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OBS-5+ System
10. Interfering Factors
10.1 Particle Size
34
pn 4576 Alkaline C-Cells Batteries
pn 20806 OBS-5+ Test Cable, 2 m (6.5 ft)
pn 21381 7-piece Allen Wrench Set, 5/64 to 3/16 Ball End
pn 21139 SS Hex Socket Screw, #2-56 x .187
Changes in sediment concentration (SSC) are the primary cause for OBS-5+
output fluctuations in the environment. In some monitoring areas, however,
factors other than SSC, will cause the OBS-5+ to indicate SSC variation that
are invalid and which the user does not wish to measure. These factors are
called interferences because they cause apparent shifts in SSC that are not real.
Interferences include particle size, shape, reflectivity, flocculation, and
disaggregation. This section summarizes some of the important ones that you
might encounter while using an OBS-5+.
Sensitivity is the change in light scattering intensity, indicated by ND and FD
counts, per unit change in SSC or an interfering factor. It is therefore a good
measure of relative interference. An interference that reduces sensitivity will
cause SSC to appear to decrease and one that makes an OBS-5+ more sensitive
will cause the opposite effect. Interfering factors are ranked by the ratio of the
sensitivity change that is caused when the factor changed over its full range in
the environment. For example, the size of suspended sediment particles in the
environment ranges from about 0.5 to 125 μm. This range causes relative OBS
sensitivity to change from 2 to 0.008, (see next section), giving a factor of 250.
Interfering factors ranked in this way are summarized in TABLE 10-1. The
ranking shows, for instance, that the size of suspended particles can affect
OBS-5+ measurements more than particle shape or NIR reflectivity by a factor
of 25. Interferences can be tolerated so long as the resulting errors fall within
acceptable limits.
TABLE 10-1. Relative magnitude of the effects of sediment
characteristics on OBS-5+ sensitivity
Size
Relative Magnitude
<250
The trend of the line on FIGURE 10-1 shows that relative sensitivity declines
at a rate inversely proportional to the particle diameter. The graph provides a
useful method for estimating the relative effect of grain size on OBS-5+
sensitivity. Using this method, for example, one gram of silt with a grain size
of 10 microns, suspended in a liter of water, might produce an OBS signal of
55,000 counts, whereas a gram of sand per liter with a grain size of 100
microns would produce only 5,500 counts, with other factors such as shape and
reflectivity being the same.
Interfering Factor
Shape
Reflectivity
Flocculation
10
10
2
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