Low-cost turbidity probe | Light-to-frequency converter
The continuous and excessive water use due to the developing industry and agricultural activities creates a strong pressure on water resources, leaving the aquatic ecosystem vulnerable to large-scale degradation. Water resources management and protection are at the core of sustainable development, and require constant monitoring and assessing, being a key element to minimizing the negative impacts on these ecosystems. One of the most important indicators of water quality is the relative cloudiness of water caused by chemical precipitates, organic and inorganic compounds, and microorganisms.
Although the increase of turbidity causes a reduction in water visibility, the effect on water quality is not limited to water cloudiness. Suspended particles present can serve as a refuge for pathogens, heavy metals, and pesticides, and for that reason, turbidity is one of the most important parameters to describe water quality.
Turbidity meters work based on optical scatters and transmit-detection techniques. The light emitted by the light source is absorbed, reflected, and dispersed by suspended particles present in the water. The light detectors, mounted at different angles from the light source, are responsible to capture the light and correlate it with the amount of suspended particles present in the water sample (Fig 2).
For example, if the light detector is positioned at 90° from the light source (Cneph), the cell will have a higher light response under higher turbidity waters (this is known as the nephelometry principle). On the other hand, if the detector is positioned aligned with the light source (Cturb), the cell will detect higher light intensity for lower water turbidity values, since most part of the light will not be scattered by suspended particles.
Commercial turbidimeters that perform continuous turbidity monitoring cost around 7000 Euros. Due to the high cost of commercial turbidity sensors, recently many low-cost turbidity meters have been proposed. The aim of this pair of studies is to describe the design and fabrication of a low-cost smart turbidity meters, and investigate the capability of this sensor to provides additional information about the water quality based essentially optical scatters and transmit-detection techniques. Thus, in this section we do not pretend to mention previous observations conducted with analog turbidity sensors that uses ADC (Analog-to-digital converter) transmission to read signals and correlate with turbidity readings. For more information about this previous designed probes, click here.
Enhancing Eutrophication Analysis with an Advanced Turbidity Meter (Abu Dhabi probe)
To test light-to-frequency converter and analog sensors based on ADC transmission, we brought into existence the Bangkok turbidity meter, similar to Paris, but with only two light detectors (LDR and light-to-frequency converter), both mounted at 90° from the light source. Differently from the light-dependent resistor (LDR) that is a analog sensor that varies the internal resistance depending on the intensity of the light, the light-to-frequency converter works as a digital sensor capable to convert the light intensity that reaches the photodiodes in a continuous digital square wave, in which the light intensity is proportional to the wavelength.
Differently from both probes, Abu Dhabi works through a similar principle of light-to-frequency converter mounted in Bangkok probe, the only difference is that it has 64 photodiodes with four different color filters (red, green, blue, and clean), which are responsible to identify the components of the RGB color space.
Firstly, our investigation has been conducted in the lab, we only inject standard turbidity solution into the probe interior. The standard turbidity solution was created similar as described before, based on a dillution of water and skimmed milk. In this next step we use a different composition of standard turbidity solution, varying the color and water composition, to check if the sensor is capable to identify other potential threats on water that may influence the water quality of aquatic systems.