When tanks are installed underground it is quite often required that these tanks have 2 walls with an air space between the walls. It is important to know that these tanks are in good condition and not leaking. With the switches configured to detect interface you can be notified of the source of any leak. If the outer tank wall is leaking we would detect ground water entering the air space. If the inner wall is compromised then we would detect the contents of the tank leaking into the air space.