LED optical sensor prototype to determine dissolved oxygen saturation in water

Water is an important resource for socioeconomic and environmental activities. Therefore, its quality should be monitored with the affordability of the system being a key factor. Dissolved oxygen is an important parameter to control when assessing water quality. We ran a preliminary study of its determination using optical techniques using two samples. One of them had saline water, while the other had fresh water. The samples were brought to 0% (with Na2SO3) and 100% (with an aerator) dissolved oxygen levels. With the samples at 0% and 100% dissolved oxygen, a spectrophotometer was used to determine the best wavelength for the detection of variations. The results show promising differences in the blue visible light (455 nm, 460 nm, 470 nm, and 505 nm) and infrared wavelengths (950 nm). The differences for infrared light showed less dispersion and similar differences for both fresh tap water and saline water (0.018 and 0.014, respectively). Both wavelength ranges could be useful. Three stability tests were conducted for the prototype, using a blue Light Emitting Diode and a photodiode with a Light Dependent Resistance. The Light Dependent Resistance showed interferences with the salinity; therefore, the final sensor should be isolated from the medium.