555 HYDROGRAPHIC MANUAL PaGE 520 



When it is expected that soundings in both feet and fathoms will be recorded 

 between bar checks the following procedure shall be followed: After the adjustment has 

 been made with the instrument operating for soundings in feet, without moving the bar, 

 the speed should be changed to soundings in fathoms and a record made of the trans- 

 mitted signal. The difference between the latter and the former shall then be sub- 

 tracted arithmetically, or graphically, from all soundings recorded in fathoms. This 



amount shall be noted on the fathogram as "Subtract fms. from soundings in 



fathoms." 



When soundings in fathoms only are to be recorded, the original adjustment may be 

 made at the fathom speed, so that the recorded position of the transmitted signal is 

 correct when scaled in fathoms. 



555. Motor Speed 



The depth scales of all echo-sounding instruments are calibrated for a selected velocity 

 of sound in sea water. For instruments used by the Coast and Geodetic Survey this 

 calibration velocity is generally 820 fathoms per second. The speed of the electric 

 motor used to operate the instrument is related to the selected calibration velocity and 

 the depth scales of the instrument. For correct results this speed must remain constant 

 at the correct value. Any variation in motor speed will cause a proportional error in the 

 registered soundings. In order to keep such an error at a minimum it is essential that 

 the speed-governing or speed-indicating device be checked periodically. 



555 J. Verification of Tuning-Fork Frequency 



The indicator motor speed of the Dorsey Fathometer Nos. 1 and 3 is controlled 

 by a tuning fork whose frequency is essentially constant. However, the frequency of 

 the tuning fork must be verified at least once each year and if it has changed it should 

 be corrected. It should be verified by one of the two following methods: 



Method 1: This method is used by vessels operating within the range of the radio station thar 

 transmits calibration signals. The signals are transmitted at the exact rate of 20.5 short pulses pet 

 second, the rate being accurately controlled. To verify the frequency of the tuning fork, the radio 

 signals are applied to the Fathometer so as to flash the 20-fathom dial indicator neon tube, as follows: 



(a) Disconnect the output of the echo amplifier from the grid circuit of the type 885 gas discharge 

 tubes that flash the indicator neon tubes, and connect the output of the radio receiver to this grid 

 circuit. If the output impedance of the radio receiver is low it may be necessary to use a step-up 

 transformer between the radio receiver and the grid circuit of the type 885 tubes. There must be a 

 condenser (about 0.1 microfarad) in series with the lead going to the grids of the type 885 tubes, to 

 prevent shorting their bias voltage to ground. In the case of the Dorsey Fathometer No. 3, the lead 

 going to the indicator, numbered 25 on the terminal strip in the echo-amplifier cabinet, should be 

 disconnected from the terminal and connected to the output of the radio receiver. 



(6) Run the Fathometer at least 15 minutes before calibration. For the Dorsey Fathometer No. 

 3 the depth-selector switch should be on position 1 and the starting-frequency switch on position 4. 

 Remove the type 59 oscillator tubes from the power supply. 



(c) The radio calibration signals will produce flashes similar to echoes on the 20-fathom dial. If 

 the tuning fork is vibrating at the correct frequency (1025 cycles per second) the flashes will appear 

 stationary at some one point on the dial, but if it is off frequency they will move around' the dial, in a 

 counterclockwise direction if the frequency is low and in a clockwise direction if it is high. If the flashes 

 make more than one revolution around the dial in 2 minutes, the frequency of the fork should be ad- 

 justed (see 5552). 



Method 2: This method should be used to verify the tuning-fork frequency of a Dorsey Fatho- 

 meter No. 3, when beyond the range of the radio calibration signals. A break-circuit chronometer is 

 employed in such a way that a flash is produced once each second on the dial of the Fathometer. The 

 direction and speed at which these flashes move around the dial will be a measure of the tuning-fork 

 frequency. 



