Optimal design of optical refractometer for seawater salinity measurement

摘要

Salinity of seawater is one of the most important ocean parameters. Salinity of seawater is mainly obtained by conductivitymeasurement using CTD (Conductivity-Temperature-Depth). Conversion accuracy between conductivity and salinityrelies on the assumption that components of seawater are fixed, as well as high accuracy and synchronism measurementsof conductivity, temperature and pressure. The study of seawater salinity based on the V-block optical refractive indexmethod provides a total different principle for salinity measurement. Achieving high resolution seawater optical refractiveindex measurements could help to study factors affecting the accuracy of salinity measurement. In this paper, the variousinstrument parameters that affect the accuracy of seawater refractive index measurement are analyzed and the opticalrefractometer is optimized based on the components on the shelf. This paper systematically analyzed the resolution andtolerance of refractive index measurement on the parameters of V-block refractometer, such as incident angle, externalenvironment and prism refractive index, etc. The optical refractometer with an air film layer on both sides of the V-blockwas proposed for seawater salinity measurement. With such an optimization, the measurement accuracy is furtherimproved and the tolerance is increased. The theoretical resolution to the seawater refractive index and salinity are1.8×10~(-16) and 0.01‰, respectively. Experimentally, we have achieved 3.9×10~(-16) and 0.021‰ respectively, and a goodlinearity. The difference between theoretical and experimental results are analyzed.