Campbell Scientific OBS-3A Turbidity and Temperature Monitoring System - Manual (Page 65)

OBS-3A Turbidity and Temperature Monitoring System
10.2 Suspensions with Mud and Sand
As mentioned earlier, backscattering from particles is inversely related to
particle size on a mass concentration basis (see FIGURE 10-2). 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. An OBS
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. The obvious thing to do is to take a lot of water samples and
analyze them in the laboratory. This is not always practical during storms
when the errors are likely to be largest. Do not rely solely on OBS sensors to
monitor suspended sediments when particle size or composition are expected to
change with time at a monitoring site.
0 50 100 150 200 250
Grain Diameter (microns)
0.2
0.4
0.6
0.8
1.0
Gain (V/g/l)
Gain = 8.3(D)
-0.6
FIGURE 10-2. Effects of particle size
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