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Turbidity Overview:
What is Turbidity and how is it measured?

Turbidity Levels in Beakers
Introduction to Turbidity
Turbidity is an optical characteristic or property of a liquid, which in general terms describes the clarity, or haziness of the liquid. Turbidity has always been based on human observation and while this phenomenon is quantifiable by many different means, much discussion still exits around the various techniques used to measure turbidities of fluids.

Turbidity is not color related, but relates rather to the loss of transparency due to the effect of suspended particulate, colloidal material, or both. A lack of turbidity results in clarity or clearness because it is, in part, the effect of these various suspended materials on light passing through a liquid.

A body of water, such as a lake, is a natural example of turbidity. All of us have seen lakes that are very clear to the eye, and are sometimes fascinated by the depth to which one can see. On the other hand you find 'murky' waters where you couldn't see your hand at arm's length. Detectable depth of a visual target, called a Secchi disk, is a measurement technique still used today for lake and river water turbidity clarity assessments. Secchi disk being lowered into a lake.
Secchi disk being lowered into a lake.

Classified as an "optical property," direction of illumination, stray light, background, and optical path length, can have an affect the turbidity measured, but not color itself. Some turbidity constituents are temperature sensitive, and may only precipitate out of solution when chilled, or may dissipate when heated. Other protein-based components may precipitate as a compound reacting with other dissolved material. While changing color will change the observed effect of the liquid on light passing through it, the actual turbidity will not change due to color change alone.


What is Process Turbidity?
Process turbidity is an assessment of a product's clarity being made or processed in a continuous or a batch production system. Typically measured against a baseline quality control parameter, or as a measurement of process performance.

Turbidity Meter TF16-N
optek in-line turbidity meter, TF16-N
Often times, samples of the process liquid are taken and the actual turbidity assessment is performed visually by lab personnel or with a multitude of bench-top style turbidity meters. Sampling allows other influences such as time, settling, and temperature to affect the sample reading. When sampling the product from a pipe to measure turbidity, it is no longer a "process" measurement. This being said, process turbidity is not a laboratory, bench-top, or grab sample measurement.

In recent years, on-line and inline turbidity meters that take an "optical" measurement off of, or inside of, a pipe carrying the product have been used more often as these process systems became more automated.

Electronic turbidity measurements are being made as the product is being made or refined in the process. The advantage of on-line or inline measurement is that the measurement results occur continuously, in "real-time", and provide a more representative assessment of the actual process conditions. Turbidity Meter TF16-N installed inlineA optek TF16 turbidity meter installed in the centrate pipe monitors clarification performance of the centrifuge and initiates de-sludging.

Immediate action can now be taken when process conditions change, or are out of spec. Process turbidity measurements can also be made in a "bypass" line or slipstream. Slipstreams have been a common practice due to line size limitations or the maintenance requirements of certain instruments. While this is considered a process turbidity measurement, the use of a bypass line is not usually desirable as variations between the process line and bypass line can change particle concentration and size distribution.

Additionally, temperature decreases due to the bypass may have a profound effect on detectable haze levels. In sanitary applications, in which process instruments clean-in-place, or CIP, bypass lines or slipstreams present an additional cast of cleaning challenges and should be avoided.

The performance of process turbidity meters are often checked, or validated, by using laboratory measurement methods. It is important to note that these correlation studies must be thoroughly evaluated for consistency of technique, temperature, time, color, sample preparation and other possible procedural errors.

Differences in optical design, wavelengths of light used, angle of measurement, and reference techniques used (or not used), between various turbidity instruments, will inevitably impact the accuracy and repeatability of the individual results. This has historically been, and continues to be, a challenge. If possible, the same measurement angles, methods, and techniques should be used in both the process as well as the lab assessments, to minimize any deviation, and create viable correlations.


More on Process Turbidity Analyzers