0.32 Hz or lower could be resolved with reasonable accuracy. This frequency 

 limit serves as a high-frequency cutoff for wind wave analysis. Third, it was 

 necessary that the wave field be statistically uniform over the length of the 

 array. The uniformity requirement meant avoiding any irregular bathymetry or 

 wave reflectors that might focus wave energy selectively at one part of the 

 array. This constraint was satisfied by placing the array well outside the 

 nearshore bar system and also seaward and updrift (for the most common high 

 energy waves) of the pier. 



89. Within the array, each gage is powered by a 28-V power supply 

 (grounds are common) located at the seaward end of the pier. Combined power 

 and signal cables run from the pier to the array, with one cable serving two 

 gages. Each pressure transducer is a Senso-Metric Model SP973(C), which has a 

 pressure -sensitive diaphragm referenced to an evacuated cavity. Diaphragm 

 displacement is sensed with a piezoelectric strain gage. Strain gage output 

 is amplified so that 5-V analog output corresponds to full-scale pressure, 

 which is 25 psi* relative to an internal, thermally compensated, electroni- 

 cally preset reference pressure of 14.70 psi. The internal diaphragm is 

 coupled to environmental pressure through a silicon oil chamber behind a 

 second, exposed diaphragm. A perforated copper cap containing copper wool 

 protects the outer diaphragm from physical damage and inhibits biofouling. 

 Signal wires lead back to the seaward end of the pier, to the landward end of 

 the pier, and into the FRF computer room. Signals are electronically filtered 

 with a 10-Hz, fourth-order, Butterworth filter, primarily to eliminate 60-Hz 

 noise. Filtered signals are then passed through an analog-to-digital signal 

 converter having 11 -bit accuracy to a Digital Equipment Corporation VAX 11/750 

 computer for data acquisition and storage. 



90. The manufacturer's stated accuracy for the pressure transducers is 

 ±0.25 percent of full scale which corresponds to an accuracy of ±0.043 m of 

 water in a static water column. Site calibrations, performed annually as well 

 as before and after deployment, indicate a stable field accuracy of ±0.006 m 

 for wave- induced fluctuations about a mean stataic water column height of 8 m, 

 the nominal operating depth of these gages. The digitization step has a 



To convert pounds per square inch to Newtons per square metre, multiply by a 

 factor of 175.1268. 



35 



