one cycle per 1,000 seconds. A strain-gauge transducer converts the 

 filtered pressure fluctuations into electrical signals, which are trans- 

 mitted to shore through a cable. 



Sea pressure is transmitted through a rubber bellows (Figure 

 VII-2). Two capillary tubes, K\ and R£, lead to two domes that contain 

 metal bellows, Cj and C^. The rubber bellows, capillary tubing, and 

 the compliant chambers between the domes and the exteriors of the 

 metal bellows are filled with silicone fluid. The interiors of the two 

 metal bellows and the transducer chamber are filled with air main- 

 tained at atmospheric pressure. The total air reservoir is large 

 relative to the volume of the metal bellows, and the variation in air 

 pressure due to the deflection of the bellows is negligible compared to 

 the changes in outside pressure. The transducer chamber and the 

 interiors of the metal bellows can be pressurized to extend the depth 

 of installation, which is otherwise limited by the bellows to ten meters 

 (one bar). 



Two identical bellows are used in the compliant chambers, but the 

 capillary tubes are adjusted so that their resistances to the flow of 

 hydraulic fluid have a ratio of about four to one. A strain-gauge pres- 

 sure transducer is connected between the two compliant chambers to 

 sense the differences in pressure. Because of the flow resistance of the 

 capillary tubing, high-frequency pressure fluctuations (ordinary gravity 

 waves) cause negligible pressure changes within the compliant cham- 

 bers and between them. For very low-frequency pressure fluctuations 

 (tides), the pressure in each chamber essentially equals the applied 

 pressure, and the pressure difference between the chambers is again 

 negligible. For intermediate frequencies the pressures in the two 

 chambers are unequal owing to the difference in resistance of the 

 capillary tubes, and an appreciable signal will be produced by the 

 transducer. (See Reference VII-23.) 



e. Knapp bottom pressure gauge (abstract) 



This bottom pressure gauge was designed for studying harbor 

 surging and developed and tested at the U.S. Naval Station, Long Beach, 

 California. A strain-gauge unit, used in connection with a pressure- 

 sensitive bellows, comprises the transducer of the pressure head. The 

 four strain-gauges in the unit are connected to form a bridge circuit 

 that is linked to the recorder by an electrical cable. A d.c. voltage is 

 applied to the bridge, and the record is obtained by recording photo- 

 graphically the unbalanced current from a magnetic oscillograph. Any 

 standard strain-gauge recorder can be used for the recording system. 



VII- 7 



