The gauge differs from other pressure gauges in that no slow leak 

 is provided to eliminate tides and long period waves. In place of the 

 slow leak, a solenoid valve is installed which is held open while the 

 instrument is being lowered or raised to prevent damage to the 

 pressure- sensing element. Once the instrument is in place, the valve 

 is closed electrically to seal the reference chamber at an average 

 pressure corresponding to the depth of the water. (See Reference 

 VII- 16.) 



f. Mark VI shore wave recorder (abstract) 



The Mark VI shore wave recorder was designed to fulfill 

 the need for an instrument with a high frequency response to record 

 subsurface pressure fluctuations in the surf zone. High frequency 

 response is obtained by using a Brush universal strain analyzer 

 that has a uniform frequency response to 100 cycles per second and 

 an underwater pressure head with a correspondingly high natural 

 frequency. (See Reference VII-24.) 



2. Floating wave gauges 



a. Electric wave staff 



This instrument, developed by the Hydrographic Office, con- 

 sists of three water-tight sections of three-inch diameter, 12-foot 

 long aluminum tubing, coupled together with one-inch threaded pipe 

 to form a 36-foot long staff. Below this, at a distance of 20 feet, is 

 suspended a three-foot diameter, corrugated, metal damping disk. 

 The staff and disk are weighted so that the staff floats vertically with 

 six feet of length exposed in calm water; thus, the effective measuring 

 length of the staff is 12 feet. However, the vertical motion of the staff 

 is such that under certain wave frequency conditions, waves as much 

 as 20 feet in height can be measured. A step-resistance gauge is mounted 

 on the upper 12-foot section of the staff and consists of a series of 

 36 contact points spaced four inches apart. This gauge is similar to 

 the type developed by the Beach Erosion Board. (See Part C,3,a of 

 this Section.) Inside the staff and between each contact stem is placed 

 an appropriate resistance in series. As a wave rises up the staff, 

 submerging additional contact points, proportionately more current 

 flows through the circuit as the parallel salt water current path reduces 

 the resistance of the gauge. The upper 12-foot section of the staff con- 

 taining the electrical circuit is coated with neoprene rubber. This 

 causes water to drain off the surface of the staff rapidly when a 

 wave has passed and prevents current leakage. 



VII- 9 



