Figure 49. Calibration of beach profiler indicator is verified on benchmark
the lowest limit that the profile indicator can reach, and is lower than the lowest
elevation on either the fixed-bed or the moveable-bed beach. After the sensor is
lowered near the horizontal floor and the sensor is motionless, the toggle button
is pressed and the second output voltage is recorded. The operator manually
enters the physical elevation of the sensor. The vertical shaft of the profiler is
raised 0.1 m, and the third calibration point is recorded. This process is repeated
in increments of 0.1 m using the vertical scale to obtain the physical elevation at
each calibration step. After all 15 data points have been collected; the toggle
button is pressed to calculate the best-fit linear equation through the data points,
and a linear equation is used represent the measured calibration data. This
completes the calibration process. Additional information on the beach profile
indicator can be obtained in Delft Hydraulics (1998).
To ensure consistency from one experiment to the next, and that the sensor
has not "drifted" since the previous survey, the calibration of the profile indicator
is verified prior to and immediately after each bathymetric survey. Experience
has shown that the profile indicator may drift electronically by as much as
+2 mm between two consecutive surveys, generally representing a time period of
24 to 72 hr. This drift offset is removed numerically when the wheel of the
profile indicator is resting on the benchmark. The Converters panel in the MTS
control software is opened and the analog channel associated with the profile
indicator is selected in a similar manner as was shown for the ADVs in
Figure 45. The value of the offset is adjusted slightly until the voltage output
with the wheel on the benchmark is identical to the voltage output obtained
during the calibration process with the wheel on the benchmark. After this is
completed, the user can begin conducting the bathymetric survey.
89
Chapter 8
Sensor Calibration, Measurement, and Data Analysis
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