Sea water salinity detection using circular fringes formation based on diffraction of laser light

Salinity describes the amount of salt dissolved in water and brininess of water makes it unsuitable for drinking, industrial and agriculture use. The sea water salinity is most often detected by total dissolved salt method in laboratory. This process is both time consuming and requires laboratory facilities. Various sensors are developed for automated measurements of salinity using light in order to establish a relationship between sodium chloride concentrations and output voltage. The main disadvantage of these voltage based sensors is that output voltage depends on temperature, ambient light and water clarity. In this paper a simple system for salinity detection in sea water is proposed and demonstrated using laser light and ultrasound. The working mechanism of the proposed system is based on the outer most circular fringe radius variations, which consequently alters refractive index. Passing laser light (650 nm) and ultrasound (9 MHz) simultaneously through sodium chloride (NaCl) solutions produce circular fringes at the output. Radii of the circular fringes decrease with increase in the concentration of NaCl dissolved in water and consequently increase the refractive index. The effectiveness of the system is tested on various sodium chloride concentrations. As the NaCl solution concentration increases from 0% to 9% (0 g/l - 90 g/I), the radius of the outer most circular fringe decreases from 16.95 mm to 16.45 mm and the refractive index increases from 1.333 to 1.373. The proposed system has more than 98% accuracy and 99% linearity.