180 



HYDROGRAPHIC MANUAL 



soundings will be inaccurate and corrections 

 must be made in the sounding record. 



The principal sources of error are: (1) 

 incorrect paper speed and speed of rotation ; 

 (2) incorrect radius of rotation of the stylus 

 arm; (3) incorrect alignment of the paper^ 

 and (4) improper phase shifting. 



There are two methods of checking the 

 operating speed of the instrument and the 

 correct balance of the tachometer. The first 

 is the speed count described in the previous 

 section. The second check is made by run- 

 ning the instrument in the feet mode and 

 measuring the rate of paper movement. With 

 the instrument properly warmed up, make a 

 fix mark on the paper and start an accurate 

 stop watch simultaneously. Exactly 4 min- 

 utes later scribe another fix mark on the 

 graph; remove the paper and measure the 

 distance between fix marks along the center- 

 line of the paper. For a calibrated velocity 

 of 820 fathoms per second, the paper should 

 have traveled exactly 8 inches; and for a 

 calibrated velocity of 800 fathoms per sec- 

 ond, the paper should have moved 7.8 inches. 

 The middle reed of the tachometer should 

 be vibrating at maximum amplitude during 

 such tests. If the speed count of the stylus 

 arm or the rate of advance of the paper fail 

 to agree with the correct speed or rate after 

 repeated tests, the tachometer should be re- 

 turned to the office for recalibration. 



Although variations in motor speed are 

 indicated by the changes in the rate of paper 

 travel, care should be taken that paper slip- 

 page is not misinterpreted as a change in 

 motor speed. The spring-belt drive on the 

 paper rewind roller should maintain proper 

 tension so that the paper is not pulled 

 through the sprocket roller. Torn sprocket 

 holes or loose paper are indications of im- 

 proper paper speed not attributable to the 

 motor speed. 



The fathogram scanner should check the 

 paper speed against the clock time recorded 

 in the sounding record. If the paper travel 

 is incorrect, the soundings shall be corrected 

 by the percentage variation from the normal 

 travel speed. Before applying speed correc- 

 tions, it should be determined that the ab- 



normal rate of travel is not due to paper 

 slippage. Proper notes on the fathogram, 

 or in the sounding record, should be made so 

 as to preclude improper consideration of 

 these discrepancies. 



Adjustment of the stylus arm length is 

 described in 3-92. Tests should be made and 

 adjustments completed before the instrument 

 is used for hydrography. The operator 

 should make frequent checks of this adjust- 

 ment by causing a fix mark to follow a 

 printed arc on the fathogram. If the arcs 

 coincide, the adjustment is correct. If the 

 two arcs coincide at both ends of the printed 

 arc, but not at the center, the needle should 

 be adjusted. 



If the arcs coincide at only one end of the 

 printed arc, the paper alignment should be 

 adjusted. Any large variation in the paper 

 alignment (see 3-93) is likely to cause er- 

 rors in the recorded soundings which are 

 difficult to correct. 



5-112 808 phase corrections. — The fatho- 

 grams used in 808 fathometers have four 

 printed scales of 55 fathoms (or feet) in 

 each scale. An overlap of 20 units is pro- 

 vided on successive scales. A phasing head 

 is provided for changing scales as neces- 

 sary to record the returning echo. Theoret- 

 ically, a sounding in the overlap area of suc- 

 cessive scales should be the same on either 

 scale. In practice this seldom occurs and 

 the differences, or phase corrections, must 

 be determined by comparison. Because of 

 mechanical imperfections in the phase shift- 

 ing mechanism it is imperative that phase 

 changes shall always be made by rotating 

 the phasing head in a clockwise direction as 

 the locking pin falls into place. When shift- 

 ing to a deeper phase, rotate the head in a 

 counter-clockwise direction with the locking 

 pin held against the spring until the pin is 

 to the left of the socket. Release the pin and 

 turn the head in a clockwise direction until 

 the pin falls into place and the head will 

 turn no farther. 



Phase comparisons shall be made at the 

 beginning of the season and once each month 

 during the season in the overlap zones of 

 the phases (scales) in use (see 3-94). The 



