Campbell Scientific OBS500 Smart Turbidity Meter with ClearSensor Technology - Manual (Page 47)

OBS500 Smart Turbidity Meter with ClearSensor™ Technology
FIGURE 8-5. Infrared reflectivity of minerals as a function of
10-Munzell Value
8.6 Water Color
Some OBS users have been concerned that color from dissolved substances in
water samples, not colored particles as discussed in Section 8.5, IR
Reflectivity—Sediment Color, produces erroneously low turbidity
measurements. Although organic and inorganic IR-absorbing, dissolved matter
has visible color, its effect on turbidity measurements is small unless the
colored compounds are strongly absorbing at the sensor 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.
8.7 Bubbles and Plankton
Although bubbles efficiently scatter light, monitoring in most natural
environments shows that OBS signals are not strongly affected by bubbles.
The side scatter measurement may be more affected. 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
exceptions to this generality and can produce erroneous turbidity values
resulting from bubbles.
OBS sensors detect IR backscattered between 90° and 165° where the
scattering intensities are nearly constant with the scattering angle. Particle
concentration has the most significant effect in this region. OBS sensors are
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