Campbell OBS-3+, Instruction Manual

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OBS-3+ and OBS300 Suspended Solids and Turbidity Monitors
FIGURE 11-2. The apparent change in turbidity resulting from
disaggregation methods
11.2 Suspensions with Mud and Sand
As mentioned in Section 11.1, Particle Size , backscattering 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. 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 is expected to
change with time at a monitoring site.
11.3 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 an OBS
sensor. As the graph in FIGURE 11-3 shows, plate-shaped particle (clay-
mineral particles for example), backscatter light about ten times more
efficiently than spherical particles and angular shapes have intermediate
scattering efficiency. OBS sensors are very sensitive to shape affects and this
makes it very important to calibrate with material from the monitoring site. It
is also essential that particle shape remain constant during the monitoring
period.
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