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Instrumentation: Calibrating Fiber Optics Distributed Temperature Sensing Systems for Soil Moisture Characterization

Author(s): Sergio Zubelzu, Leonor Rodriguez-Sinobas, Daniel Segovia-Cardozo, Freddy Canales Ide

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Keywords: distributed temperature sensing; soil moisture; active heated fiber optic; hydrology; temperature; surface thermometers


High accuracy and precision of fiber optic distributed temperature sensors allow field accurate characterization of complex processes such as the soil water movement. The Active Heated Fiber Optic (AHFO) is the method used to estimate soil water content derived from the measurement of the soil temperature with an armored fiber optic cable. The metal wire, enveloping the fiber optic cable, is heated during a given time producing a heating and cooling process. The soil water content is calculated studying the heat transference pattern from the heated fiber optic to the surrounding soil. The accuracy and precision of the measurements depends on the calibration process of the fiber optic cable and the distributed temperature sensor. This fact becomes relevant in the estimation of soil water content and soil water flow since higher sensitivity is required to determinate small soil water content variations. During calibration, fiber optic coils are submerged into calibration baths of known temperature measured with external probes. The parameters values required for calibration are deduced matching the external reference temperature to its measurement in the fiber optic coil. However, this methodology does not provide the best solution for AHFO applications since the reference temperature does not coincide with the temperature of the active heated cable which misleads the calibration process. This paper presents a calibration methodology that fulfills this gap by using, as reference temperature, surface thermometers attached to the steel jacket of the armored fiber optic cable. This method allows the estimation of the cable temperature during the heating and cooling periods reducing the length of the cable section under controlled temperature conditions. The calibration parameters were calculated yielding satisfactory results with accurate calibrated temperatures however the best results were obtained with protocols using three external temperature references.


Year: 2019

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