I've found this scheme for a water-level.
It has only visual alarm but with a NPN it's possible to connect a buzzer through pin12.
What I want to do is connect a pump that works like scheme:
ON - when level
This circuit will have problems with electrolytic corrosion on the probes as well as the problems of varying conductivity of the water.
I would not use this circuit for anything other than a temporary installation.QUOTE]
That would be true with most materials. However, if stainless steel probes and hardware were used along with the proper sealing devices at the junctions I don't think there would be a problem for decades at minimum. I know this from my own experience as a journey electrician in my own community before I went back to school for my degree. The water handling systems I installed at schools, hospitals, processing plants and wastewater facilities are still functioning as designed.
Regarding the conductivity of the water medium, that is a matter of degree that can be overcome by the sensitivity of the circuit employed, and the isolation of the probes within and without the medium being sensed.