5553 HYDROGRAPHIC MANUAL PaGE 522 



If the frequency of the fork is too high, it should be lowered by filing with a fine 

 half-round file in the crotch of the fork. The fork need not be removed for this but 

 all the filings should be collected after the operation. File equal amounts from the 

 crotch on each side of the centerline. Count the number of file strokes so that equal 

 amounts will be removed from each side. Repeat the filing operation until the fork 

 is properly tuned. 



5553. Verification and Adjustment of 312 Fathometer 



The motor speed of the 312 Fathometer is indicated by a reed tachometer (see 

 5234). The tachometer shall be removed from the instrument and forwarded once 

 each year to the Washington Ofiice for calibration. During the field season the cali- 

 l)ration of the tachometer shall be verified at least once each month by the following 

 method and the result recorded in the Sounding Records. 



The transmitted signals of the 312 Fathometer will record on the chronograph 

 (see 673), when the latter is operated as for Radio Acoustic Ranging. The Fathometer 

 should be run for a warm-up period of about 30 minutes before making a test. If the 

 calibration of the Fathometer is for a velocity of sound of 820 fathoms per second, 4.1 

 signals per second should be transmitted at fast speed; and if the calibration is for a 

 velocity of sound of 800 fathoms per second, exactly 4 signals per second should be trans- 

 mitted. For the test the chronograph should be run at least 1 minute, the 312 Fath- 

 ometer being operated at fast speed, with its speed continuously adjusted to give 

 maximum vibration of the middle reed of the tachometer. 



If the tachometer is found to be in error, a reed other than the middle one may 

 indicate the correct velocity. Test other reeds at maximum amplitude until one is 

 found which most nearly indicates the correct velocity and operate the Fathometer 

 on this basis until the tachometer can be corrected. Measure the speed of index revo- 

 lution with this reed vibrating at maximum amplitude, and calculate the corresponding 

 Telocity of sound. Use this as the calibration velocity of the instrument in computing 

 the correction factors (see 5613) for all soundings taken under these conditions. 



Where a chronograph is not available, the middle reed of the tachometer may 

 be checked, with a fair degree of accuracy, by timing a certain number of revolutions 

 of the slow speed dial with a stop watch. It should be not^d that the tachometer is 

 equally effective at the slow and fast speeds of the Fathometer. The accuracy of the 

 stop watch should be verified by comparison with a chronometer of known rate. The 

 Fathometer should be run for at least a half-hour before testing the frequency of the 

 tachometer. The Fathometer should be operated at slow speed using the automatic 

 cutout so that a signal will be transmitted every alternate revolution of the index disk. 

 The gain of the amplifier should be reduced so that the echo cannot be heard. For the 

 test the echo-listening device should be used to listen to the transmitted signals. The 

 stop watch should be started at the beginning of some signal and, calling this zero, 204 

 signals should be counted and the watch stopped at the beginning of the 205th signal, 

 for an instrument calibrated for a velocity of sound of 820 fathoms per second. Through- 

 out this test the middle reed of the tachometer must be kept vibrating at maximum 

 amplitude. If the frequency of the middle reed is correct, the corrected stop watch 

 reading should be exactly 10 minutes. 



