Calibration and zero reference. Pore pressure gauges were calibrated 

 weekly by filling or emptying the wave channel and recording the output 

 signals from the instruments. The reference was a video record of a 0.1 -ft- 

 (3.0-cm-) division staff, painted on the side of the channel. The staff can be 

 read to ±0.01 ft (± 0.3 cm). Calibrations for the nearshore gauges were 

 contaminated by hydraulic gradients due to drainage of the beach. Post- 

 SUPERTANK calibrations demonstrated that all instruments remained within a 

 few percent of the gain-set calibration constants of 0.5 ft (0.15 m) of head per 

 volt. Table 12-3 summarizes the post-SUPERTANK calibration constants. 



Zero references for the pore pressure readings occur at the beginning of 

 each run, before waves arrive at the beach. These zero references were 

 augmented by 10-min time series of zeroes run at the beginning and ending of 

 each day, after the first week of testing. 



Sampling and time reference. The ten pore pressure gauges were 

 sampled simultaneously at a rate of 16 Hz. The samples were synchronized 

 with wave profile and velocity measurements through a common data 

 acquisition system. All samples were synchronized with WWV radio 

 broadcast time signals, as discussed in Chapter 13. 



Pre-processing and filtering. All pore pressure gauges were differentially 

 driven to avoid d-c drift. Each channel of data was analog filtered prior to 

 digitizing. A 10-Hz, fifth-order Bessel filter was used to avoid aliasing prob- 

 lems in the digitally sampled data. 



Table 12-3 



Pore Pressure Gauge Calibration Constants 



Channel No. 



Gauge I.D. 



Calibration Constant 

 ft of head per volt 



17 



A 



0.4984 



18 



B 



0.4963 



19 



C 



0.4985 



20 



D 



0.4915 



21 



E 



0.4762 



22 



F 



0.5056 



23 



G 



0.5010 



24 



H 



0.4915 



25 



J 



0.5018 



26 



K 



0.5044 



244 



Chapter 1 2 Pore Pressure and Sediment Density Measurements 



