; 5" box has a volume of about 1640 cc, or 0.00164 m3.

Dry air at 25&deg;C has a density of about 1.184 kg/m3.

Therefore, you have about 1.94 g of air in your box.

The dew point is about 2.0% mass of water vapor at 25&deg;C.

Therefore, it would only take about 38.8 mg of water to raise the humidity inside the box to the dew point, starting with perfectly dry air.

It took 15 days (21600 minutes) for that to occur, representing an infiltration rate of about 1.8 &micro;g per minute.



On the other hand, raising the pressure inside the box by 0.1 PSI, starting from 15 PSI, requires adding 0.667%, or 12.9 mg of air.

Over a test period of 2 minutes, this would require an infiltration rate of no less than 6450 &micro;g per minute, more than 3 orders of magnitude greater than that of the water test.



Given the design lifetime of 10 to 20 years, you're going to need to test for an infiltration rate that's a factor of 500&times; less than what you're getting now. Cut that in half again if you're starting out at 45% RH.



Silica gel can absorb about 30% of its mass of water. Adding 100 g of silica gel inside the box would allow your 20-year infiltration rate to be about 1000&times; higher than it can be without it. In other words, this could mean your current seals are good enough.

You also might consider pressurizing the box with 10 PSI of dry nitrogen before you drop it in the water.

A high-quality sealing grease on your mating surfaces and seals would probably help a lot, too.