571 



HYDROGRAPHIC MANUAL 



Page 544 



The following instructions apply generally to all fatliograms, but particularly to 

 the 808 Fathometer. The fathogranis illustrated to which reference is made in the 

 text were all recorded on the 808 Fathometer. 



a. Effect of gain adjustment. — A variation of the gain control causes a decided change in the 

 recorded depth. (See 57(a).) Tests of one 808 Fathometer in depths up to 50 feet have proved that 

 a change of one division in the adjustment of the gain control in the operating range (7 to 8) causes 

 a change of about one-half percent in the recorded depths; also, a change of about 0.1 foot in the 

 recorded position of the transmitted signal. As repeatedly emphasized in this chapter, the instrument 

 must be operated as much as possible at a constant gain setting — the maximum possible without the 

 introduction of excessive strays. Corrections should be computed for any variation of sensitivity 

 for recorded soundings in critical depths, if such corrections affect the soundings appreciably. 



No. 31 GRAPHIC RECORD 



Sheet No. P,^, .".?.-? 5? /. . . Recorder No. . .S. 9. ?. 7 A P. . . 



locality ^?.? F.9. ...^..^9.t. .M.9!P.'? 



Vessel. . . . ^.f.kU. ...^P.tp.C . . ^.9/!?r.. 

 From Pos. No. . . /.T.?. . .Date. .v'.V./.V . . JMi. I.^.fA. 

 To Pos. No.. ..?<5 /v.. Date. ...s/.^./.V. ../?.'.. {?.l^. 

 Jagged profile (not) caused by seas. 



■.l'.iS.:.U,^.?A'.'?.^. Operator 



Tide reducers by : . .V^.-. d . ?.. Checked by: . ."J: Ai N: . 



Figure 116. — Facsimile of stamp used on fathograms. 



Improper gain adjustment is illustrated at (5) in figure 117 by the prolonged record made by the 

 stylus after the receipt of the echo, and previously in the upper right corner of A by the excessive 

 width of the transmitted signal. In an attempt to obtain a more clear-cut record the amplifier gain 

 was increased excessively, causing oscillation of the amplifier. Excessive gain is also illustrated in 

 / in figure 119. Note the strays caused near (34) in figure 119 by this excessive gain. 



Four changes in gain adjustment are apparent in L in figure 119. At (38) the gain was increased, 

 as is evidenced by the wider and darker record of both the transmitted signal and the echo; a further 

 increase in gain resulted in oscillation of the amplifier at (37) ; a decrease in gain at (36) and an increase 

 at (35) caused a lightening and darkening, respectively, of the trace of both the transmitted signal 

 and the echo. 



6. Variation in motor speed. — A variation from the correct motor speed produces a directly 

 proportional error in the recorded soundings. It is, therefore, important for the hydrographer while 

 sounding, and the cartographer while plotting the smooth sheet, to check the travel speed of the paper 

 occasionally in accordance with 5554. 



Irregular speeds caused by poor governor control may result in a record very similar either to one 

 produced by echoes from marine growth or to one produced while sounding in a short regular chop. 

 A sudden sharp peak in the record may be caused by a momentary sticking of the governor con- 

 tacts of the naotor. Such a fault can cause' a record, as in (27) in figure 118, which resembles a pin- 

 nacle. Note that this is not a stray because the double echo is plainly evident at (28) and this evidence 

 would be lacking if (27) were a stray. Or such a fault can cause a sudden depression in the record. 

 Such a change in speed is readily detectable by ear, although the noise of the launch engine may make 

 it go unnoticed. When such a change is detected however, it should be noted on the fathogram. 



c. Faulty instrumental operation.- — A sudden sharp depression or elevation in the record of the 

 bottom, where accompanied by a change in the recorded position of the transmitted signal, is not a 

 hole or shoal on the bottom, but an instrumental fault probably caused by a sudden shifting of the 

 transmitting contact due to a looseness in the locating pin or shifting of the phasing head. Such an 

 instrumental fault is probably illustrated at (39), (40), and (41) in figure 119. Note that each time 

 this occurs in the record of the bar check it also occurs in the record of the transmitted signal. Further- 

 more, that the instrument is operating poorly is evidenced by the ragged appearance of the line 

 recorded by the transmitted signal. 



