I was wondering what's the best way to waterproof the actual sensors? My current plan is to use silicon caulking over all the components but will the actual wires running 8 inches into the ground need to be waterproofed as well?
I know these are corrosion-prone and plan to have them in the ground for 3 hours.
We plan to have long wires so our Arduino board will be above ground.
You have a resistive sensor. The corrosion does not come only from the water but from the way it's being used (ie not just because it is in contact with the soil but also because there is a DC current flowing which causes electrolysis of the sensors)
if you get a capacitive sensor you won't have that issue
Three hours is hardly a problem as to corrosion. Also if you embed the sensor legs in anything the sensor won't work. The legs need to be clean and exposed. If you want to waterproof anything I would use silicone caulking and after connecting your lead wires caulk the terminals. I would use a jacketed wire for my leads. I would likely choose something like Alpha Wire 2414C and use only 3 of the four conductors but your choice. Again for a three hour test or even a 24 hour test I would not be concerned about corrosion.
Heck, if you can solder I would remove the screw terminals and just solder to the sensor board.
