Distributed pH and water detection using fiber-optic sensors and hydrogels

This paper reports on the design, construction and test of a sensor for detecting the presence of water as a function of position along an optical fiber length. The technique combines Optical Time Domain Reflectometry (OTDR) with chemically sensitive water swellable polymers (hydrogels). Initial experiments have shown that the sensor is capable of detecting water ingress points as small as 50 cm in length over lengths of sensor in excess of 100 m. The sensor has been assembled into a robust cable configuration which allowed it to be installed into a representative bridge structure without the need for special handling precautions. Once installed the sensor gave meaningful information on the location of voided regions during a cement grouting process. The present gel system is primarily a water detector however through varying the composition of the gel the sensor can be tuned to respond to different trigger stimulus such as pH response