Campbell Scientific OBS-3A Turbidity and Temperature Monitoring System - Manual (Page 55)

OBS-3A Turbidity and Temperature Monitoring System
7.2.1 Equipment and Materials
• Dry, completely disaggregated bottom sediment or suspended matter from
the monitoring site,
• 1-gallon (4 L) brown Nalgene polypropylene bottle with top cut off,
• 1-liter, Class A volumetric flask,
• 2 gallons filtered distilled water (purified water from the super market
works fine),
• Hand-drill motor,
• Paint stirrer.
7.2.2 Sediment Preparation
Sediment preparation is a critical factor in calibration quality. It is most
convenient to use dry material because it can be accurately weighed with an
electronic balance. However, this only works well for clean sand because
disaggregation produces a sediment size different than existed in the field. For
example, deep harbors with weak currents often have cohesive (sticky) mud
with high organic-rich flocculation. Disaggregation of the flocs will reduce the
particle size and change the OBS response. Sediment or suspended solids
concentration is the dry weight of sediment divided by the weight of the
sample (expressed in ppm) or by the volume of sample in liters (expressed as
mg/L). Usually the disaggregated particles will be finer than untreated
sediment. When dried sediment is used, verify that field estimates are accurate
by comparing the OBS results with direct samples of suspended matter. (See
Section 11, References.)
FIGURE 7-1 shows how different methods of disaggregating sediment can
change the relationship between turbidity and the concentration of suspended
material. This occurs because vigorous disaggregation produces more small
particles than less vigorous methods as well as more OBS signal per unit of
mass concentration. The result is higher signal levels for a given
concentration.
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