Solids concentrations in liquids and gases are determined with standardized methods based on filtration and centrifuging, which require separation, drying, and weighing of the solids fraction. These laboratory methods are cumbersome because they can take hours to perform and are practically impossible to automate.
By contrast, the turbidity caused by the solid particles, an optical phenomenon, is well-suited to continuous measurement. The turbidity value is determined by measuring the scattered light, either as attenuation of the transmitted light (scatter coefficient) or as scattered light intensity at a specific lateral angle.
A clear correlation exists between the turbidity value and the solids concentration of a suspension if the particles' properties, i.e. their refractive index and especially their particle size, are constant. The standard suspension of formazine used for calibration provides a good example of unchanging properties.
Another example is shown in the following Figure: the correlation between the turbidity value in FTU and suspensions of titanium oxide and iron oxide (rust).
But even less homogeneous media, such as surface water and groundwater, exhibit a very dependable correlation of solids content with the scattered light. The reproducibility of results is enhanced by the fact that many common separation processes (such as filtration, sedimentation, centrifuging, gas scrubbing, etc.) tend to narrow down the particle size distribution.
In the treatment of exhaust gases, for example, scattered light measurement following electrostatic filters and gas scrubbers yields highly reproducible values for measuring the dust concentration.
In any case, even where compositions fluctuate sharply as in the case of certain industrial wastewaters, continuous turbidity measurement has proven to be a reliable tool for signaling concentration jumps.