Page 473 i!:cHO sounding 524 



(m) Faulty echo-amplifier gain-control operation — caused by salt water running down the gain- 

 control shaft. This may be prevented by sliding a rubber sleeve over the gain-control shaft so the 

 water cannot get to the gain-control resistor inside the cabinet. 



(n) Lack of high d-c voltage from either of the generators — may be caused by worn brushes or a 

 dirty commutator on either the motor or generator end of the unit. 



(o) Leaky cable connectors — caused by salt encrustations. The interior part of the connector 

 plugs should b3 removed and cleaned. Future trouble of this nature can be prevented by. wrapping the 

 plugs with rubber tape at the joints. When the fish is stowed on the open deck a cap provided for the 

 purpose should be screwed over the ends of the plugs to prevent salt water entering the openings. 



(p) Disagreement of soundings recorded on different depth phases, in depths falling within 

 the overlap between these phases. This is because of mechanical imperfections in the phasing 

 mechanism. The amount of this discrepancy should be determined by an accurate bar check (see 557) 

 in smooth water, or by comparative soundings from a stationary vessel at a place where the bottom is 

 smooth and flat and in calm water. The results of the tests should be adequate for use in correcting 

 the soundings for any such errors. Corrections should be made by reference to phase A. Since any 

 such error is due to a mechanical fault, its effect on the recorded depths is six times as great for depths 

 recorded in fathoms as for those recorded in feet; that is, if the error for a certain phase on a certain 

 instrument is yi foot for a fathogram in feet, the error for the same phase and instrument will be % 

 fathom for a fathogram in fathoms. 



(q) The take-up fathogram roll is driven by a coil-spring belt from the shaft on which the sprocket 

 roller rotates. The tension of this belt is critical. If the coil-spring belt breaks or loses its tension, 

 it must be replaced with a standard replacement part; neither any other type of belt nor a repaired 

 coil-spring belt can be used. The tension must be such that the paper will be tightly rolled on the 

 take-up roller, but so that the belt will slip and never cause the fathogram to travel faster than the 

 sprocket roller moves it. (See also 5233 and 5554.) 



524. DoRSEY Fathometer No. 1 



The Dorsey Fathometer No. ] is a shoal-water, supersonic, visual type of echo- 

 sounding instrument. Its depth range is from a few feet to 150 fathoms, although 40 

 fathoms is about the maximum to which it is normally used in hydrography. Numerous 

 features have been incorporated in the instrument to make it capable of measuring shoal 

 depths with great accuracy, especially for hydrographic surveying. The most important 

 of these features are: 



(a) The use of supersonic frequencies, facilitating shoal measurements. 



(6) A combined transmitting and receiving unit, known as a transceiver, to eliminate the separation 

 error. 



(c) High damping of the transceiver to suppress persistent diaphragm vibration, which is a 

 deterrent to shoal-water soundings. 



(d) An efficient high-gain echo amplifier, especialh' designed to be stable and capable of rapid 

 recovery from excessive input voltages. 



(e) Tuning-fork control of the indicator-motor speed to better than 99.9 percent of the correct 

 value, thus eliminating errors from this source. No adjustment of the motor speed is necessary. 



(/) Photoelectric tube keying to eliminate moving contacts with their inherent time variations. 

 (g) An expanded depth scale so that readings can be estimated to Ko foot under ideal conditions. 

 (h) Soundings at the rate of 20)4 per second are spaced only a few inches apart along the bottom, 

 even at high vessel speeds. 



The operation of the Dorsey Fathometer No. 1 is illustrated schematically in figure 

 104. The operating cycle in producing a sounding can be traced as follows: A beam of 

 light reflected from a rotating mirror strikes a photoelectric tube which, by its photo- 

 electric action in conjunction with associated circuits, activates a vacuum-tube oscillator 

 for a fraction of a millisecond. The amplified electric oscillations energize the magneto- 

 strictive transceiver, which transmits a supersonic sound signal. The echo is received 

 on the same transceiver and amplified by the echo amplifier to a sufficient voltage to 



4©.5382 — 44 32 



