557 HYDROGRAPHIC MANUAL PaGE 526 



the index must not be used as a reference for making compensatory adjustments, with 

 the instrument in operation. The method described in 5543 for adjusting the index of 

 the 312 Fathometer avoids any effect of the separation error, since the readings are 

 made while the index disk is turned manually and very slowly. In making index 

 adjustments, the introduction of an error due to separation must be avoided in the 

 Veslekari, the 312 Fathometer, and in the Dorsey Fathometer No. 3 should it be 

 adjusted using the sonic system only. For the proper methods of adjusting these 

 instruments, see 5544, 5543, and 5542. 



In using a graphic-recording instrument that utilizes a transceiver, or separate 

 acoustic units so close together that the separation is negligible, the depth measured 

 below the units is recorded on the fathogram by the difference in the recorded positions 

 of the transmitted signal and received echo. This is not so if there is an appreciable 

 separation between the acoustic units, because the transmitted signal will be recorded 

 too late by one-half the distance between the units in terms of the depth scale, and this 

 fact must be taken into consideration in scaling the fathograms. 



557. Bar Check 



The bar check is a method whereby the accuracy of an echo-sounding instrument 

 may be verified. It consists of lowering a flat bar horizontafly below the acoustic 

 units to serve as a reflector, thus forming a nearly perfect artificial bottom at an accur- 

 ately known depth, with which the registered depth can be compared. By using the 

 bar check, the index of an echo-sounding instrument operated at a constant velocity 

 may be adjusted to compensate for the draft of the acoustic units and the instrmnental 

 error without knowledge of their amounts (5545) ; and under certain conditions, for a 

 velocity of sound differing from the calibration velocity of the instrument (5617). At 

 the time of the bar check the index should be adjusted for settlement and squat if 

 appreciable and known (see 553). 



Rehable bar checks can be made only in a calm sea and are usually limited to a 

 depth of about 15 fathoms. With difficulty bar checks can sometimes be made to • 

 greater depths but the resiflts are likely to be unrehable. In a moderate or choppy 

 sea the maximum bar-check depth may be as little as 2 fathoms, owing principafly to 

 the difficulty of keeping the bar horizontal, and vertically below the acoustic units. 

 The minimum bar depth at which echoes can be recorded usually depends on the echo- 

 sounding apparatus. 



Because bar checks are most advantageously used in connection with shoal soundings, 

 the method is principally used to verify and adjust shoal-water semiportable echo- 

 soimding instruments, such as the 808 Fathometer and the Hughes MS 12 D. 



5571. Bar Check Apparatus 



The bar, used as a reflecting surface, is a rectangle supported at its ends by lines 

 in such a way that it is held in a horizontal position when submerged. The bar may 

 be of metal or wood, metal being preferred because it will sink of its own weight. The 

 bar must be rigid enough so that it does not sag and it must be rigged so that it can be 

 suspended in a horizontal position, at a known depth, and be kept vertically under the 

 acoustic units. The bar should be as long and wide and heavy as can be conveniently 

 handled, for then there wifl be less difiiculty in keeping it in position below the acoustic 

 units. For the 808 Fathometer the minimum size of bar which should be used is about 

 3 feet long, 9 inches wide, and ji inch thick if iron and % inch thick if wood. If numerous 



