A kilometre-range distributed relative humidity sensor

Abstract

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.

Optimising the design, cost, and performance of a distributed humidity and temperature fibre sensor

“…The experimental setup of this study is based on a phase-sensitive OTDR that is fully described in a previous study. 10 The technique is employed to measure the static Rayleigh backscattering signal produced by random variations in the longitudinal refractive index (RI) of an fibre optic. These variations occur due to entropic fluctuations in the structure of the fibre.…”

“…In many state-of-the-art studies, researchers often rely on either use commercially available optical interrogators, 9 which can be costly due to the use of highprecision optical components and advanced signal processing algorithms, or on in-house interrogators assembled using large laboratory devices such as oscilloscopes and pulse generators. 10 However, the latter option is limited in terms of portability.…”

Distributed Humidity Sensor for Moisture-Front monitoring in Soils

“…The small size of fiber and the capability to be embedded, together with the high spatial resolution achieved make them a good candidate for moisture monitoring. For this application, other authors explored the distributed humidity sensing before [9], [10]. In particular, in [10] was characterized the sensitivity of optical fibers with different buffer using an optical backscatter reflectometer (OBR).…”

Distributed Fiberoptic Sensor for Simultaneous Humidity and Temperature Monitoring Based on Polyimide-Coated Optical Fibers

“…Humidity belongs to the principal environmental factors that is of importance in various fields, e.g., agriculture, civil engineering, structural health monitoring (SHM), or industrial process control. Distributed humidity/water monitoring is sought after for numerous applications including soil moisture measurement in agriculture [3], concrete condition monitoring in civil engineering [4], leak detection in sewage industry [5], corrosion prevention in pipeline industry [6], environmental monitoring for particle accelerator detectors [7], or SHM of large structures such as dykes or dams [8]. Despite notable applicational demand, available technical solutions for fiberoptic distributed humidity sensing (DHS) are rather scarce.…”

“…Nevertheless, such optical fibers with thick PI coatings remain commercially unavailable as application of PI on the fiber relies on thermal curing and is limited to few µm layer thickness in a single step. Instead of interrogation with OBR, which is limited to relatively short distances (tens of meters), Neves et al used phase-sensitive OTDR to extend the measurement range up to 10 km [7]. The increase in monitoring range, however, comes at the price of decreased spatial resolution.…”