Page 455 echo sounding 52 



The Frahm type of tachometer consists of a series of metallic reeds, supported at one end and 

 free to vibrate at the other, each having a natural period of vibration determined by its length and 

 other physical properties. At its natural period a reed will vibrate at maximum amplitude. If there 

 is a direct relation between the frequency of a reed and the motor speed at calibration velocity, the 

 motor speed can be verified by observing the reed's amplitude of vibration — when it is a maximum it 

 indicates that the motor is operating at the correct speed. A reed tachometer is usually composed 

 of seven reeds, ranging by a few percent above and below the calibration velocity, which is indicated 

 on the middle reed. The reeds of higher and lower frequency serve to indicate the amount and 

 direction of the motor-speed deviation. The reed tachometer is usually operated by some unbalanced 

 mechanical force, or is actuated by an electromagnet that receives impulses from a commutator 

 coupled to the indicator motor. (See 5234.) 



When alternating current is used to drive the indicator motor a frequency meter calibrated in 

 revolutions per minute is sometimes used. 



52. ECHO-SOUNDING INSTRUMENTS 

 USED BY THE COAST AND GEODETIC SURVEY 



The Coast and Geodetic Survey uses a number of different echo- sounding instru- 

 ments, sometimes several on one vessel, depending on the nature of the surveys expected 

 to be made. For surveying in deep water, such instruments as the Dorsey Fathometer 

 Nos. 2 and 3, the 312 Fathometer, and the Veslekari Recorder are used. For a vessel 

 used principally for surveying in shoal to moderate depths, the sounding equipment 

 normally includes a Dorsey Fathometer No. 1 or a permanently installed 808 Fathometer. 

 Since the range of the Dorsey Fathometer No. 3 includes both shoal and deep water, 

 and since this instrument embodies all the principles and accuracy of the Dorsey No. 1, 

 there is no need for a special shoal-water instrument on a survey vessel equipped with a 

 Dorsey No. 3. 



Some vessels are equipped with two or more instruments so that, if the principal 

 instrument fails to function, another may be substituted immediately. Furthermore, 

 the availability of two or more instruments permits the use of the particular instrument 

 best suited for the depths and character of the bottom at the time. The advantages of 

 two instruments on a vessel are further discussed in 545. 



Besides those instruments mentioned above, a large number of semiportable graphic 

 recorders are now being used by the Coast and Geodetic Survey for surveys in shoal 

 water. Because of their portability they may be transferred from one vessel or field 

 party to another as the need arises. 



All of the instruments in use in 1941 are described in this section. Because of the 

 constant progress and development of echo sounding it must be expected, however, 

 that other types of instruments will be in use, perhaps in the near future. 



521. 312 Fathometer 



The 312 Fathometer may be classified as a visual, sonic type, echo-sounding instru- 

 ment intended for depth measurements in water ranging from moderately shoal to deep. 

 This instrument is manufactured by the Submarine Signal Company of Boston, Massa- 

 chusetts, and has been used by the Coast and Geodetic Survey since 1925, and is still 

 used for surveys in deep water on a number of the vessels of the Bureau. The optimum 

 depth range of the 312 Fathometer is from 20 to 4,000 fathoms, although it has been 

 used in depths as shoal as 12 fathoms with very good results. In actual use the range 

 will vary, depending on the installation, the vessel, and other conditions affecting echo 

 sounding. The maximum range of 4,000 fathoms is obtained only from a perfect 

 installation and under the most favorable echo-sounding conditions. 



