Study on near infrared reflectance sensor for high sediment concentration measurement

Abstract: The sediment concentration measurement is one important part in monitoring soil and water loss. The research on development of sediment measurement sensor is of a great significance for real-time measurement，especially for high sediment concentration. In order to achieve the measurement of high sediment concentration, the paper analyzed the optical reflection and absorption features of sediment-laden water to lights of different wave lengths, and developed the related measurement device based on a near-infrared sensor with 1 800 nm (strong water reflection wave length) and 1 940 nm (strong water absorption wave length). Laboratory experiments were conducted to calibrate the sensor with three different sediment types collected from Yellow River, Weihe River and Tianshui, and the relationship of sediment concentration with sensor's output was worked out. The results show that all the reflection intensity of two wave lengths near-infrared light increases with the sediment concentration. The sensor outputs are strongly and linearly correlated with sediment concentration when the sediment concentration is less than 300 kg/m3, and the coefficients of determination of the built linear model are greater than 0.9. In order to verify the accuracy of the model, the paper compares the measured sediment concentration with the model computed sediment concentration. The results show that both the regression coefficient close to 1, the relative error is less than 10%. It indicates that the sediment concentration which is computed by the linear model can be used as the measured values. The sensor is capable of measuring sediment concentrations from 1 to 300 kg/m3. When sediment concentration is greater than 10 kg/m3, the measurement accuracy is greater than 90%. The measurement accuracy of near-infrared sensor is also affected by other factors in stream flow，such as sediment types，dissolved NIR-absorbing matter and bubbles. The design of the sensor can meet the measured requirements of high sediment concentration in severe soil erosion area and has good adaptability. The near-infrared reflection sensor can be applied in real-time monitoring system and will be beneficial for the assessment of soil and water loss.