oscillator, the frequency of which was varied between 16 and 26 

 kilocycles by a variable capacitor driven by a Bourdon tube. A hydro- 

 phone was towed at the surface 1,000 to 1,500 feet behind a ship, 

 away from noise of the ship. Signals could be heard from a 1,500- 

 foot depth while underway and from a 3,200-foot depth when not under- 

 way. Early field trials indicated an accuracy between five to eight 

 percent of the actual depth. This accuracy was improved to about one 

 percent of full scale (1,500 feet). 



Bourdon-actuated depth measuring elements purchased by the 

 Hydrographic Office are giving erratic results in the field. Two models 

 were purchased: a 200-foot model, and a 10,000-foot model. The 

 manufacturer states a full-scale accuracy of ± 2 percent. Calibrations 

 by this Office show that the meters do meet the specification. However, 

 this two percent of full scale may show up, and does, at all depths. 

 Thus, for the 10,000-foot instrument, the reading at a calibration 

 pressure of 200 feet may be 400 feet. The calibration data for meas- 

 urements from the 10,000-foot instrument indicate that the meters 

 should be subjected to much more detailed examination in the labora- 

 tory. The calibrations (and techniques) are too sketchy to permit any 

 definite conclusions concerning the repeatability (or reliability) of 

 the meters, and some of the calibration data indicate a degree of 

 erratic behavior. The calibration data for the 200-foot instruments 

 are more satisfactory, but these data also are insufficient to permit 

 conclusions concerning repeatability. Field tests aboard the USS 

 LITTLEHALES showed good agreement between three 200-foot meters. 

 However, conditions precluded controlled measurements to determine 

 accuracy. Both models have performed erratically in the field. To date 

 insufficient quantitative data exist to help determine the causes of 

 error. The very small scale on the face of the indicator is undoubtedly 

 one source of error: A scale division of one- sixteenth inch equals 100 

 feet. 



b. Electrical resistance change (strain gauge) 



Essentially, the element of a strain gauge is an electro- 

 mechanical device which transduces minute changes of displacement 

 to sensible resistance changes proportional to the displacement. The 

 transducer is electrically and mechanically symmetrical. In the center 

 of a stationary frame, an armature is supported rigidly in the plane 

 perpendicular to the longitudinal axis so as to allow free movement 

 along this axis. Connected between rigid pins mounted in the frame and 

 armature are four filaments of strain- sensitive resistance wire which 

 comprise the four elements of a Wheatstone bridge. As the armature 



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