5513 HYDROGRAPHIC MANUAL PaGE 510 



The draft of the acoustic units must be recorded in the Sounding Record in the 

 appropriate space in rubber Stamp No. 33, Sounding Apparatus (see fig. 176). This 

 stamp provides a separate entry for the draft of the oscillator and the hydrophone 

 used. Where a transceiver is used, or where it is more convenient to determine the 

 mean draft of the transmitting and receiving units, the one entry will suffice if the 

 facts are clearly indicated. 



If there are separate transmitting and receiving units, it is their mean draft which 

 is used for the purpose of draft adjustment. 



5513. Frequency of Draft Measurements 



The variation in the ship's draft and the depth of the soundings are the two factors 

 which determine the frequency of draft measurements. 



For soundings of 10 fathoms or less the draft should be known within one-fourth 

 foot and should be measured with sufficient frequency to ensure this. Depending on 

 the variation in the ship's draft, this may require a measurement once each day, 

 usually at the beginning of the day's work, so that the echo-sounding instrument may 

 be adjusted before any soundings are taken. 



For soundings deeper than 10 fathoms the draft should be known within at least 

 one-half percent of the depth of water. 



For soundings in deep water where the possible error due to erroneous draft 

 adjustment is only a very small percentage of the total depth, an accurate knowledge 

 of the draft is unimportant and echo-sounding instruments are usually adjusted for 

 a mean draft. 



552. Instrumental Error 



Errors caused by instrumental time lags are inherent in all echo-sounding instru- 

 ments, and some have mechanical lags. Their effect on echo sounding is to increase 

 the registered depth to a value greater than the actual depth. Such errors differ in 

 magnitude for each type of instrument, and are unlikely to be exactly the same even for 

 instruments of the same design. Furthermore, such errors are variable in which changes 

 may be attributed to: (1) Variation of tuning and gain of the echo amplifier; (2) varia- 

 tion in strength of the transmitted and echo signals; (3) adjustment of the keying 

 ■circuit; and (4) deterioration of tubes and other parts. There are numerous other minor 

 ■causes of such errors, but they are usually of small magnitude, and are relatively con- 

 stant and therefore need not be considered for correction purposes. 



Perhaps the most troublesome source of instrumental time lag is that due to a 

 variation in the strength of the registered echo signal. As its strength decreases, the 

 v^alue registered on the dial increases and is always too large. This may be the result 

 of varying the gain of the amplifier or of a variation in the strength of the received echo 

 signal before amplification. A variation in the gain of the amplifier affects the regis- 

 tered depth and it has been repeatedly emphasized in this Manual that the amplifier 

 must be operated at the highest gain practicable without introducing too many strays. 

 A reduction of gain may increase the registered depth by an appreciable amount. (See 

 5163 and 5237.) The variation in the strength of the echo signal before amplification 

 is complex — it is partly a function of depth but may be due also to the character of the 

 bottom or the characteristics of the water. This latter variation cannot be compen- 

 sated for by a simple instrumental adjustment, but if the range of depth in the project 

 area is limited, this error is small as compared to other errors. 



