A wide range turbidity measurement system based on the light scattering method and light transmission method was developed to measure the turbidity of liquids. In order to solve the problem of accuracy attenuation of the light scattering method in the case of measuring high turbidity and the light transmission method in the case of measuring low turbidity, and meet the requirements of large-scale measurement in one system, it is proposed to use the light scattering method to measure low turbidity and the Beer-Lambert law of light transmission method to measure high turbidity methods. Use an avalanche photodiode (APD) to detect the intensity of scattered light or transmitted light, and convert the intensity of light into the number of photons based on single-photon counting technology. In order to obtain a reasonable measurement range of the light scattering method and the light transmission method in this system, a series of experiments were carried out. Under a specific incident light intensity, the experimental results show that the reasonable detection range of the light scattering method is 1 NTU-324 NTU, within this range, the system exhibits good stability. The reasonable detection range of the light transmission method is 325NTU-815NTU, within this range, the system still shows good stability. The widerange turbidity measurement system in this article has broad applicability and can be used to monitor urban drinking water, factory sewage discharge, and other industries.
A high-resolution fiber turbidity sensor has been developed to measure the wide dynamic range of turbidity. To improve the accuracy of measurements, the high sensitive single-photon detection technique is adopted in the sensor by using an avalanche photodiode (APD) to detect the total intensity of transmitted light and quantize it to the corresponding number of photons. A theoretical model for turbidity measurement is proposed by combining the B-L transmission law with the single photon counting theory. In order to cover a wide dynamic range measurement based on only one measurement system, an optimal optical power for the sensor must be chosen. Therefore, an experiment is designed to measures the different turbidity solutions by gradually changing the power of incident light. The test results show that, with the increasement of light intensity, the measured data fit better with the theoretical one when the measured turbidity is lower than 0.1NTU. However, the light intensity cannot be increased without limit in high turbidity region, especially when the turbidity is high than 1NTU, since there is a corresponding upper limit for the detection of avalanche photodiode (APD). After the processing and analysis of test data, the test results indicated the sensor can obtain the best measurement accuracy when the optical power of incident light at 11mW. The measured turbidity range is as wide as 50dB from 0.01 to 1000NTU. The proposed fiber sensor is robust and can be used for the wide dynamic turbidity detection for drinking water or some remote monitoring of water pollution.
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