27 July 2004 Distributed fiber optical HC leakage and pH sensing techniques for implementation into smart structures
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Interaction of target molecules with the evanescent wave of light guided in optical fibers is among the most promising sensing schemes for building up smart chemical sensor technologies. If the technique of optical time domain reflectometry (OTDR) is combined with silicone-clad quartz glass fibers distributed chemical sensing is possible. Hydrocarbon (HC) detection and location is done by automated identification of the position of the corresponding step drop (light loss) in the backscatter signal induced by local refractive index increase in the silicone cladding due to a penetrating HC compound. A commercially available mini-OTDR was adapted to sensing fibers of up to nearly 2-kilometer length and location of typical HC fuels could be demonstrated. The instrument is applicable for fuel leakage monitoring in large technical installations such as tanks or pipelines with spatial resolution down to 1 m. A similar technique using measurements in the Vis spectral range is being developed for health monitoring of large structures, e.g., for early detection of corrosion caused by water ingress and pH changes in reinforced concrete. Here, a pH indicator dye and a phase transfer reagent are immobilized in the originally hydrophobic fiber cladding, leading to a pH induced absorption increase and a step drop signal in the backscatter curve. The configuration of the distributed sensing cables, the instrumental setups, and examples for HC and pH sensing are presented.
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Jochen M Buerck, Jochen M Buerck, Bernhard H. Vogel, Bernhard H. Vogel, Siegmar Roth, Siegmar Roth, Sasan Ebrahimi, Sasan Ebrahimi, Karl Kraemer, Karl Kraemer, "Distributed fiber optical HC leakage and pH sensing techniques for implementation into smart structures", Proc. SPIE 5384, Smart Structures and Materials 2004: Smart Sensor Technology and Measurement Systems, (27 July 2004); doi: 10.1117/12.538240; https://doi.org/10.1117/12.538240

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