In this paper, three common measuring methods for the principles of water turbidity, namely, scattering, transmission and the ratio method are introduced. A method combined with modulation and demodulation techniques based on pulse width modulation (PWM) pulse for turbidity measurement using light intensity modulation is proposed. The experimental system that is, the realizing circuit of modulation and demodulation is described in detail. The experimental system was used to measure Formazine Standard Solution turbidity of different values. The experimental results are shown, and then turbidity values are calculated using the three methods. Through the analysis and comparison of the experimental results, it is shown that when using the ratio method combined with the modulation technique, the interference of background light can be eliminated effectively, so the measuring accuracy is significantly improved.

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