On-line turbidimeters are currently used for monitoring particle concentration in water and gas treatment processes. For small particle concentration, the intensity of scattered light is a linear function of the particle concentration, as long as a number of other parameters are kept constant: the refractive indexes of the particles and the surrounding medium, size, measuring angle and wavelength of the light. An international standard has been created in order to define the characteristics of the turbidimeters and the calibration suspension. The effects of the parameters are described and quantified. The special aspects of the measurement of very low turbidity values in water are treated: the zero value of water and the definition of the calibration conditions. A measuring method is presented which grants long-term stability without recalibration.

 The first industrial application for on-line turbidimeters was the monitoring of beer filtration in breweries, which started about 50 years ago. In the meantime, turbidimeters have been used in very diversified fields such as dust measurements in stacks, visibility in road tunnels and filtration control in the chemical industry. However, their main application today is the monitoring of drinking water treatment plants: control of the flocculation process by measuring the variations in the raw water (rivers, lakes, ground-water, sources) and the resulting properties, sand filtration survey and final quality control.

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