spooling technique ensures that no drag is exerted on the wire because of 

 the ship's travel or the falling probe and the probe falls vertically. 

 Thus a low strength, small diameter wire may be used. 



Changes in the resistance of the thermistor, due to temperature 

 changes in the water are recorded on shipboard via the wire link. The 

 rate of descent is precalculated and so depth can be recorded. At some 

 depth, say 500 meters, the wire supply is exhausted and the probe is 

 expended. 



The advantages of these expendable systems may be summarized as 

 follows : 



1. They can be used from a vessel travelling at high speed. 



2. A suspect record can be easily checked by a second sensor. 



3. They can be employed in weather conditions which would forbid 

 the use of conventional equipment. 



4. Since the vessel need not slow to take a measurement there will 

 be economical and tactical advantages. 



5. They exhibit greater accuracy and precision than the mechanical 

 BT. 



6. The cost per observation is low. 



Considerable detail on the mechanical and electrical design of 

 these instruments is given in References 3, 22, 23. A detailed descrip- 

 tion of the results of an experimental evaluation of the accuracy, precision 

 and reliability of the three instruments, which was carried out under the 

 management of the U.S. Navy Underwater Sound Laboratory, is given in 

 Reference 24. A detailed discussion of the economic and tactical advan- 



23 



Arthur l.HittlcJnr. 



