149 



The Mark VIII is relatively light, stable, rugged, and is simple to con- 

 struct and assemble. Lowpass characteristics are easily adjusted. If desired, 

 a flat response from low acoustic frequencies to trans-tidal frequencies is ob- 

 tainable. The vacuum compliance model is expected to show no effect from wa- 

 ter temperature changes. 



The measuring unit, a Statham strain gage, is especially adapted for use 

 in this pressure head by the manufacturers, the Statham Laboratories Inc. , of 

 Beverly Hills, California. 



The calibration has shown a straight-line relationship of pressure to out- 

 put over the working range. The signal output is 0.2 millivolts per foot of wa- 

 ter per 1.5 volts on the bridge. A total of 15 volts can be used on the bridge, 

 resulting in a response of 10 chart inches per foot of pressure on the standard 

 2-millivolt Speedomax, and a least reading of 0.005 foot of water. 



Knapp Bottom Pressure Gage - A bottom pressure gage designed for studying 

 harbor surging was developed and tested at the U.S. Naval Station at Long 

 Beach, California (Knapp, 1951). A Statham strain gage unit, used in' connec- 

 tion with a pressure-sensitive bellows, comprises the transducer of the pres- 

 sure head. Four strain gages in the Statham unit are connected to form a 

 bridge circuit which is attached to the recorder by an electrical cable. A DC 

 voltage is applied to the bridge, and the record is obtained by photographically 

 recording the unbalanced current with a magnetic oscillograph. Any standard 

 strain gage recorder could be used for the recording system. 



The gage differs from other pressure gages in that no slow leak is pro- 

 vided to eliminate tide and long period waves. In place of the slow leak, a 

 solenoid valve is installed which is held ppen while the instrument is being 

 raised or lowered to prevent damaging the pressure-sensing element. Once the 

 instrument is in place, the valve is electrically closed, which seals the refer- 

 ence chamber at an average pressure corresponding to the depth of the water. 



INSTALLATION OF SHORE WAVE RECORDERS 



Selecting the Site - The general location of a wave gage depends on the pur- 

 pose for which it is intended. If data are needed to compile statistical informia- 

 tion describing the general wave action along a section of coastline, the gage 

 should be located so that it is well exposed to the open sea. There should be 

 no islands, bars or prominent points to interfere with the waves before they 

 reach the instrument. In this case, the most suitable sites are off long, 

 straight beaches and off exposed points. The wave data obtained from gages 

 exposed to the open sea can be used to estimate the amplitude of waves acting 

 at a particular site along the same section of the coast. Principles of refrac- 

 tion and diffraction of ocean waves can be used to calculate wave height at a par- 

 ticular site providing the wave height, period and direction in deep water, and 

 the contours of the bottom over which the waves must pass to reach the site are 

 known. 



Gages installed to study the waves acting at a harbor entrance, at a pier, 

 or along a breakwater should be located near the site. Careful attention should 

 be given to the local refraction to determine the relation between the waves at 

 the gage and the site studied. Often within a few hundred feet a noticeable dif- 

 ference in wave action can be observed. Gages not exposed to the open sea are 

 seldom used in estimating the wave conditions offshore. 



Pier-mounted Gages - Piers provide supporting structures from which gages 



