the points of view of cost, reliability, precision and time available for 

 measurements has led to recent interest in and development of expendable 

 devices. These consist of a free falling transducer which returns its 

 output to the ship by a cable link. XBT is a generic name for expendable 

 bathythermographs . 



The design of expendable instruments illustrates nearly all the 

 problems and advantages which can occur with particular oceanographic 

 instrumentation. Thus expendable instrumentation offers the advantage of 

 speed for tactical situations because the ship does not have to slow down 

 to take the measurement. However, the device must be inexpensive and, 

 further, problems may occur in the telemetry of the information and the 

 depth location of the device. For details of XBT design see Reference 22. 



The use of a pressure sensor to indicate depth with the required 

 accuracy and uniformity would add greatly to the cost. Consequently, depth 

 is computed by relating elapsed time after launching to the sink rate. 

 This method has received wide application with expendable devices and is 

 discussed in the references. 



The current XBT is wire connected to readout and recording 

 equipment on board ship. In December 1964, it was announced that the Navy 

 Bureau of Ships was evaluating and testing expendable BT's from three 

 instrument companies, Bissett-Berman, G.M. Defense Research Laboratories, 

 and Francis Associates. The basic designs of these XBT's are similar. 

 The instruments consist of a thermistor probe in a housing containing a 

 spool of wire which connects to a second spool of wire in a launching 

 unit. When the probe is launched the wire from both spools is free to 

 unwind allowing the probe to fall at a precalculated rate. The dual 



22 



artbur 2l.1Uttlc.llnr. 



