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An optical Solution for Simultaneous in-situ Sea Water Salinity and Turbidity Measurements

Abstract : Salinity and turbidity are two important seawater properties for the physical oceanography. The study of physical oceanography requires a compact high-resolution in-situ salino-turbidi-meter. The main objective of this work is to propose, design and implement an optical solution to simultaneously measure the seawater salinity and turbidity. Our first study is carried out to design a high-resolution refractometer based on a laser beam deviation measurement by a Position Sensitive Device (PSD). The refractive index measurements obtained by the voltage value delivered by PSD have been evaluated to quantify the performances of the sensor. According to the obtained results, it is clear that this PSD-based refractometer is attractive for innovative applications in metrology. However, PSD lacks the capability to retrieve the power distribution information of laser beam, which is related to the turbidity measurements. On the contrary, Charge-Coupled Device (CCD) gives much more information of laser beam than PSD. In the second part of the thesis, a performance comparison between PSD and CCD combined with a centroid algorithm are discussed with special attention paid to the CCD-based refractometer. According to the operating principle of CCD-based system, five factors of CCD-based system: image window size, number of processed images, threshold, binning and saturation are evaluated to optimize the CCD-based refractometer. By applying the optimized parameters, the performance of CCD-based refractometer is better than PSD-based refractometer in measuring the refractive index. Furthermore, by applying different post-processing algorithms, CCD-based system possesses the capability of measuring the power distribution sensitive quantities. To show this advantage of CCD-based system, the attenuation measurement method is used to measure turbidity without modifying the refractometer configuration. The turbidity measurement and salinity measurements influence each other in a refractometer. To overcome these influences, a CCD combined with a new location algorithm is used to measure both the refractive index and the attenuation. Several simulations and experiments are carried out to evaluate this new method. According to the results, the way to improve the resolution is discussed as well. Comparing to the nephelometer specified by the NTU standard, our method has been proved as a valid method to measure turbidity. By studying the performances of CCD-based refracto-turbidi-meter, 3 new prototypes are proposed to improve the salinity and turbidity measurement performance at the end of this thesis.
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Submitted on : Friday, July 20, 2012 - 11:16:56 AM
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  • HAL Id : tel-00719584, version 1

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Bo Hou. An optical Solution for Simultaneous in-situ Sea Water Salinity and Turbidity Measurements. Optics [physics.optics]. Télécom Bretagne, Université de Bretagne-Sud, 2012. English. ⟨NNT : ⟩. ⟨tel-00719584⟩

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