Conformal Coating
It is very important that the board be coated before inserting it into the ground.
In the short term, we've seen uncoated boards experiencing drifts in the reading (#2, #50, #77), likely due to water creeping in under the solder mask and between the copper layers. If left uncoated for a long time, boards will eventually look like this:
See #79 for a community discussion on options and tests.
Quick drying, easy to use spray can.