Monitoring instrument of the water Chlorophyll-a concentration with laser-induced fluorescent

Water quality monitoring is of great significance to maintaining human health, environmental protection and sustainable economic development. Much attention has also been drawn to water quality safety in recent years. Among them, the concentration of chlorophyll-a is one of the most important indicators of water quality in the fields of aquaculture, fishery management, and environmental protection. Multi-point deployment and continuous monitoring are then required to accurately assess the overall evolution of water quality in large-scale water areas. However, the current instruments of chlorophyll detection are limited for the implementation of large-scale water body monitoring, due to the high price and low degree of automation. In this study, a monitoring device was designed to detect the water chlorophyll-a concentration using laser-induced fluorescence technology. An ambient light correction and data processing were also proposed for the real-time monitoring of chlorophyll-a concentration. The laser light was emitted with a wavelength of 440 nm into the water body, in order to induce and excite chlorophyll-a for the fluorescence with a central wavelength of 680 nm. The concentration of chlorophyll-a was then monitored using the visible light sensor in the device, indicating the 680 nm fluorescence intensity excited by chlorophyll-a. There was a low overall manufacturing cost of the device. Low-power device design introduced the standby and serial port wake-up modes for online automatic monitoring. The two-way communication module was used to realize the data interaction. Data processing was evaluated to reduce the influence of the non-uniform distribution of chlorophyll-a in the water body on the measurement using statistical analysis. The mean value was calculated in time and space. A stable and accurate measurement was achieved in the instrument. An ambient light correction was proposed to reduce the impact of ambient light changes on the instrument measurement. A similar corrected value of the 680 nm light intensity was obtained in the water body chlorophyll-a with the same concentration under 6 groups of ambient light. Experiments show that this device effectively reduced the interference of ambient light on the measurement. The instrument stability test was carried out to observe the power supply circuit of the laser module and the 440 nm light intensity value, where the standard deviation coefficients were both 0.001 after 4 minutes, and the Pearson correlation coefficient was 0.84. It indicated that the laser output was controlled by the current intensity. A short time was obtained for the laser module power supply current to reach a stable state after precisely controlling the current, further reducing the overall power consumption of the instrument. 20 groups of water samples were configured with different chlorophyll-a concentrations using Chlorella chlorophyll in the instrument calibration test, referring to the range of chlorophyll-a concentration under different water bloom levels. The fitting model was established using the linear relationship between the fluorescence value at 680nm and the concentration of chlorophyll-a, where the coefficient of determination R² was 0.996. In the performance verification test, the green algae Chlorella and Microcystis aeruginosa were used to prepare 6 groups of water samples to be tested with different dominant algae species. The average measurement error of the instrument was within 5.48% under 2 groups of different light intensities. The test results show that the instrument performed stably to effectively detect the samples to be tested. There was no significant effect of the chlorophyll content in the water samples on the detection, due to the different types of algae, indicating the high detection accuracy. The finding can provide more convenient, cost-effective equipment support in the field of water quality monitoring, in order to realize all-weather monitoring chlorophyll-a concentration.