Seawater Density Measurement Based On Optical Refraction

Seawater density is one of the most important marine parameters.It provides important physical parameters for seawater circulation,climate change,biochemistry and marine ecology.Seawater density is calculated by using the measured conductivity,temperature and depth(CTD)parameters through an algorithm.Accurate calculation of seawater density relies on high accuracy and synchronized measurement of conductivity,temperature and pressure.However,due to the unsynchronized measurement between the temperature and salinity of CTD,the density spike may occur when calculating the density,which leads to a deviation between the measured value and the true value.The salinity is calculated based on conductivity measurement,that will be influnenced by the suspended conductive particles in sea water.In addition,the CTD method can not detect the contribution of silicate and other non-electrolytes in the seawater to salinity,which result in the calculated sea water density smaller than the actual density.The study of seawater density based on the optical refractive index method in the V-block provides a total different principle with conductivity method.By measuring the optical refractive index of seawater,combining the calibrated temperature and pressure parameters,using the relationship between refractive index and density,the corresponding seawater density could be obtained.For the research goals,The main contents of the paper are as follows:based on the principle that different densities of seawater have different refractive indices.When the laser is incident to different density seawater at a fixed angle,the deviation angle of the output light will be different.The lateral displacement of the beam changes with the density of seawater.The displacement can be detected by postion sensitive device(PSD),and the refractive index of different density seawater can be obtained.According to the calibration curve of "PSD position and seawater density",the density of seawater is determined.In addition,the influence factors affecting the refractive index measurement of seawater by light refraction method are analyzed.The theoretical simulation of the laser incidence angle,the position of the incident point,the receiving distance,the influence of the external environment.The key influence factors such as the incidence angle,the receiving distance and the position of the incident point are measured experimentally.Innovations in this article are as follows:In the refraction index measurement system of seawater,the paper proposes the "front air film" and "rear air membrane" structure,which can greatly reduce impact of the incident angle,and can improve the measurement resolution of the seawater refractive index.For the innovations,the paper compares with and without air(the method currently used by the best V-block density meters in the world).The optimized with air V-block the simulation results can have higher resolution and lower sensitivity of incidence angle than the current commercial V-block.In theory,the "front air film" structure can reduce the incident angle sensitivity by 80%,and the "rear air film" structure can increase the measurement resolution by 40%.Experimentally,When the resolution of PSD resolution is 3?m,the resolution to the seawater refractive index is 3.8×10-6RIU,the resolution to seawater salinity is 0.02PSU,and the resolution to seawater density is 0.015kg/m3,and a good linearity.Although the results of the paper hasn't exceeded the best international V-block density meter,the reason is that the PSD resolution of this paper is ten times bigger than the best.If the best used the same PSD,the paper would have better results.The advantages of the scheme are as follows:direct measurement of seawater density without the need for conductivity,temperature and pressure conversion;The interference and error caused by suspended particles in seawater can be effectively avoided.The influence of silicate and other non-conductive organic compounds in the seawater can be measured.In addition,by calibratied the curves between the index of seawater and different parameters can extend multi-parameter measurement(temperature,salinity,etc.).With the maturity,stability and wide application of the method,it is expected to overcome the shortage of the electrical measurement method,it would have a major influence on international salinity standards and ocean observations.