Handbook: Optical Measurement of In-Process Fluids

In 1974, the National Oceanic and Atmospheric Administration recommended, after due deliberation, to call turbidity a "non-technical appearance descriptor." While turbidity certainly has become a common description, relied on in many industries, it still is accurately defined as an "appearance descriptor."

What this means is that turbidity is a human phenomenon. It is the "reduction" of transparency due to the effect of particles. Clarity or clearness is the lack of turbidity. Turbidity is, only in part, the effect of suspended particles on light passing through a liquid.


Turbidity is not just an optical measurement of percent solids. It is an "optical property" of the liquid. Turbidity is not color, but it is affected by intensity and hue of color, direction of illumination, background and path length. If you change the color of a liquid by dissolving a dye into the liquid, you will change the observed effect of the liquid on light passing through it, but you will not change the turbidity, were turbidity considered only particles, but its optical properties would certainly change.

Note, however, that light absorption or scatter signals will often follow the concentration levels of particles in the liquid. This relationship is often used to analyze concentrated levels from near zero to 60% concentration (by weight or volume).

Turbidity is also not empirical. You cannot run a liquid through filter paper, or evaporate it by some other means and measure the turbidities that remain (e.g. color aspect). Turbidity is not, therefore, absolute. If you want to change or maintain the turbidity properties of a liquid from day to day, then you must first agree upon how you'll measure, and use a similar measurement technique each time.


The use of the term process to modify turbidity is generally taken to mean that the visual measurement of a pipe carrying product is being made "full time" or continuously.. Process implies here that there is a product being made in either a continuous or batch method. Turbidity measurements are made on this product "on-line" or "in-line-across the pipe" as the product is made.

If you take a sample of the product from a pipe to measure the turbidity, it is no longer a "process" measurement. Process turbidity measurement, therefore, is not "laboratory," "table top," or "grab sample" measurement. We have noted significant variations between the actual process optical measurement and the subsequent laboratory results which are often attributed to the laboratory sampling and handling procedures.

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