Appendix D. SDI-12 Sensor Support
E-5
FIGURE E-5. Infrared reflectivity of minerals as a function of
10-Munzell Value
E.6 Water Color
Some OBS users have been concerned that color from dissolved substances in
water samples, not colored particles as discussed in Appendix
, 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.
E.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
sidescatter 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
