This paper describes a simple and economical sensor which employs fiber optics to detect the presence of hazardous liquid chemicals, particularly undiluted hydrocarbons. The device is best suited to monitor the interstitial space of double walled underground storage tanks. Because the sensor is plastic and is situated at the end of a passive and insulating optical fiber the sensor can be considered inherently safe. The optical fiber used for this device can be up to several hundred meters long.
"Fiber Optic Detector For Liquid Chemical Leaks", Proc. SPIE 1120, Fibre Optics '89, (29 October 1989); doi: 10.1117/12.961007; https://doi.org/10.1117/12.961007