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OBS-3+ and OBS300 Suspended Solids and Turbidity Monitors
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FIGURE 11-4. Response of an OBS sensor to a wide range of SSC
11.5 IR Reflectivity—Sediment Colo
u
r
Infrared reflectivity, indicated by sediment colo
u
r, has a major effect on OBS
sensitivity because with other factors remaining constant, it changes the
intensity of light scattering. Although OBS sensors are colo
u
r blind, tests have
shown that “whiteness”, colo
u
r, and IR reflectivity are correlated. Calcite,
which is highly reflective and white in colo
u
r, will produce a much stronger
OBS signal on a mass-concentration basis than magnetite, which is black and
IR absorbing. Sensitivity to colo
u
red silt particles varies from a low of about
one for dark sediment to a high of about ten for light gr
e
y sediment; see
11-5
. In areas where sediment colo
u
r is changing with time, a single
calibration
curve may not work. Resulting errors will depend on the relative
concentrations of colo
u
red sediments.