Campbell Scientific OBS-3+ and OBS300 Suspended Solids and Turbidity Monitors - Manual (Page 37)

OBS-3+ and OBS300 Suspended Solids and Turbidity Monitors
FIGURE 11-5. Infrared reflectivity of minerals as a function of
10-Munzell value
11.6 Water Color
Some OBS users have been concerned that color from dissolved substances in
water samples, not colored particles discussed in Section 11.5, IR Reflectivity—
Sediment Color, produces erroneously low turbidity measurements. Although
organic and inorganic IR-absorbing dissolved matter has visible color, its effect
on OBS measurements is small unless the colored compounds are strongly
absorbing at the OBS wavelength (850 nm) and are present in high
concentrations. Only effluents from mine-tailings produce enough color to
absorb measurable IR. In river, estuary, and ocean environments
concentrations of colored materials are too low by at least a factor of ten to
produce significant errors.
11.7 Bubbles and Plankton
Although bubbles efficiently scatter light, monitoring in most natural
environments shows that OBS signals are not strongly affected by bubbles.
Bubbles and quartz particles backscatter nearly the same amount of light to
within a factor of approximately four, but most of the time bubble
concentrations are at least two orders of magnitude less than sand
concentrations. This means that sand will produce much more backscatter than
bubbles in most situations and bubble interference will not be significant. Prop
wash from ships and small, clear-mountain streams where aeration produces
high bubble concentrations are an exception to this generality and can produce
erroneous turbidity values resulting from bubbles.
OBS sensors detect IR backscattered between 90 o and 165 o where the
scattering intensities are nearly constant with the scattering angle. Particle
concentration has the most significant effect in this region. OBS sensors are
more sensitive, by factors of four to six, to mineral particles than particulate
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