298 



VOLKMANN 



[CHAP. 13 



The pingers in use to date are essentially identical to those first developed 

 and described by Swallow (1955). They consist of a relaxation oscillator timed 

 to discharge a relatively high energy capacitor through a cold cathode trigger 

 tube into a 10-kc magneto -strictive toroid. A pulse length of about 2 msec dura- 

 tion and a pulse rate of a ping about every 2-3 sec has been found optimum for 

 this system. While a longer pulse would permit narrower filtering and increased 

 signal-to-noise ratio, the present method of measuring the time delay visually 

 in the two -hydrophone listening system and power considerations made the 

 2 msec pulse a good compromise. 



The instrument case is made of extruded aluminum tube closed by "O" ring 

 sealed-end caps through one of which are led the wires for the external magneto - 



6 8 10 



Pressure lb/in 2 (xlO 3 ) 



14 



16 



Fig. 1. Collapse pressure for cylinders of various aluminum alloys against the ratio of wall 

 thickness to outside diameter. The curved section represents buckling of the cylinder, 

 the straight sections represent metallic yield. (By courtesy of L. D. Hoadley.) 



strictive sound source. Aluminum, while not unique, has two properties essen- 

 tial to the operation of the pinger. First, it is light enough to carry a pay load 

 to deep depths without collapsing. The figure (Fig. 1) shows the collapse 

 pressure for tubes of various alloys against the ratio of wall thickness to outside 

 diameter. The bigger this ratio, of course, the less the flotation per foot, and for 

 aluminum a ratio of about 0.106 gives no flotation. Secondly, the aluminum 

 tube is less compressible than water and if made slightly heavy at the surface 

 will gain buoyancy as it sinks. By adjusting the density of the pinger before 

 release it can be made neutrally buoyant at any desired depth. The density of 

 sea-water is well known and the change in volume of the aluminum cylinder, 

 owing to temperature and pressure changes, can be computed from standard 



