Measurement of seawater refractive index and salinity by means of optical refraction

Abstract

The aim of this work has been to construct and test a compact refractometer for accurate measurement of seawater refractive index and salinity. The specific goal was to measure the refractive index of the water sample with an accuracy of 10-6 refractive index units (RIU). The size goal was set to be a cylindrical container with a height of 80 mm and a diameter of 36 mm. The realization of the instrument resulted in a cuboid measurement chamber with a length of 70 mm and a width of 40 mm and an additional elec- tronics compartment. The instrument's basic concept is a laser beam which is transmitted through a prism setup. The water sample is brought into the sample compartment by using a water pump. The lateral displacement of the beam, which changes according to the refractive index of the water sample, is then de- tected by a position sensitive light detector and the temperature of the water sample is measured with a thermistor. The photocurrent from the detector is amplified and acquired to a computer along with the temperature voltage from the thermistor. A computer simulation has been made to accurately design and describe the instrument. The experiments were conducted by measuring the laser beam displacement and temperature of 10 seawater samples with practical salinity ranging from 0 to 36, as well as 10 samples of crystalline sodium chloride dissolved in pure water with concentration ranging from 0 to 4 g/100g. The refractive index of the samples were then calculated from empirical algorithms. The results show that the instrument is capable of measuring the refrac- tive index of seawater with an precision of 1.5 · 10-4 RIU over long time periods, up to several hours. The main contribution to this uncertainty has been iden- tified to be a temperature dependent drift in the position detector output. The precision within intervals of minutes was found to be 1.2 · 10-5 RIU.