Sensor…. Not seltzer!
Actually - the “sensor” could be as simple as an Alka Seltzer wafer. I placed them in the basement in between two strips of a beer can and wired to a battery hooked to a buzzer. The wafer dissolves in the flooded basement (water leak) and the alarm goes off! Luckily my patches to the old galvanized pipes lasted until I got the plastic pipes in.