The increase in resistance is precisely the result of
electrolysis and that's why devices that measure the resistance of wet
substances like soil or the presence or absence of water use AC.

        There are tilt sensors filled with fluid that one is not even
supposed to check with an ohm-meter because that will start
electrolysis and ruin the sensor.

        I would think that a PIC would be able to drive a dew sensor
by alternating the state of two pins so that one is high and the other
is an input, pulled to ground via resistor.  The idea would be to put
a resistor on each pin and then bring one high while the other was the
input and then change the input to output and bring it high while
changing the first output to an input and reading it.  I don't know if
the wave form of the AC is important or not, but it does need to be AC
unless you want to end up with some electro plating.

Martin McCormick