I'm planning to do a sump pump depth gauge too at some point as an emergency "pump is busted!" alarm - very interested if you don't mind sharing your thoughts for that design.
The measurement of percent humidity is dependent on temperature - cold air can hold less moisture. For the same moisture content, the lower temperature will have a higher percentage of its capacity. If you want a measure that is independent of temperature, look into something like Dew Point.
I'm planning to do a sump pump depth gauge too at some point as an emergency "pump is busted!" alarm - very interested if you don't mind sharing your thoughts for that design.