Extended Study of Fiber Optic-Based Humidity Sensing System Performance for Sewer Network Condition Monitoring

This article reports on an extended (20-month) period of monitoring of humidity in situ at two locations in the sewer network operated by Sydney Water using a fibre optic network into which a series of Bragg Grating-based sensors had been installed. The locations (Eustace Street in Manly, Sydney and Old Toongabbie at Oakes Reserve, western Sydney, Australia) both had different operating environments and thus conditions for evaluating the sensor system. It was designed to provide a solution to enable long term, low cost and more reliable monitoring in the harsh conditions of the sewer environments in terms of high relative humidity >95% and a broad range of hydrogen sulfide levels. The results of the study show that even after ~20 months of use, the same sensor is reliably recording humidity and temperature in the sewer environment-overcoming the problems seen with conventional electrical sensors, which typically fail within a couple of weeks of use in this continuous high acid/high humidity environment. The data, recorded constantly from the sensor system, were stable throughout the full monitoring period and further, a comparison with the changing weather conditions was made over the different seasons during the study. The sensor system developed was battery operated and had 4G connectivity for data transfer and debugging. These features have enabled the system to be installed in situations where power is not available and operate successfully with minimal human operation, thus allowing for additional systems to be integrated to the measurement system in the future.