Section V. PRESSURE-SENSITIVE GAGE 



1. Theory of Operation of Pressure-Sensitive Gage 



The pressure-sensitive wave gage operates on the principle that 



when a wave crest passes a given point there will be an increase in water 



depth, and with an increase in the height of the water column there will 

 be an increase in the pressure at the bottom of the column. 



While a wave crest is not exactly equivalent to closed water column, 

 the change in water level related to a wave crest or trough will cause a 

 pressure change at the ocean bottom. If a pressure-sensitive device is 

 placed near the ocean bottom, it will sense the pressure change caused 

 by the wave. 



The signal from the pressure-sensitive device may be carried to a 

 shore location over an electrical cable, and recorded on a paper-strip 

 chart or magnetic tape recorder. Since the signal at the recorder is 

 produced by the wave crest and trough, it is directly related to the wave. 



The pressure change produced by a wave train of constant amplitude 

 and constant period will decrease as the pressure sensor is placed deeper 

 and deeper in the water. If the wave period is made shorter, the pressure 

 from the same wave height will also be reduced at a constant water depth. 

 Ratios for conditions of pressure, depth, wave height and wave period 

 have been established, and may be used to correct the recordings from a 

 pressure-sensitive wave gage to provide a usable measurement of wave 

 conditions. 



Ripples and small sharp surface changes will be filtered out of the 

 wave record due to the pressure-period attenuation outlined above. This 

 filtering will influence the wave spectra analysis so that there will be 

 apparent differences when comparing spectra data taken at the same time 

 and location with both pressure and staff gages. 



The change in tide at locations where pressure wave gages are used 

 must be known. The increased water depths due to tide is, in effect, an 

 increase in water depth, which must be used in correcting the wave record. 



For these reasons a pressure-sensitive gage is not an ideal device 

 for gathering true data on waves. This gage is recommended only for those 

 locations where the installation of a step-resistant staff-type gage is 

 impracticable due to the cost of a mounting structure or where a mounting 

 structure would cause a navigational hazard. 



The CERC-designed pressure-sensitive gage uses a Sylphon bellows 

 that changes its length with an increase in pressure. The bellows move- 

 ment is coupled to the core of a linear differential transfornjer by a 

 permanent magnetic, steel ball, universal joint. 



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