This method is slightly different in setup for tanks and piping.
Tanks: Tank systems use a sensor installed within the interstitial space at the bottom of the tank toward the end that is lowest in elevation. If a leak occurs from the inner tank the liquid will flow to the sensor. When the liquid contacts the sensor the sensor sends a signal to the monitoring console, which triggers an audible and/or visible alarm to alert the operator of a problem with the tank.
Piping: Piping systems must be designed so that any leak from the inner wall of a double-wall piping system must direct the leak either back to a sump located at the tank or a dispenser, or to a sensor located along the piping. When the liquid contacts the sensor it will trigger an audible or visible alarm to alert the operator of a problem.