How Is Turbidity Measured - LaMotte 2020 Instruction Manual

HOW IS TURBIDITY MEASURED?

Light passing through clear water will travel in a straight line. Particles in turbid
water will cause the light to scatter giving it a "cloudy" appearance. The turbidity
of a sample is determined by measuring the amount of scatter when a light is
passed through a sample. The higher the turbidity, the greater the amount of
scatter.
Turbidity can be measured in many ways. Visual methods include, the
comparative methods, the Secchi disk method and the Jackson Candle method.
Comparative methods are used in shallow water and determine turbidity by
matching the turbidity of a water sample to a standard of known turbidity either
with a "target" at the bottom of a tube or with a turbidity comparator. In the
deeper waters of lakes, ponds, rivers and estuaries the Secchi disk is often used to
measure turbidity. The Secchi disk is a disk about eight inches in diameter that is
either white or is marked with black and white quadrants. The disk is lowered
into the water on a calibrated line and the depth is noted where the disk just
disappears from sight. The disk is then raised until it is visible. The average of
these two distances is known as the "Secchi depth".
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