AVR-Chat

Maybe you could implement this:

http://ww1.microchip.com/downloads/en/AppNotes/00512e.pdf

This circuit is designed for a PIC but I am in the process of implementing 
it with an AVR.  I have used the PIC version to detect the water levels in 
boreholes and mine voids with the probes attached to a cable and lowered 
as much as several hundred feet down. My PIC version also includes an LCD 
to display the resistance measured. But you could get as simple or as 
elaborate as you want. My probes are 6-inch lengths of stainless steel 
rod 3/32 inch in diameter spaced 1 inch apart.  But you could make 
smaller probes to detect the water level in any sort of reservoir. In a 
metal radiator you could even use the radiator as one terminal and a small 
probe as the other. Probes out of water should measure many megohms, and 
probes in water that is not distilled should be on the order of tens to 
thousands of ohms.

Zack