duel sensor detector tube systems are installed along the path length of a pipeline , storage tanks , or other fuel system to be monitored for leaks . one sensor detector tube system is constantly evacuated and passed through a sensor detector to test for presence of vapors or gaseous indications of large leaks . concurrently , the other sensor detector tube system is permitted to lie dormant for a predetermined period to absorb and receive diffusion of vapors or gases from medium to small leaks that are too small to be detected by the continuous air flow system of the first tube . periodically the flow of the two sensor detector tube systems is interchanged to evacuate the dormant tube , and it then becomes the continually aspirated system . signal output from the sensor detectors provide data to the control alarm system which compares concentration profiles of current on - going tests with recorded profiles of prior tests , and any significant deviation is recognized as a problem , sounding an alarm . the system provides direct data for location of small leaks , with significant indications for location of large leaks .