load cell. This increase in output voltage (i.e., electronically induced increase in
weight) was accurately measured in the electronics laboratory prior to installing
the load cells in the sediment trapping system. During the "shunt cal" procedure
(prior to an experiment) the "gain" of the linear calibration equation is adjusted
so that the increase in output voltage exactly matches the increase in voltage that
was measured prior to installation. In other words, the "gain" in the calibration
equation is slightly adjusted to compensate for the effect of a change in
temperature on the load cells, cabling, and other components. This procedure has
been automated and can be conducted in approximately 10 min at the beginning
of each experiment. From theoretical considerations alone, this procedure only
needs to be performed when the ambient temperature has changed significantly
since the last "shunt cal" was conducted. However, it is prudent to perform this
procedure at the beginning of each experiment, because it provides a means of
verifying that all 60 load cells are functioning properly.
Furthermore, the "shunt cal" procedure is only performed after the sediment
traps have been dredged and the 20 traps are essentially empty of sand. This is
necessary because the calibration file associated with the "shunt cal" procedure
adjusts the offset of each calibration equation so that each trap weighs 0.0 kg
after the procedure is complete. If the traps each contain a considerable amount
of sand, it is necessary for the operator to input the most recently measured
weight on each load cell (i.e., measured at the end of the last test segment). This
process is somewhat tedious, because 60 values must be transferred from the
most recently measured data file to the calibration file. Therefore, the standard
procedure is to conduct a "shunt cal" only after the traps have been dredged.
Measurement of sediment trap data
The physical process of sand accumulating in the sediment traps is a
relatively slow process. A typical test segment lasts from 1 to 3 hr of continuous
wave forcing and current recirculation. During the design process for the
sediment traps, the following tentative assumptions were made. The first
assumption is that during later stages of an experiment, after the beach profile has
approached an equilibrium shape, the rate of accumulation of sand in a given trap
is relatively constant (i.e., increase linearly) throughout a test segment. The
second assumption is that during the early stages of an experiment, as the beach
profile is adjusting more rapidly, the rate of accumulation in a given trap may not
increase with a linear trend during the test segment. Therefore, the data
acquisition system for the sediment traps was designed with sufficient capacity to
sample the 60 load cells at a frequency of 20 Hz. This rate should provide the
capability to quantify the change in sediment transport rate during the early
stages of an experiment. In addition, a data acquisition system with this capacity
has the possibility to quantify the change in sediment transport rate associated
with, for example, a large group of waves in an irregular wave spectrum.
Based on these tentative assumptions, the following measurement
methodology is used during a typical experiment. First, all 60 load cells are
sampled continuously throughout the entire duration of the test segment.
Normally, sand trap data collection begins approximately 1 min prior to wave
generation and ends approximately 1 min after the wave generators are stopped.
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Chapter 8
Sensor Calibration, Measurement, and Data Analysis
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