A new compact optical refractometer is presented to improve the measurement of the refractive index of seawater. These measurements are useful in oceanography to calculate density and salinity of oceans from empirical relations. This refractometer shows a lower temperature dependence and obtains a better absolute accuracy on salinity compared to the conductivity sensors which are used nowadays to assess seawater salinity. Tests and calibrations have been made in a temperature-stabilized seawater tank. They show that the prototype is capable of measuring seawater refractive index with a resolution of about ±4 × 10−7, equivalent to a salinity resolution of ±2 × 10−3 g kg−1.

Absolute salinity measurement of seawater has become a key issue in thermodynamic models of the oceans. One of the most direct ways is to measure the seawater refractive index which is related to density and can therefore be related to the absolute salinity. Recent advances in high resolution position sensitive devices enable us to take advantage of small beam deviation measurements using refractometers. This paper assesses the advantages of such technology with respect to the current state-of-the-art technology. In particular, we present the resolution dependence on refractive index variations and derive the limits of such a solution for designing seawater sensors well suited for coastal and deep-sea applications. Particular attention has been paid to investigate the impact of environmental parameters, such as temperature and pressure, on an optical sensor, and ways to mitigate or compensate them have been suggested here. The sensor has been successfully tested in a pressure tank and in open oceans 2000 m deep.

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Optical salinity sensors described here measure directly the seawater refractive index and thus enable a measurement of the seawater density and composition variation. We detail the measurement dependence to environmental parameters (in particular temperature and pressure) compared to conductivity sensors, and demonstrate that it may be advantageous to directly measure refractive index rather than electrical conductivity and so obtain a more direct route to density and absolute salinity.