C. Depth Recorders (A. L. McCahan) 



Commercial depth recorders, as currently manufactured and in- 

 stalled, are adequate for use as navigational aids for performing 

 routine duties. However, these depth recorders have been used in the 

 past for hydrographic survey operations where the degree of accuracy 

 required was higher than that which was obtained. 



The basic principle of the depth recorder is the measurement of the 

 time interval between the transmission of the original sound and the 

 reception of the returning echo. The only feature designed into routine 

 depth recorders for time resolution is a synchronous motor which 

 depends upon a constant 60- cycle frequency source. It is readily appar- 

 ent that a slight frequency variation of one cycle causes a depth recording 

 error of 1.6 percent, or 76.8 feet, at the standard velocity of 4,800 

 feet per second. Thus, the loss of one cycle while recording depths 

 in 1,000 fathoms produces an error of 16 fathoms and in 2,000 fathoms 

 an error of 32 fathoms. 



It is considered that a precision depth recorder (PDR) more 

 adequately satisfies the requirement ^ for detail and accuracy in the 

 recorded sounding than does any of the routine depth recorders. 

 The accuracy of the vertical recording is approximately one part in 

 3,000 with a time accuracy of one part in a million. However, the preci- 

 sion depth recorder only performs the recording function. The timing 

 precision is made possible by the tuning-fork- controlled, slip-free 

 drive of the facsimile-type recorder. Assuming that one millimeter 

 is the smallest readable unit, the depth as read from the record is 

 accurate to ±1 fathom. 



The PDR, as presently designed, is considered to be an adequate 

 instrument for recording depth detail with accuracy. The major cause 

 of operative failures is the transmitter used at present with this instru- 

 ment. 



It is recommended that a depth recorder be designed specifically for 

 the purpose of hydrographic surveying. It is further recommended that 

 the better features of such recorders as developedby the oceanographic 

 laboratories be incorporated with those of PDR-type recorders into one 

 precision recorder. Further, it is recommended, where other agencies 

 might be doing developmental work on depth recorders, that the opera- 

 tional personnel of such agencies be consulted concerning the problem. 



Encl. (3) to Code 5401-BEO/bah memo of 23 Jul 1958 



B-13 



