Page 507 echo sounding 55 



system have been installed on some vessels. Although the two instruments are not 

 interchangeable as to depth range, they were interconnected so that the 312 Fatho- 

 meter could be operated independently. Figure 110 shows the circuit arrangement by 

 which the sonic-oscillator and echo-amplifier system can be switched to either the 312 

 Fathometer or the Dorsey Fathometer. When the switch is down the complete 312 

 Fathometer system is in operation. When the switch is up, the oscillator and echo 

 amplifier are operated in conjunction with the Dorsey Fathometer, and the soundings 

 will be registered on the dial of the Dorsey Fathometer indicator. 



55. ADJUSTMENT AND VERIFICATION OF ECHO-SOUNDING INSTRUMENTS 



For utmost accuracy all depths measured by echo-sounding instruments must be 

 corrected instrumentally, or otherwise, for certain variable factors and conditions; 

 although, practically, some inherent errors are so insignificant that they may be ig- 

 nored, or are so nearly constant that an initial adjustment of the instrument may 

 suffice. 



Where practicable, compensating adjustments should be made to the echo-sounding 

 instruments for certam factors, so that the soimdings as registered will have been 

 corrected by the appropriate amounts. This eliminates a subsequent arithmetical 

 reduction with its possibility of error, and reduces the office time required to complete 

 the records. 



Draft (551), instrumental error (552), and settlement and squat (553) are factors 

 for which compensation may be made by an appropriate adjustment of the instrument. 

 The effect of each of these is determined individually and then combined algebraically 

 into a total for which allowance is made by setting the index of the instrument according 

 to 554. 



All echo-sounding instruments in use by the Coast and Geodetic Survey in 1941 are 

 driven by motors which must be maintained at a constant speed, or the registered depths 

 will be erroneous. In some echo-sounding instruments a special test is required to 

 detect a variation in motor speed of appreciable effect, the test varying with the type of 

 instrument. This test must be made periodically to ensure that the instrument 

 operates at the correct speed, and an adjustment must be made if necessary. (See 555.) 



If soundings are obtained with instruments using separate acoustic units for 

 transmitting and receiving, an error due to the horizontal distance between the two 

 units must be avoided by compensation or correction. This is known as the separation 

 error (see 556). 



551. Draft 



The depth of water registered by an echo-sounding instrument used in hydro- 

 graphic surveying should be the depth below the water surface, and not the depth below 

 the submerged acoustic units. The common method of allowing for the draft of the 

 transmitting and receiving acoustic units, is to delay the transmission of the signal by 

 a time equal to twice that which would be required for the sound to travel from the 

 surface of the water to a depth equal to the draft of the acoustic units, which in turn is 

 equal to a reading of this depth on the dial of the instrument. Draft with reference to 

 echo-sounding corrections shall be understood to be the mean depth of the transmitting 

 and receiving units below the surface of the water when the ship is not underway. 



Provision must be made, or special instruments must be installed, to measure the 

 draft on ships having the acoustic units permanently mounted in the hull. An internal 



