Seawater Salinity Refractometer With Improved Index Sensitivity

Seawater salinity is one of the essential quantity of ocean observation.The measurement of seawater salinity is of great significance in the study of ocean circulation and climate prediction.However,the commonly used conductivity method don’t take into account the effects of the non-ionic components,such as Dissolved Organic Matters,which leads to unknown errors in the estimation of absolute salinity of seawater.Unknwon measurements errors exist in offshore and seawater areas where the composition of seawater deviates from standard seawater.Due to certain limitations,the accuracy of the measurement results is affected by the composition of seawater.Refractive index of seawater is sensible to all the constituents of the fluid and is therefore a good way to assess the seawater absolute salinity.Among the current optical salinity measurement methods,the V-block method has become produte due to its simple structure and easy integration,but at the same time it faces the following problems:(1)The sensitivity is not high enough to distinguish the low-content organic matter in seawater,the system sensitivity is determined by the system angle sensitivity and the sensing distance,the angle sensitivity is limited by the V prism structure,and the sensing distance is limited by the system size.(2)There is a lack of systematic research on the accuracy and reliability of optical methods for measuring seawater salinity.In view of the above problems,this paper has done the following research:(1)Firstly,the optical principle of measuring the refractive index of seawater by the Vblock is analyzed.Based on the empirical equation of seawater,the relationship between refractive index and salinity and the theoretical value of the influence of pressure and temperature on the refractive index are studied.In order to effectively carry out temperature compensation,two kinds of glass with opposite signs of "refractive index temperature coefficient" are selected as the material of V-block.(2)In order to further improve the sensitivity of the V-block method to the change of the refractive index of seawater,a V-block with novel structure seawater salinity system was designed.The sensitivity of the V-block with novel structure and the change of the sensitivity with the incident angle were simulated and analyzed;the V-block with novel structure was further fabricated,and the V-block seawater measurement system was built to verify the system angle sensitivity and repeatability and stability.The experimental results show that the angle sensitivity of the V-block with novel structure can reach 3.5× 10-3 rad/PSU.which is 2.7 times higher than that of the traditional V-block,and the stability of the salinity measurement system is better than ±0.0023 PSU.(3)In order to systematically study the accuracy and reliability of optical methods to measure seawater salinity,a prototype of the in-situ measurement system for seawater salinity based on V-block was designed.The optical path structure in the prototype was optimized to improve the angular sensitivity of the prototype probe;the angle dependence of the sensing probe was studied;the mechanical shell and watertight parts were designed for the integrated prototype;the influence of seawater pressure on the probe sensitivity was analyzed,and the stability and repeatability of the in-situ measurement prototype are verified in the seawater temperature and salinity environment simulation system.The test results show that the angular sensitivity of the prototype is 1.4035×10-3 rad/PSU,which is slightly better than the world’s best product NKE.The stability of the prototype is better than±0.0023 PSU,and the repeatability is better than±0.298 PSU.(4)The influence of turbidity on the output light intensity was tested by using the V-block system,and the influence of different turbidity standard liquids on the refractive index measurement results was clarified.The innovations of this paper are as follows:(1)Based on the principle of near total reflection,a V-block with novel structure is designed on the basis of the traditional V-block,which improves the angular sensitivity of the V-block and verifies the sensitivity and stability of the system.(2)A prototype based on V-block was designed.In the marine environment simulation system,the stability and repeatability of the system were studied,and the influence of turbidity on the measurement results was also studied.It lays a solid foundation for studying the measurement accuracy of seawater salinity sensors based on optical principles.