3. EQUIPMENT AND INSTRUMENTS 



83 



the needle length and paper alignment are 

 correct. If the fix mark agrees with the 

 printed arc at one end only, the paper align- 

 ment is incorrect and should be adjusted. 

 If the fix mark agrees at both ends of the 

 printed arc but is out in the center, the 

 needle length should be corrected. A con- 

 tinuous variation of the initial is an indication 

 that the chart paper may not- be running 

 true. This is usually caused by uneven wind- 

 ing of the chart on the take-up reel, and 

 can be corrected only by starting over with 

 another reel (see 5-111). 



3-94 Phasing differences.— The 808 re- 

 corder is designed to record depths to 160 

 feet or fathoms in four overlapping phases 

 designated A, B, C, and D. The initial is 

 recorded only on the A phase. The instru- 

 ment is made to record on the proper scale 

 or phase by means of a phasing head which 

 is rotated to the proper position and locked 

 in place by a spring operated pin. Phasing 

 errors may be caused by small errors in con- 

 struction of the instrument, by natural wear 

 on the pin and holes in the plate, or by 

 wearing down the stylus needle while oper- 

 ating on a scale other than A. The fathom- 

 eter operator should be trained to make 

 phase changes in a clockwise direction, and 

 to be certain that the locking pin is firmly 

 in place (see 5-112). During hydrographic 

 survey operations the phasing head should 

 be shifted and locked in exactly the same 

 manner used in calibration by bar check or 

 phase comparison. When sounding on any 

 of the 3 deep phases, the recorder should be 

 shifted back to the A scale occasionally to 

 check the initial setting and adjust it if 

 necessary. 



3-95 Voltage and gain. — The recorder 

 operates from a 12-volt storage battery. Volt- 

 age may be allowed to vary to a limited 

 extent ; however, significant errors will result 

 when voltage drops too low. The operator 

 should notify the hydrographer if the volt- 

 age falls below eleven volts. The gain should 

 always be set as high as possible without 

 producing spurious traces on the fathogram. 

 The gain should be the same for survey op- 



erations as for bar-check or comparative 

 sounding calibrations (see 5-110). In areas 

 of heavy kelp or grass where the bottom 

 trace may be lost, the gain may be tem- 

 porarily adjusted, but should be reset when 

 the area is cleared. 



3—96 Echo sounder transducers. — As its 

 name implies, an echo sounder measures the 

 time required for a sound wave to travel 

 from its point of origin to the bottom and 

 return, and converts time to distance or 

 depth. Various instruments and methods 

 have been devised to produce the sound and 

 receive its echo. The transmission of sound 

 used in echo sounding is dependent on cer- 

 tain properties of the water and on the re- 

 flecting surface. Ideally the water should 

 possess constant physical characteristics 

 throughout the entire depth, thus resulting 

 in constant velocity of sound from surface 

 to bottom ; there should be no attenuation of 

 sound ; and there should be 100 percent re- 

 flection from a bottom parallel to the sur- 

 face. These ideal conditions never exist. 



That part of the echo sounder which con- 

 verts electrical energy to sound energy is 

 called a transducer. The same part, or one 

 like it, is used to convert the echo to elec- 

 trical energy to be amplified and recorded 

 as a measured depth. An echo sounder trans- 

 ducer is usually designed to operate at a 

 specific frequency. When echo sounding 

 equipment is classified according to the fre- 

 quency of the transmitted sound, it is said 

 to be either sonic or supersonic. Devices that 

 utilize acoustic waves of a frequency that 

 are audible to the human ear are generally 

 classified as sonic instruments, and when 

 higher frequencies are used they are called 

 supersonic or ultrasonic instruments. In 

 practical application equipments using fre- 

 quencies of 20 kilocycles or less are called 

 sonic. The frequencies most commonly used 

 for echo sounding can also be divided into 

 three groups : low frequencies, or those below 

 15 kilocycles; medium frequencies, or those 

 between 15 and 50 kilocycles ; and high fre- 

 quencies, or those above 50 kilocycles. At 

 its designed or resonant frequency the trans- 

 ducer is most efficient in producing sound 



