5542 HYDROGRAPHIC MANUAL PaGE 516 



The 1,000-fathom dial has a built-in adjustment, but it is provided with insufficient 

 movement, so adjustment must be made by loosening the screw E in the clamp that 

 engages the collar which is pinned to the shaft and turning the entire assembly to ad- 

 just the index. 



Control knobs ^ to Z> are provided with lock nuts that serv^e to hold the index in 

 adjustment and these have to be loosened before adjustment and retightened after- 

 wards. 



When making adjustments with knobs C and D, the photoelectric tubes must 

 be kept within the illumination of the exciter lamps. If the proper index adjustment 

 cannot be so made, proceed as follows: Turn the adjusting knob until the photoelec- 

 tric tube is in the center of the illuminated area. Loosen the screw that clamps the 

 shutter disk to the collar keyed to the shaft, and turn the disk until the neon tube 

 flashes at the desired point on the depth scale. The adjusting knob can then be used 

 for the final index adjustment. 



Caution must be observed when sounding with this instrument to ensure that the 

 correct starting-frequency and depth-selector switch combinations are used. Failure 

 to use the correct combination of these switches will result in erroneous soundings. 

 The correct combination is determined at the time of adjustment of the instrument and 

 normally should be as follows: The depth-selector switch positions 1,2,3, and 4 must 

 be used only with starting-frequency switch position 4, and depth-selector switch po- 

 sition 5 only with position 5 of the starting-frequency switch. 



The index adjustment of the Dorsey Fathometer No. 3 may be made by either of 

 two methods. Method 1 should be used wherever practicable and particularly for 

 surveys in regions of regular bottom such as are quite often found on the Atlantic and 

 Gulf Coasts. Method 2 may be substituted where method 1 is impracticable, especially 

 for surveys in regions of irregular bottom such as are generally found on the West Coast 

 and in Alaska waters. In both methods the index of the 20-fathom scale is first ad- 

 justed to register the algebraic sum of the instrumental error, draft, settlement and squat 

 by means of adjusting knob A in figure 111. The depth-selector switch must be on 

 position 1 and the starting-frequency switch on position 4 while making this adjustment. 



Then in method 1, the indices are adjusted until the depths registered on one scale 

 agree with those registered on another as outlined below. But in method 2, the indices 

 are merely adjusted to corresponding values, without reference to the depths. 



Method 1: After the index of the 20-€athom scale has been adjusted, the other circuits are adjusted 

 by a comparison of soundings. With the ship stopped in appropriate depths, where the bottom is 

 smooth and level, the instrument may be switched from one dial setting to another by means of the 

 depth-selector switch and the depths registered made to agree by use of the appropriate adjustment 

 knob. Table 19 outlines this method of adjustment. 



For example, if it is desired to adjust the index of the 100-fathom scale, the depth-selector switch 

 should be set on position 3 and the starting-frequency switch on position 4. Using adjusting knob C, 

 in figure 111, the depth registered on the 20-fathom dial should be adjusted to agree with the depth 

 registered on the same dial when the depth-selector switch is turned to position 1, with the starting- 

 frequency switch still on position 4. This adjustment should be made in a depth of water between 

 30 and 50 fathoms. In making this adjustment it is essential, as stated previously, that the photo- 

 electric tubes remain fully in the beam of light. 



Method 2: After the index of the 20-fathom scale has been adjusted, with the starting-frequency 

 switch still on position 4, the indices for positions 2, 3, and 4 of the depth-selector switch are adjusted 

 by means of adjusting knobs B, C, and D, in figure 111, to agree with the index for position 1 of the 

 depth-selector switch. 



