Campbell Scientific OBS-3+ and OBS300 Suspended Solids and Turbidity Monitors - Manual (Page 30)

OBS-3+ and OBS300 Suspended Solids and Turbidity Monitors
see FIGURE 8-4. For the OBS300, mount the sensor so that the laser
diode is submerged just below the water surface to maximize the distance
from the detector and the bottom of the container.
FIGURE 8-4. Portable sediment suspender (left) and OBS beam
orientation in suspender tub (right)
SSC = Wt
s
[V
w
+ Wt
s
/ρ
s
] –1 ; where: Wt
s
= total sediment weight in tub, in mg;
V
w
= volume of water in liters; ρ = density of water (ρ = 1.0 kg l –1 at 10 o C);
and ρ
s
= sediment density (assume 2.65 10 3 mg L –1 );
8.2.3.3 Procedure
1. Record and log the clean-water signal as in Step 1, Section 8.1.3,
Procedure; see FIGURE 8-2.
2. Move the OBS sensor to the suspender as described in setup.
3. Weigh 500 ± 10 mg of sediment in a weigh boat and transfer it to the
teacup. Record the weight on the calibration log sheet and add about 10 cc
of water from the suspender tub to the teacup and mix the water and
sediment into a smooth slurry with the teaspoon.
4. Add the sediment slurry to the tub and rinse the teacup and spoon with tub
water to get all the material into the suspender.
5. Turn the suspender ON and let it run for 10 minutes or until the OBS
signal stabilizes.
6. Take one-minute averages of the low- and high-range signals with the
DMM or datalogger and enter them on the calibration log sheet.
7. Calculate the sediment-weight increment as follows: Wi = 2500 mg
(RNG/Vx); where: W i = the incremental weight of sediment, RNG = the
range of the OBS sensor (2.5V, 5V, or 16 mA), and Vx = the average
output signal from step 6. Note: if the output is 4-20 mA, Vx will equal
the output minus 4 mA. The resulting weight gives the amount of
sediment to add to get five evenly spaced calibration points.
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