Measurement of seawater salinity, with a high accuracy,has become a key issue in ocean control. One of the mostdirect ways is to measure the seawater refractive indexwhich can be then related to the salinity. Refractometry is the most direct and simplest way to measure refractive index. Recent advances in high resolution position sensitive devices (PSD) impose now to consider the physical limits and resolution of beam deviation measurement methods such as refractometers in particular when additional constraints such as compactness are required. This paper proposes to assess what can be achieved with this technology according the current technical state of the art. In particular, we present the resolution dependence to refractive index variations and consequently derive the limits of refractive angle measurements for seawater sensor applications.