Campbell Scientific OBS500 Smart Turbidity Meter with ClearSensor Technology - Manual (Page 37)

OBS500 Smart Turbidity Meter with ClearSensor™ Technology
7.5.2.2 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, and for this reason, sediment and water
samples should be stored at about 4°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calibration requires that water samples be withdrawn from the suspender after
each addition of sediment for the determination of SSC (suspended sediment
concentration) by filtration and gravimetric analysis.
7.5.2.3 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 OBS500 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.
7.5.2.4 Performing a Dry-Sediment Calibration
Materials and equipment: OBS500 with test cable; dry, disaggregated
sediment from the location where the OBS500 will be used (sediment should
be in a state where grinding, sieving, or pulverization does not change its
particle-size distribution); datalogger with 12 V power supply; 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 10 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; and teaspoon.
1. Check the balance with calibration weights; recalibrate if necessary.
2. Connect the OBS500 to a computer or datalogger so that the measured
values can be observed.
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 7.5.1, Turbidity),
mount the OBS500 so that the sensor end is 50 mm above the bottom of
the suspender tub and secure it in the position that minimizes reflections
from the wall; see FIGURE 7-8.
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