A pilot study: Evaluation of sensor system design for optical fibre humidity sensors subjected to aggressive air sewer environment

摘要

Research has identified that humidity plays a fundamental role on the conversion of hydrogen sulphide into sulphuric acid during corrosion of concrete in gravity sewers and that potentially minor reductions in humidity can reduce corrosion rates. The high levels of hydrogen sulfide and the high relative humidity (typically > 98%) that leads to condensation in sewer air makes humidity measurement within such environment difficult. The work presented here aims to tackle this issue by applying a more durable packaging that could withstand the harsh environment. The humidity monitoring element in the probe is based on a moisture-sensitive polymer coated fibre grating in series with an uncoated grating for temperature compensation. To optimize the device to be operational, two designs of the probe assembly were configured using different material, thus aiming to provide long term durability. The aim of the probe design evaluated was to achieve both a good sensitivity to humidity and 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. The packaged sensors were trailed in situ over a period of 5 months, during which the sensors were constantly subjected to the prevailing high, but varying levels of humidity and hydrogen sulfide gas. The results show the significant promise for tailor made packaging which will protect the sensor element from the harsh environment while retaining good sensitivity. These outcomes show a promising future for optical fiber sensors to be employed for the measurement of humidity in the long term in harsh environmental applications.