148 



The low impedance (750 ohms) and high power dissipation (1 watt) of the 

 transducer potentiometer enables the pressure head to be used with practically 

 any type recorder available. An Esterline-Angus recording milliameter con- 

 nected in a 24-volt Wheatstone bridge circuit, of which the pressure head forms 

 two legs, is used by the University of California as a standard recording system. 

 Other equipment has been designed for use with this system including (1) a tele- 

 phone telemetering system which provides telemetering over standard telephone 

 circuits, (2) an ordinate distribution analyzer to provide automatic analyses of 

 wave height and (3) an amplifier (now being developed) with a hyperbolic frequen- 

 cy characteristic to convert the pressure record to a surface wave record. 



Mark V Shore Wave Recorder (University of California) - The transducer of the 

 Mark V recorder comprises a 32-junction thermopile installed in a gas-filled 

 rubber bellows (Isaacs & Wiegel, 1950). The reference junctions are in ther- 

 mal contact with the sea through the pressure head case, while the active junc- 

 tions are in thermal contact with the gas. The pressure fluctuations act upon 

 the bellows to produce temperature fluctuations in the gas which generate volt- 

 ages through the thermopile. Because of the relatively short thermal time con- 

 stant of the pressure head, long period pressure variations, such as tides, are 

 ignored by the gage. The shore installation connected to this pressure head is 

 a Leeds and Northrup recording millivolt meter. 



The Mark V pressure head is of simple design and inexpensive construc- 

 tion but has the disadvantage of being difficult to calibrate. The pressure head 

 must be calibrated for each wave period and for each depth of water in which it 

 is installed for research work. This pressure head is designed for military 

 applications where a rugged and easily handled unit is required and precise cali- 

 bration is not important as critical wave heights can be visually related to the 

 signal. 



Mark VI Shore Wave Recorder (University of California)- Work on this record- 

 er was prompted by the need for a high frequency response instrument to record 

 sub-surface pressure fluctuations in the surf zone (Snodgrass, 1952). High fre- 

 quency response was achieved by using (1) a Brush strain gage recorder with flat 

 frequency response to 100 cycles per second and (2) an underwater pressure 

 head with a natural frequency that is correspondingly high. 



The pressure head consists of an air-backed brass diaphragm to which 

 strain gages are attached. Elastic deformation of the diaphragm due to the wa- 

 ter pressure is measured by 4 strain gages attached to the air-backed surface of 

 the diaphragm, 2 near the rim; and 2 near the center. A continuous record is 

 obtained by connecting the strain gage to a Brush strain gage recorder. Unbal- 

 ance of the strain gage bridge due to the static depth of the water can be neutra- 

 lized by rebalancing the recorder after the pressure head is installed. The sys- 

 tem is then an effective differential pressure recorder which responds to pres- 

 sure fluctuations generated by the waves. 



The strain gage system is not satisfactory as a total pressure recorder 

 (even though the diaphragm is strained according to the total pressure) because 

 of the instability of the recorder's zero setting. Similarly, the recorder cannot 

 be used to record such long period pressure variations as tides, because of the 

 zero drift. 



Mark VIII Shore Wave Recorder (Scripps Institution of Oceanography) - This re- 

 corder is a differential meter which utilizes axial strain wires to sense the dif- 

 ferential pressure between an exposed chamber and a compliant chamber. The 

 pressures in the compliant chamber stabilize at the average pressure by the flow 

 of liquid silicone through a capillary. 



