Campbell OBS-3+, Instruction Manual

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OBS-3+ and OBS300 Suspended Solids and Turbidity Monitors
8. Perform 2nd-order polynomial regressions on the calibration data to get
the coefficients for converting OBS signals to NTU values.
FIGURE 8-3. OBS-3+ (left) and OBS300 (right) in 500-NTU AMCO
Clear ® turbidity standard in 100-mm black polyethylene calibration
cup
8.2 Sediment
There are three basic ways to calibrate an OBS sensor with sediment. These
are described in the following sections, however, only the procedures for dry-
sediment are explained in this manual.
8.2.1 Dry-sediment Calibration
Dry-sediment calibration is a calibration performed with sediment that has
been dried, crushed, and turned to power. This is the easiest calibration to do
because the amount of sediment can be determined accurately with an
electronic balance and the volume of water in which it is suspended can be
accurately measured with volumetric glassware. Of the three methods, dry-
sediment calibration causes the greatest physical and chemical alteration of the
sediment. Alteration of the sediment size as a result of processing can
significantly affect the calibration slope. FIGURE 11-1 shows that reducing
the grain size by a factor of two during grinding can increase OBS sensitivity
by a factor of two (see definition of sensitivity).
8.2.2 Wet-sediment Calibration
Wet-sediment calibration is performed with sediment obtained from water
samples or from the bed of a river that has not been dried and pulverized.
Consolidation and biochemical changes during storage and processing cause
some alteration of wet sediment during storage, and for this reason, sediment
and water samples should be stored at about 4
o
C prior to use. The wet
sediment is introduced into the sediment suspender as it comes from the field.
This kind of calibration requires that water samples be withdrawn from the
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