Another way:

use 2 isolated electrodes and build a capacity measuring bridge.
Then any other medium (water, oil, ...) between change the capacity. It works independend from conductivity, electrolisis, temperature, dust or other reliability reducing factors.

Its a simple dual-Op-Amp (one half to generate AC, the other half as bridge detector), some R and C around and a poti to calibrate it to the air capacity.


Peter