In its simplest terms, turbidity is the optical measurement of scattered light resulting from the interaction of incident light with particulate material in a liquid sample. Typically, the liquid is a water sample and the suspended material causing the light to be scattered can be composed of a broad variety of components.
Examples of particles include suspended solids such as silt, clay, algae, organic matter, various microorganisms, colloidal material, and even large molecules that are dissolved in the sample such as tannins and lignins.
Particulate matter in a water sample will cause the incident light beam to be scattered in directions other than a straight line through the sample. The scattered light that returns to the detector causes a response correlating to the level of turbidity in the sample. A higher level of scattered light reaching the detector results in a higher turbidity value.
The measurement of turbidity is not directly related to a specific number of particles or to particle shape. As a result, turbidity has historically been seen as a qualitative measurement. In an attempt to make turbidity methods more quantitative, standards and standardization methods can be used.
Although interferences have a dramatic and ever-present impact on turbidity measurements, the type and magnitude of the interference often depends on the turbidity level being measured. When performing low-level turbidity measurements, primary interferences are stray light, bubbles, ambient light, and contamination. For high turbidity testing, a greater impact from color, particle absorption, and particle density is seen.
In an attempt to minimize interferences, several new turbidity measurement methods have been developed. Many of these methods have been designed to maximize sensitivity and minimize the effects of interferences. It is important to understand and identify the prominent interferences in the sample stream. Doing so can help identify the instrument design that will provide the most accurate and “interference-free” measurement.
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