5. HYDROGRAPHY 



179 



changed to 2 fathoms thus introducing an 

 error of 10 feet. A change in mode of op- 

 eration does not affect the position of the 

 initial mark on the graph. 



5-110 Bar checks. — Reliable bar checks 

 (see 3-109) can be made only under the 

 most favorable conditions. When the sea is 

 calm and there is little current to displace 

 the bar from a position vertically below the 

 transducer, bar checks can be obtained in 

 depths as great as 15 fathoms, but under 

 less favorable conditions the maximum bar- 

 check depth may be as little as 2 fathoms. 

 In moderate depths where bar checks and 

 vertical cast comparisons can be obtained 

 over the full depth range, it is possible to 

 determine the corrections to be applied for 

 the difference between the calibrated velocity 

 of the instrument and the actual velocity of 

 sound in the water. 



All launches and small boats using echo 

 sounders in hydrographic surveying shall 

 make and record bar checks under the fol- 

 lowing conditions and at the following 

 frequencies : 



(a) In protected waters where there is 

 every reason to believe the results of the bar 

 checks are dependable, and the range of 

 depths sounded can be covered by the bar- 

 check range, bar checks shall be made on 

 the descent and ascent of the bar at each 10 

 feet throughout the range of depths sounded 

 three times daily — at the beginning and 

 end of the day's work and once near the 

 middle of the day. 



(b) Where most of the depths sounded 

 are beyond the range of the bar check, other 

 conditions being as in (a), one check shall 

 be made at the deepest depth at which a 

 dependable result can be obtained, and one 

 check shall be made at a depth in the over- 

 lap between phases — three times a day as 

 in (a). 



(c) In exposed waters, where dependable 

 results cannot be obtained, a bar check shall 

 be made at 10 feet or 2 fathoms (preferably 

 in a protected place) at least twice, and 

 preferably three times daily — but no other 

 bar checks shall be made. 



(d) The index or initial shall be set to 



coincide with a line on the fathogram which 

 closely approximates the draft of the trans- 

 ducers when sounding in feet. The initial 

 should be set and maintained on the zero 

 line on all launches when sounding in fath- 

 oms, or when it is necessary to sound part 

 of the area in feet and part in fathoms. 



(e) Where the results of bar checks are 

 to be used for correcting or compensating for 

 the velocity of sound, not less than three 

 complete bar checks per day shall be made, 

 the bar check must cover at least 75 percent 

 of the range of depths sounded, and the bar 

 checks must be most accurate and depend- 

 able (see 5-115). 



When an 808 fathometer is used, a speed 

 count should be observed and recorded. The 

 stylus arm should make 123 revolutions in 

 66 seconds in the fathom mode when the cali- 

 bration velocity is 820 fathoms per second 

 and with the middle reed of the tachometer 

 vibrating at maximum amplitude. For a 

 calibrated velocity of 800 fathoms per sec- 

 ond the arm should make 120 revolutions in 

 66 seconds. A well-regulated stop watch shall 

 be used when making this check (see 3—91). 

 When the speed count fails to agree with 

 these standards, the paper speed test de- 

 scribed in 5-111 should be made. 



If an EDO-255 echo sounder is used the 

 frequency meter reading shall be recorded. 



The gain setting shall be recorded for 

 each depth of the bar. 



When making a bar check in depths of 

 20 fathoms or less, a leadline sounding shall 

 be taken and compared with the echo sound- 

 ing observed simultaneously. 



All bar check data shall be recorded under 

 Stamp No. 35. 



5-111 Sources of errors in 808 echo 

 soundings. — The circular sweep system of 

 recording and the mechanical method of 

 phase shifting employed by 808 type echo 

 sounders introduce sources of error which 

 are not found in other designs. The 808 

 fathometer is described and its operating 

 characteristics are stated in Chapter 3. Un- 

 less the instrument is adjusted and operated 

 in accordance with those specifications, the 



