We want to waterproof an electronic device and the specifications call for IP-65 protection. To this end, we have selected a sealed polycarbonate box to put the electronics in. The external connectors are reduced to a minimum and we use automotive IP-67 connectors. We tested a few prototypes and they cope with submersion. I'm closing on any remaining issues that may cause water damage in operation.
I'm worried that atmospheric air that gets trapped inside when the device is fully assembled might condense if cooled significantly. Especially if assembly is in mid-summer.
It seems that this problem is tackled in the industry by displacing ("backfilling") the air in the enclosure with nitrogen, helium and/or clean dry air. I don't have any experience with these, but they seem to be expensive operations that only make sense for medium- to large-scale production, when you have the equipment required.
What production technique can be used for small-scale production (10-100s/month) to keep the humidity of the air low enough so that it doesn't condense?
The device has a display which needs to be visible through the transparent box, so condensation-resistance is a must. Conformal coating alone won't suffice.
The device is battery-powered, so keeping the innards at elevated temperatures is also not an option.
I've read the excellent question about waterproof enclosures and unfortunately the "leaks more on the bottom than the top" strategy cannot be used here. Moreover, its answer specifically assumes "this is not meant for production" :)