3. EQUIPMENT AND INSTRUMENTS 



81 



255 cannot be switched from feet to fath- 

 oms operation as has been the custom with 

 the 808 fathometer. When it becomes neces- 

 sary to change modes, the stylus drive motor 

 must be repositioned and the fathoms cali- 

 bration switch placed in the ON position. 

 The change shall be made by a qualified tech- 

 nician, and unskilled personnel are not au- 

 thorized to attempt it. The hydrographer 

 should plan his work so that changes, if 

 required, shall be kept to a minimum. 



3-88 Operating manual. — A technical 

 manual is furnished with each EDO-255 

 recorder and contains complete instructions 

 for servicing and adjusting the instrument. 

 Personnel assigned to operate the recorders 

 should be instructed in procedures for chang- 

 ing paper rolls and stylus needles as well as 

 normal operating procedures. All other serv- 

 icing or repair shall be done by electronic 

 technicians. A record of servicing and re- 

 pairs should be kept for each instrument, 

 and should accompany it if transferred to 

 another survey vessel or party. 



3-89 808 Fathometer.— The 808 Fathom- 

 eter (Fig. 27) is a semiportable, supersonic, 

 graphic-recording, echo-sounding instrument 

 designed for hydrographic surveying in shal- 

 low to moderately deep water from vessels of 

 all sizes. Its range is from about 2 feet be- 

 low the transducer to 160 fathoms. The 808 

 Fathometer has been the standard shoal wa- 

 ter sounder since 1939. The sounder records 

 on a graph through a stylus operating in a 

 circular sweep. The range of scale of the 

 record paper is 55 divisions, and by means of 

 a phasing arrangement with a 20-unit overlap, 

 three other ranges may be recorded thus pro- 

 viding for an expanded scale covering a range 

 of to 160 units, either feet or fathoms. 



A contract has been awarded to reengineer 

 the 808 Fathometer to incorporate several 

 new features which will improve its capabili- 

 ties and correct some of the operating de- 

 ficiencies of the old models. The specifications 

 are not fixed, and a detailed description is not 

 available. 



3-90 Operating characteristics of 808 



Fathometer. — The fathogram paper used on 

 808 depth recorders is designed for the fol- 

 lowing conditions of operation : 



Calibrated for velocity 



of sound. 

 Center reed of tachom- 

 eter. 

 Motor armature speed 

 Stylus arm speed, feet 

 Stylus arm speed, 



fathoms. 

 Time per revolution, 



fathom scale. 

 Effective length of the 



stylus arm (4.419 



inches). 

 Chart paper speed, 



foot scale. 

 Chart paper required . 



820 fm/sec 



67.1 cps . 



4,026 rpm 

 671 rpm . . 

 111.833 rpm 



0.5365 sec 



11.224 cm 



2 in/min . 



15, 15UE 

 or ES-9. 



800 fm/sec. 



65.5 cps. 



3,928 rpm. 

 654.6 rpm. 

 109.104 rpm. 



0.5232 sec. 



11.224 cm. 



1.95 in/min. 

 TCI-3008. 



3-91 Motor speed. — The accuracy of the 

 recorded depth is directly related to the 

 motor speed, and the motor must always ro- 

 tate at calibration speed. The motor speed 

 is controlled by means of a centrifugal type 

 governor attached to one end of the motor 

 frame. A Frahm vibrating-reed type of 

 tachometer provides a visual monitor of the 

 motor speed. The tachometer is composed of 

 seven reeds; the middle reed, vibrating at 

 maximum amplitude, indicates correct motor 

 speed for the calibrated velocity of sound in 

 seawater. Tachometers may be constructed 

 for any desired velocity, but only two are 

 used: 800 or 820 fathoms per second. The 

 fathometer operator must be constantly alert 

 and adjust the motor speed when necessary 

 to keep the middle reed vibrating at maxi- 

 mum amplitude (see 5-55). The fact that 

 the motor speed is correct should be recorded 

 at frequent intervals. Variations in motor 

 speed are indicated by the tachometer and 

 the paper speed, but the paper speed is not 

 always a true measure of the motor speed 

 and care shall be taken that paper slippage 

 is not misinterpreted as incorrect motor 

 speed. Daily checks should be made on paper 

 and speed and rpm count against a stop 

 watch, and recorded (see 5-110 and 111). 



3-92 Stylus arm length. — Accuracy of 

 recorded depth also depends on the rate of 

 travel of the stylus over the fathogram, and 

 the radial distance to the stylus contact must 

 be 11.224 cm (4.419 inches). An abnormal 



