## 2 Literature Review
### Our Proposed Solution  
A performance comparison of several techniques used for pure and salt water salinity measurement is detailed in the Table 1. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Reference** & **Method/Technique** & **Advantages** & & **Disadvantages/Limitations** \\ \hline \multirow{8}{*}{\begin{tabular}{c} [10; 11; 12; 13] \\ \end{tabular} } & & & \(\bullet\) & Accuracy is not perfect. \\  & & & \(\bullet\) & A dramatic shift of the working range of \\  & & & \(\bullet\) & immobilized lucigenin is required. \\  & & \(\bullet\) & Fast and easy implementation & \(\bullet\) & Lucigenin fluorescence is quenched by \\  & Chemicals (lucigenin, & technique. & & chloride, bromide, salicate, thiocyanate, \\  & luminophores & \(\bullet\) & Chloride measurement over a & & and iodide. \\  & membranes \(\ldots\) ) & & wide range of salinities. & \(\bullet\) & The sensor calibration changed with \\  & & \(\bullet\) & Dynamic range can be adjusted. & & sensing fluorophores photo bleach at \\  & & & & \(\bullet\) & Not suitable to many applications that \\  & & & & & require unchanged solution properties. \\ \hline \multirow{4}{*}{\begin{tabular}{c} [14; 15] \\ \end{tabular} } & & \(\bullet\) & Suitable for pure and seawater & \(\bullet\) & High cost and a poor anti-interference \\  & Acoustic/density & & detection. & \(\bullet\) & Design is not perfect (not easy to \\  & & - & Simple concept. & & implement). \\ \hline \multirow{8}{*}{\begin{tabular}{c} [18; 19; 20; 21; 22; 23; 24; 25] \\ \end{tabular} } & & \(\bullet\) & Simple and easy & \(\bullet\) & Highly reliable pump is needed to flush \\  & & \(\bullet\) & implementation. & & water through the conductivity cell at a \\  & & \(\bullet\) & Fast response and low cost. & \(\bullet\) & Water velocity dependent technique which \\  & & - & Widely employed in & makes the system complicated and difficult \\  & & various industrial & & to miniaturize. \\  & & applications. & \(\bullet\) & All dissolved material in seawater should \\  & & & & be conductive. \\ \hline \multirow{8}{*}{\
\begin{tabular}{c} [29; 32; 33] \\ \end{tabular} } & & \(\bullet\) & Achievement of highest & \(\bullet\) & The gold layer thickness affected the \\  & & wavelength interrogation & & sensing performance. \\  & & sensitivity among fiber-based & \(\bullet\) & The noise captured complicated the \\  & Surface Plasmon & & SPR sensors. & detection process. \\  & Resonance & \(\bullet\) & Simultaneous measurement of & \(\bullet\) & The holes filled with the fluid limited the \\  & & seawater salinity and & & performance of the sensor design. \\  & & temperature. & \(\bullet\) & Number of holes in the sensor design \\  & & \(\bullet\) & Possibility of avoiding metal & & limited the light propagation and reduce \\  & & coating in the fiber holes. & & the core-guided light. \\ \hline \multirow{8}{*}{
\begin{tabular}{c} [34; 35] \\ \end{tabular} } & & \(\bullet\) & Verified technique in real ocean & \(\bullet\) & Raman scattering is neglected in the blue \\  & Raman scattering & water under normal solar & region of the ocean water. \\ \cline{1-1}  & & illumination. & \(\bullet\) & Pigment concentration data missing to \\ \cline{1-1}  & & & & confirm the technique hypothesis. \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Comparison table detailing the advantages and limitations of techniques used for pure and salty water salinity measurement. Indirect measurement techniques are the most commonly used, especially for applications where the chemical properties of solutions should stay unchanged. Among the indirect measurement techniques, optics methods are the most accurate, cost effective, and easy to implement, especially the refractive index measurement technique that provides an advantageous alternative solution [44] as it is correlated to the density and thus to the absolute salinity, pressure, temperature, and wavelength of light used [45; 46; 47; 48; 49; 50; 51; 52; 53; 54; 55].