Hi,
Why don't you use an dedicated logic IC to manage all this!
I have developped in my company a custom IC that allow you to control level probes by the resistive principle.
So you do not have to worry about any thershold nor debounce, etc...
The probes are just inox wires that comes in contact with the liquid. external resistor to generate the freq. and one capacitor to have the AC voltage.

Juan-Carlos