Salinity is one of the most important characteristic parameters of seawater. The absolute salinity 𝑆𝐴 is defined to be the “Total amount of dissolved material in grams in one kilogram of seawater”, and it is directly related to the seawater density. However, the absolute salinity is difficult to be measured in practice. At present, practical salinity 𝑆𝑃 is calculated by using conductivity, temperature and pressure measured by CTD sensors. The conductivity of seawater is dependent on the ions concentrations. However, the non-ionic components cannot be detected, though they may have significant influence on density. The optical refractive index is sensible to all dissolutions of seawater, therefore it is a good proxy of the concepts of salinity/density. In this paper, urea was used to study the influence of urea on the seawater refractive index based on optical refractive index method. The changes of seawater refractive index versus the urea concentration were studied by a Vgroove refractometer. This method provides a new method for measuring urea in seawater.
Liquid refractive index (RI) is one of the important optical parameters closely related to density, concentration, and temperature. The measurement of liquid RI is widely used in chemistry, medicine, physics, and biology research. A Fabry- Perot interferometer-based liquid RI sensor by using PMMA polymer material is proposed and demonstrated experimentally. The influence of cavity length and reflectivity on the sensor sensitivity is theoretically investigated. A long cavity length gives rise to a small free space range. In the experiment, high reflectivity of F-P cavity is made by evaporating metal film (aluminum film). An open F-P interferometer is realized by punching a hole in the PMMA plate, which is convenient for liquid exchange and in situ measurement. Experimental results show that the FSR the F-P cavity is about 5 nm with a fringe contrast of 3 dB. The designed sensor is compact, low-cost, easy fabrication and flexible replacement. Keywords: Fabry-Perot interferometer, Refractive index sensor, polymethyl methacrylate.
The optical properties of seawater are of great significance to the monitoring of marine safety and marine environment by means of optical sensing. In order to further study the relationship between the refractive index (RI) of different salt solutions, temperature and salinity, two experiments are carried out. The effect of temperature and mass fraction on RI of single component solution, and the effect of type and quantity of ions on the RI of mixed solutions when the mass fraction is kept constant at 3.5% are studied. Experiment results show that mass fraction coefficient of NaCl, MgCl2 and MgSO4 are 1.74×10-4 , 2.54×10-4 and 1.94×10-4 , respectively. Mass fraction coefficient of NaCl is lower than that of MgCl2 and MgSO4. In the second experiment, it is found that the effect of valence state on RI is higher than that of ion number, and the RI of bivalent solution is higher than that of monovalent solution. Not only the temperature and salinity of seawater, but also the components of seawater salt ions have different effects on the RI. Since the intrinsic relationship between RI and salinity is not suitable for different sea areas, the study of the relationship between the RI of single solution or a binary mixed solution and temperature and salinity is of great significance for the modulation process of optical salinity sensing.
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