quantifying the longshore uniformity of the net volumetric change along the
beach. More specifically, it also is helpful in quantifying the degree of erosion at
the updrift end of the beach, as well as the rate of growth of the submerged shoal,
which tends to form immediately updrift of the downdrift wave guide.
Examples of these types of analysis will be discussed in detail in Chapter 10.
These examples are based on the results from two sets of moveable-bed
experiments, one with wave conditions characterized by spilling waves and the
other for plunging waves.
Sediment Trap Data
The design of the sediment trapping system was discussed previously in
Chapter 4. This section will discuss additional information related to calibration
of the load cells, the methodology used to obtain an accurate measurement of the
weight of sand in each trap, and the data analysis procedures used after a test
segment is complete. As mentioned previously in Chapter 4, the term
"submerged weight" will be used rather than "mass," because submerged weight
the quantity measured by the traps.
Calibration of load cells
Each of the 60 load cells that support the 20 sediment traps consist of high
accuracy, shear beams constructed inside of a stainless steel, air-tight, and splash-
proof enclosure. The manufacturer's performance specifications indicate a
maximum combined error of +0.05 percent of the rated capacity of the load cells,
considering the combined effect of linearity, repeatability, and hysteresis.
Therefore, the maximum combined error for the 250-, 500-, and 1,000-kg load
cells is 0.125, 0.25 and 0.5 kg, respectively. The linear calibration equations
were verified in the electronics laboratory prior to installation, and the 60 load
cells met the manufacturer's performance specifications. Therefore, the linear
calibration equations supplied by the manufacturer were initially used.
These specifications, however, do not consider the effect of seasonal
temperature fluctuations, which can vary by as much as 40 C in the Vicksburg,
MS, area. The manufacturer's specifications indicate a variability of +0.007
percent of the rated capacity of each load cell, per degree centigrade change in
ambient temperature. Assuming a 40-C change in ambient temperature, the
temperature induced variability for the 250-, 500-, and 1,000-kg load cells is 0.7,
1.4, and 2.8 kg, respectively. This temperature induced potential error is 5.6
times larger than the errors associated with linearity, repeatability, and hysteresis.
To minimize the errors induced by seasonal temperature fluctuations, and
therefore improve the overall accuracy of the sediment trapping system, the data
acquisition system was designed so that a "shunt cal" procedure can be
performed on the 60 load cells at the beginning of each experiment. The
procedure works by electronically applying a known resistance to the 60 load
cells. This additional resistance increases the output voltage from each load cell
by approximately one-half of the total voltage range (or rated capacity) of each
93
Chapter 8
Sensor Calibration, Measurement, and Data Analysis
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