Campbell Scientific OBS-3+ and OBS300 Suspended Solids and Turbidity Monitors - Manual (Page 29)

OBS-3+ and OBS300 Suspended Solids and Turbidity Monitors
suspender after each addition of sediment for the determination of SSC by
filtration and gravimetric analyses.
8.2.3 In situ Calibration
In situ calibration is performed with water samples taken from the immediate
vicinity of an OBS sensor in the field over sufficient time to sample the full
range of SSC values to which a sensor will be exposed. SSC values obtained
for these samples with concurrent recorded OBS sensor signals and regression
analysis establishes the mathematical relation for future SSC conversions by an
instrument. This is the best sediment-calibration method because the particles
are not altered from their natural form in the river (see Lewis, 1996). It is also
the most tedious, expensive, and time-consuming method. It can take several
years of water sampling with concurrent OBS measurements to record the full
range of SSC values on a large river.
8.2.3.1 Materials and Equipment
• OBS sensor with test cable
• Dry, disaggregated sediment from the location where the OBS sensor will
be used (sediment should be in a state where grinding, sieving, or
pulverization does not change its particle-size distribution)
• Averaging DMM or datalogger with test leads, 12 V gel cell
• Sediment suspender (if a suspender is not available, use a 200-mm I.D.
dark plastic container and a drill motor with paint-mixing propeller)
• Electronic balance calibrated with ten-mg accuracy
• 20-ml weigh boats
• Large black polyethylene plastic tub for measuring the clear-water points
• 1-liter, class-A, volumetric flask
• Tea cup with round bottom
• Teaspoon
8.2.3.2 Setup
1. Check the balance with calibration weights; recalibrate if necessary.
2. Plug the test cable to the OBS sensor; connect the red and black leads to
the battery and clip the DMM or datalogger test leads across the blue (+)
and green (–) leads.
3. Add three liters of tap water to the suspender tub with the volumetric flash.
4. After measuring the clear-water signal (Step 1, Section 8.1.3, Procedure),
mount the sensor in the suspender tub. The sensor placement depends on
whether the sensor is an OBS-3+ or an OBS300. For the OBS-3+, mount
the sensor so that its end is 50 mm above the bottom of the suspender tub
and then secure it in a position that minimizes reflections from the wall;
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