This paper presents the design, cost optimization and performance analysis of a distributed humidity and temperature fibre optic sensor for environmental monitoring. The sensor utilises a 1-metre spatial resolution phasesensitive Optical Time Domain Reflectometry (OTDR) interrogator and employs a pair of fibre optic cables as sensing elements. One cable is coated with polyimide for humidity sensing and the other is coated with acrylic for temperature sensing. The sensor is designed to be reliable, accurate and cost-effective, enabling its use in various industrial environments. New software was developed for fast data acquisition and processing, and the hardware was assembled to allow measurements to be taken at thousands of different locations over the same fibre optic. The cost of the current version and the acquisition time have been reduced by half compared to the reference version. The sensor’s performance was evaluated in both a controlled laboratory environment and in a real-world deployment. The results indicate that the sensor effectively measures relative humidity (RH) and temperature across a broad range of conditions while preserving the precision of the previous version. Additionally, it utilizes cost-effective hardware and has a significantly faster response time.
Fibre optics sensors have been identified as very good candidates for environmental monitoring inside the silicon detectors operated at CERN’s Large Hadron Collider. In this study, we present the results from the first highly sensitive relative humidity distributed sensor with kilometres sensing range. The setup is a 70 cm spatial resolution phase-sensitive Optical Time Domain Reflectometry (OTDR) and is able to monitor fibre lengths up to 10 km. The coating effect is also evaluated, analysing different coating thicknesses, number of coating layers, different manufacturing and different materials. Relative humidity tests were performed at two different temperatures (25°C and 42°C). Polyimide coated fibres show in general a higher humidity sensitivity then a standard acrylate coated fibre, while acrylate fibres offer the fastest response and settling time. The system is able to resolve 0.1% RH and all tested fibres proved to be good candidates to be employed in a distributed relative humidity sensor. If the requirements are a fast time response and short settling time at room temperature, the standard acrylate coated fibres are the best candidates. However, if the requirements are high sensitivity and measurement stability at different temperatures, the polyimide-coated fibres offer advantages on several aspects.
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