WAVWEPS REPORT 7650 



circuits when the instniment is in the horizontal position. In this 

 position, the batteries are connected to the tvo pins of the output 

 plug in the pressure case so that charging current can be applied. 

 When the instrument is in the vertical position, the batteries are 

 switched to the electronic circuits and the instrument is operable. 



Figure 8 shows the internal construction of the instrument. An 

 inner shell, attached to the lower plate, supports the electronic 

 circuits, wiring, and batteries, and permits the instrvunent to be 

 calibrated and tested with the pressure case removed. 



SHIPBOARD INSTALLATION 



Winch and Cable . The winch used with the SVTP instrument handles 

 approximately 6,000 feet of 0. 189-inch diameter, single-conductor poly- 

 ethylene covered steel cable. The inner conductor of the cable is 

 terminated electrically at the winch in a slip-ring commutator. The 

 6,000 feet of cable weighs 159*8 poxmds in air, but because of buoyancy 

 it weighs only 65. 1 pounds in sea water. The cable has ^7 strands of 

 30-gage and one strand of 28-gage steel wire, which are sealed with 

 silicone paste and covered ;^^lth DFD-6015 polyethylene. The breaking 

 strength is 9OO pounds . The direct-current resistance is approximately 

 28.5 ohms per 1,000 feet, and capacitance to sea water about O.O5 micro- 

 farads per 1,000 feet. When the cable is on the winch, it has an 

 inductance of 3IO millihenries. With this winch and cable, the instru- 

 ment can be lowered at a maximum rate of about 5 ft/sec and raised at 

 about 3 ft/sec . 



Signals from the instrument are attenuated by resistance, capaci- 

 tance, and inductance in the cable and winch. With 6,000 feet of the 

 cable described above on the drum, resistance is approximately I70 ohms . 

 The cable acts like a coaxial cable when in sea water, and therefore, 

 has an attenuation (Ref . 9): 



a ..■-CR 



= )" 



where a, is the attenuation in nepers per unit length, and C and R are 

 capacitance and resistance per unit length. 



The greatest attenuation occurs at the greatest depth. At a fre- 

 quency of 9-8 kilocycles, the value a becomes 0.23 nepers per 1,000 feet 

 or a voltage attenuation of 2.0 decibels per 1,000 feet. When all the 

 cable is wound on the drum, the capacitance to sea water is small, but 

 inductance reaches 3IO millihenries. 



16 



