I've recently been playing around with one of these sensors for a home project, and just wondering how it works internally. (The linked pages mention a reed switch and capacitor, but I'm no clearer to understanding the internal circuit).
Providing the contacts are immersed (or shorted) it appears to conduct at any input voltage from around 0.6V upwards. Output is roughly equal to 90% of the input less that initial 0.6V (diode drop?). One terminal is electrically connected to the positive wire, but the other terminal appears isolated whatever the condition.