APPENDIX A: HYDROPHONE-DEPTH 

 MEASURING SYSTEM 



A block diagram of the pressure module with 

 associated equipment is shown in figure Al. The pressure 

 module is a waterproof stainless-steel cylinder designed 

 to withstand pressures at least as great as any to which 

 the hydrophone will be subjected (see fig. A2). The module 

 contains a strain-gage pressure transducer, a differential 

 amplifier, a VCO, and its own battery supply. The pres- 

 sure transducer is calibrated by a resistor which is con- 

 nected in parallel across one of the legs of the bridge. 

 This lowers the resistance of the leg and corresponds to 

 a known pressure change. 



The cylinder was designed to be inserted between 

 the 50 -foot cable and the longer sea cable. With the pres- 

 sure module in the system, the hydrophone -preamplifier 

 obtained its power from the internal battery supply of the 

 module. This was done so that the output of the VCO 

 could be placed on the leads which are normally used to 

 bring power to the hydrophone preamplifier. 



The frequency of the VCO (12. 5 to 14. 5 kHz) was 

 then measured with an HP model 52 IE frequency counter 

 and recorded by a digital recorder model HP 561B. The 

 digital clock is a model HP 57 IB. 



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