Campbell Scientific OBS-3A Turbidity and Temperature Monitoring System - Manual (Page 12)

OBS-3A Turbidity and Temperature Monitoring System
Light transmission in water is attenuated by scattering (deflection by water
molecules, and suspended matter) and absorption, which converts light to heat.
Attenuation, absorption, and scattering are inherent properties of water that are
affected only by impurities such as color and suspended organic matter.
Optically pure water is not readily available; however deionized water that has
passed through a 0.2 µm filter is adequate for most practical purposes.
There are dozens of turbidimeter designs, however most are configured in one
of the ways shown in FIGURE 4-2. These include: forward-scatterance, 90°
scatterance, and backscatterance nephelometers. Some instruments combine
two or more of these configurations and blend signals to produce a useful
output. The transmissometer measures attenuation, an inherent optical
property, but is not approved for turbidity measurements except by ISO 7027.
OBS sensors have superior linearity in turbid water but a transmissometer is
more sensitive at low concentrations (<~25 mg/L). Data from turbidimeters
made by different companies should be compared cautiously. Inconsistencies
between instruments results from variations in light sources, detectors, optical
configurations, and turbidity standards.
Can turbidity be converted to suspended solids concentrations and vise-
versa?
In most situations, conversions between turbidity and suspended solids
concentrations will give misleading results because the conversion equates to
an apparent optical property, in relative units, with one precisely defined in
terms of mass and volume; these are “apples and oranges”.
Conversion of turbidity to suspended solids concentration is recommended
only when:
• Measurements are made with the same turbidimeter.
• The turbidimeter is intercalibrated with a turbidity standard and suspended
matter from the waters to be monitored.
• Particle size and composition do not change over the monitoring period.
Compliance with the last condition is crucial but virtually impossible to verify
in the field because it is difficult to sample particles in their natural state and
preserve them for laboratory analysis in a consistent and meaningful way.
5. Specifications
Features:
• Measures turbidity with patented, field-proven OBS technology,
• Runs up to 8,000 hours on three D-cell batteries,
• Monitors sediment concentrations up to 5,000 mg/L and turbidity up
to 4,000 NTUs,
• Logs depth, wave height, wave period, temperature, and salinity.
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