Instrument for Real-Time Measurement of Low Turbidity by Using Time-Correlated Single Photon Counting Technique

A real-time turbidimeter based on time-correlated single photon counting (TCSPC) was developed to measure the low-level turbidity for drinking water. To improve measurement accuracy, we use a single-photon avalanche diode (SPAD) with high sensitivity to accurately detect the intensity of weak scattering light. A novel statistics principle-based TCSPC technique was applied in this system to reduce the fluctuation of measurement and improve the stability of turbidity measurement. Thanks to the SPAD with short response time and the digital output of single-photon detecting module, the real-time and steady measurement of low turbidity is finally implemented. Experimental tests for the turbidimeter´s performance were described and the results showed that 0.1 Nephelometric Turbidity Units (NTU) can be measured stably in the range of 0-400 NTU within 1 s. On the basis of the theoretical analysis, a turbidity measurement model was proposed. It was found that a tradeoff between the high measurement resolution and wide linearity range should be considered adequately depending on the practical applications. By adjusting the system parameters, we demonstrated that the linear range of measurement could be expanded in the regime of low turbidity, while maintaining high resolution of this system. The proposed turbidimeter has advantages of high resolution, wide linear range, and short response time, which is sufficient for many applications, including the real-time online turbidity or particle concentration monitoring.