Campbell Scientific OBS-3+ and OBS300 Suspended Solids and Turbidity Monitors - Manual (Page 35)

OBS-3+ and OBS300 Suspended Solids and Turbidity Monitors
0 20 40 60 80 100 120 140 160 180
Scattering Angle
0.01
0.1
1
R
el
at
iv
e
Sc
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g
In
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Cubes
Plates
Spheres
OBS-3+
FIGURE 11-3. Relative scattering intensities of grain shapes
11.4 High Sediment Concentrations
At high sediment concentrations, particularly in suspensions of clay and silt,
the infrared radiation from the emitter can be so strongly attenuated along the
path connecting the emitter, the particle, and the detector, that backscatter
decreases exponentially with increasing sediment concentration. For mud, this
occurs at concentrations greater than about 5000 mg L –1 . FIGURE 11-4 shows
a calibration in which sediment concentrations exceeding 6,000 mg L –1 cause
the output signal to decrease. It is recommended not to exceed the specified
turbidity or suspended sediment ranges, otherwise the interpretation of the
signal can be ambiguous. For example, a signal level of 2000 mV (FIGURE
11-4) could be interpreted to indicate SSC values of either 3,000 or 33,000
mg L –1 . Factory calibrations are performed in the linear region designated ‘A’
on the graph.
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