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.

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.

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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