Campbell Scientific OBS500 Smart Turbidity Meter with ClearSensor Technology - Manual (Page 43)

OBS500 Smart Turbidity Meter with ClearSensor™ Technology
8. Factors that Affect Turbidity and Suspended-
Sediment Measurements
This section summarizes some of the factors that affect OBS 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 OBS was factory-calibrated with sediment from your survey site,
you only need to skim this section to confirm that no problems have been
overlooked.
8.1 Particle Size
The size of suspended sediment particles typically ranges from about 0.2 to
500 μm in surface water (streams, estuaries, and the 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.
Results of tests with sediment shown in FIGURE 8-1 indicate a wide range of
sensitivity is associated with fine mud and coarse sand (about two orders of
magnitude). The significance of these results is that size variations between
the field and laboratory and within a survey area during monitoring will
produce shifts in apparent TU and SSC values that are unrelated to real
changes in sediment concentration. FIGURE 8-2 shows the difference in
apparent turbidity that can result from different ways of disaggregating
sediment.
10.0 100.0 1000.0
Median Particle Size (D
50
)
0.01
0.10
1.00
O
B
S
Se
ns
iti
vt
y,
S
(m
V
pe
r m
g
l -1
)
S ~ 1/D50
FIGURE 8-1. Normalized sensitivity as a function of grain diameter
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