vertical chamber, maintaining a homogeneous concentration of sand within the
vertical chamber. The 19 sensors on a vertical array are calibrated
simultaneously, and each sensor is sampled at a frequency of 16 Hz for a
duration of 120 sec.
The first data point is obtained with no sand in the water. A second data
point is obtained after an accurately measured mass of sand is added to the
mixing chamber to obtain a sediment concentration of 10 g/liter. This process is
repeated for sediment concentrations ranging from 0 to 80 g/liter, in increments
of 10 g/liter. The FOBS were calibrated using the same sand that was used to
construct the moveable-bed beach in the LSTF.
Figure 47 shows the calibration results for Sensor No. 1 (i.e., the lowest
sensor) on Vertical Array No. 1. The manufacturer of the FOBS has found that
for clean beach sand, the magnitude of the output signal from the sensors
(measured in "counts") is a nearly linear analog of suspended sediment
concentration, over the range from 0 to 100 g/liter. However, it was found that
for the sand used in the LSTF (D50 = 0.15 mm, with a narrow particle size
distribution), a third-order polynomial is needed to match the low and high range
of the calibration data. In addition, the scale used to represent the magnitude of
the output signal is quite large (i.e., counts ranging from approximately 2,000 to
4,000), and more significant digits are needed to define the coefficients of the
polynomial with sufficient accuracy. Based on the results from a sensitivity
analysis, seven significant digits are used for each coefficient in the equation,
although only five are shown in the figure for clarity.
100
y = 1.68931E-08x3 - 1.41899E-04x2 + 4.24464E-01x - 4.25622E+02
90
R2 = 9.95005E-01
80
70
60
50
40
30
Sensor 1
20
Linear (Sensor 1)
y = 0.0418x - 94.617
10
R2 = 0.9665
Poly. (Sensor 1)
0
0
1000
2000
3000
4000
5000
Counts (-)
Figure 47. Calibration results for FOBS No. 1 on Vertical Array No. 1
85
Chapter 8
Sensor Calibration, Measurement, and Data Analysis
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