Appendix B. Factors that affect
turbidity and suspended-
sediment measurements
This appendix summarizes some of the factors that affect turbidity measurements and shows
how ignoring them can lead to erroneous data. If you are certain that the characteristics of
suspended matter will not change during your survey and that your sensor was factory-
calibrated with sediment from your survey site, you only need to skim this section to confirm that
no problems have been overlooked.
B.1 Particle size
The size of suspended-sediment particles typically ranges from about 0.2 to 500 μm in surface
water (streams, estuaries, and ocean). With size, shape, and color remaining constant, particle
area normal to a light beam will determine the intensity of light scattered by a volume of
suspended matter.
B.2 Suspensions with mud and sand
Light scattering from particles is inversely related to particle size on a mass concentration basis.
This can lead to serious difficulties in flow regimes where particle size varies with time. For
example, when sandy mud goes through a cycle of suspension and deposition during a storm,
the ratio of sand to mud in suspension will change. A turbidity sensor calibrated for a fixed ratio
of sand to mud will, therefore, indicate the correct concentration only part of the time. There are
no simple remedies for this problem. One solution is to take many water samples and analyze
them in the laboratory; however, this is not always practical during storms when the errors are
likely to be largest. Do not rely solely on turbidity sensors to monitor suspended-sediment
particles when particle size or composition is expected to change with time at a monitoring site.
B.3 Particle-shape effects
In addition to size and flocculation/aggregation, particle shape has a significant effect on the
scattering intensity from a sample and calibration slope of a turbidity sensor. Plate-shaped
particle (clay-mineral particles, for example) backscatter light about ten times more efficiently
ClariVUE™10 Side-Scatter Turbidity Sensor
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