For FRP-Steel double-wall tanks, we install a SFLS-2DA/2C double-wall tank leak detection sensor into the lowest point of the interstitial space which is usually on the bottom of the tank. Any leak of the tank occurs, it will trigger an alarm. Usually, the leak of outer wall will lead water from the soil flow into the bottom of the interstitial space, and the leak of inner wall will lead fuel from the inside of the tank flow into the bottom of the interstitial space, then the liquid will be detected by the SFLS-2DA/2C double-wall tank leak detection sensor and trigger an alarm.