Evaluation of the Durability and Performance of FBG-Based Sensors for Monitoring Moisture in an Aggressive Gaseous Waste Sewer Environment

Measurements of the rate of corrosion in concrete sewers need to take into consideration the humidity in the environment, and, thus, its accurate measurement becomes critically important. Introducing a novel approach to do so, tailored fiber Bragg grating (FBG)-based humidity sensors have been evaluated in situ to examine their durability, time response, and stability when used in measurements over an extended period of time under the aggressive gaseous environment of a gravity sewer, experiencing high levels of both humidity and hydrogen sulfide gas. The critical humidity monitoring element in the probe is based on a moisture-sensitive polyimide coated FBG, using the calibrated and reproducible peak wavelength shift in response to moisture variation, in this case operationally in the sewer. To optimize the device for this environment, two different designs of the probe assembly were configured using different material, thus aiming to provide durability in the harsh environment in the long term. The aim of the probe design evaluated was to achieve good sensitivity to humidity as well as to protect the sensing elements from the aggressive environment and which had rendered ineffective the electrical sensors placed in the sewer and used for cross comparison. A full evaluation of the packaged sensors in situ was undertaken over a period of five months, during which the sensors were constantly subjected to high, but varying levels of humidity and wet hydrogen sulfide gas. The results are highly encouraging, showing superior performance of the configured fiber optic sensors used over a conventional electrical sensor when the results of the cross-comparison study of the performance were evaluated. These outcomes show a promising future for optical fiber sensors to be employed for measurement of humidity in the long term in harsh environmental applications such as this.