Campbell OBS-3+, Instruction Manual

Page: 33 / 54

OBS-3+ and OBS300 Suspended Solids and Turbidity Monitors
11. 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 over
looked.
11.1 Particle Size
The size of suspended sediment particles typically ranges from 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. Results of tests
with sediment shown on FIGURE 11-1 indicate a wide range of OBS
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 in a survey area during monitoring will
produce shifts in apparent NTU and SSC values that are unrelated to real
changes in sediment concentration. FIGURE 11-2 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 11-1. Normalized OBS sensitivity as a function of grain
diameter
23