Universal Reservoir Sensor 
Use  a  Universal  Reservoir  Sensor  with  hydrostatically  monitored 
tanks. The Reservoir Sensor monitors the level of the liquid in the 
reservoir of a double-walled tank. The sensor has a single float that 
detects abnormally high or low liquid levels within the reservoir. If a 
leak occurs in either wall of a tank, it causes the liquid in the reservoir 
to rise or fall. When liquid reaches the upper or lower limit on the 
sensor, the sensor activates.