an optimal balance over the length of the slope, where the profiling rods were out of 

 water at the crest and partially submerged at the toe. 



The power input to, and analog output from, each of the potentiometers was 

 controlled by a personal computer (PC)-based data acquisition system. Analog-to- 

 digital data conversion and signal conditioning and recording were performed using an 

 Optim MEGADAC 5000. The MEGADAC had 4 MB of intemal memory and could 

 sample up to 250 kHz from 20 channels. The heart of the MEGADAC consisted of 

 AD682SH modules that included 12-bit differential input analog-to-digital converters 

 and programmable Bessel filters. The MEGADAC also included power amplifiers to 

 provide power to the potentiometers. The digital voltages were recorded to a hard disk 

 within the PC at 20 Hz. The speed of the carriage over the structure varied, so this 

 sampling rate produced a variable spatial sampling resolution. But, in general, the 

 horizontal cross-shore sampling resolution was approximately one sample per 

 millimeter. Optim, Inc. TCS software was utilized to control data acquisition. 



Prior to each day's tests, the profiler was calibrated by first recording the 

 rotational potentiometer voltages with the rods positioned horizontally on a jig that was 

 set using high-resolution tiltometers. Then the rods were swung through a known angle 

 and the voltage differences recorded. The resulting voltage-to-angle ratios provided a 

 daily specific calibration coefficient for each potentiometer. Prior to each profile, the 

 output voltage for each potentiometer was set to zero on the fixed bar just above the 

 breakwater crest. 



78 



