Don't use ac in the sensing circuit.

As I mentioned, the best sensor technology to use is OPTICAL. Its very low power, non-contact, and doesn't risk vapor ignition as with some liquids.

The sensor uses an LED emitter detector pair sealed inside the special plastic or glass dome. If air surrounds the LED dome, it shines through and the detector gets no feedback.

When water/liquid surrounds the LED dome, the specially constructed facet of the dome working with the surrounding water, reflects the LED light back into the detector. Its basic physics.

aka J