5165 HYDROGEAPHIC MANUAL Page 454 



to one revolution can be computed if the velocity of sound in sea water is known. With the depth 

 scale appropriately graduated and the index revolving at normal speed, if a sound impulse is produced 

 in the water just when the index is correctly referenced to the zero of the scale, then the position on 

 the scale to which the index will have advanced, when the echo makes the neon tube flash, will indi- 

 cate the depth. The index slot revolves continuously at a predetermined speed depending on the 

 assumed velocity of sound for which the instrument is calibrated and the range chosen for the depth 

 scale. A sounding is normally made once each revolution of the index, and where the instrument is de- 

 signed for shoal sounding w ith a scale graduated to 20 fathoms, for instance, approximately 20 soundings 

 will be obtained per second. In this case, because of the persistence of vision, the index will appear 

 as a continuously illuminated line of light. For depth scales with greater ranges, where the index 

 rotates more slowly, a succession of single flashes will appear at a point corresponding to the depth. 



B. GRAPHIC RECORDER 



Graphic-recording devices register depths by recording marks on a paper with or without a printed 

 scale, at the beginning and end of a signal epoch. The record is made by a stylus that passes over the 

 surface of the paper at a constant velocity, and marks permanently on it the times of transmission 

 of the signals and reception of the echoes. The time intervals are measured either by a scale printed 

 on the record paper or by reference to a scale held against the paper. 



One of the principal problems in the development of recording instruments has been to find a 

 suitable paper and method of recording the soundings thereon. At various stages of the development, 

 methods involving all of the following have been used more or less successfully: smoked paper, wax- 

 coated paper, ink and paint traces, charring or perforating the paper, and chemically treated paper. 

 This last is used almost exclusively in European countries at present and is known as electrolytic 

 record paper. In the United States a black-bodied paper is used whose surface is covered with a 

 light-colored coating of special composition which disintegrates when an electric current is passed 

 through it, exposing the black background at that point. 



More detailed descriptions of graphic-recording devices are included in section 53, and in 523, 527, 

 and 528. 



5165. Motor Speed 



The accuracy of eclio-soundiiig instruments is largely dependent on a constant and 

 accurately known speed of revolution, either of the stylus of the graphic recorder or of 

 the index of the visual indicator. In modern equipment this revolution is ordinarily 

 produced by an electric motor whose speed is usually controlled within the limits of 

 accuracy required. 



a. Centrifugal governor — A centrifugal type of governor is commonly used for speed control. 

 Electric contacts on the governor mechanism control the motor current in such a way as to compen- 

 sate for any change of motor speed. Since a change of speed must occur before the governor can 

 function, this type of control at best can only maintain the proper average speed, whose instantaneous 

 rate is not known. However, a carefully designed and properly functioning governor will maintain 

 a speed within such narrow limits as to give the accuracy required in most hydrographic surveys. 



b. Tuning-fork control. — An alternating-current generator of constant frequency can be used to 

 operate a synchronous motor at constant speed, and at the same time nearly eliminate instantaneous 

 fluctuations of speed. Such generators, maintained at constant frequency by means of a tuning fork, 

 are used in some types of echo-sounding equipment in conjunction wdth a synchronous motor. Where 

 properly designed and operated in a stable electric circuit, the tuning fork will control continuously 

 the frequency of an alternating current, essentially independent of changes in supply voltage, load 

 torque, and temperature. This constant-frequency alternating current is then amplified and used 

 to run a synchronous motor at a constant speed, regardless of load or friction. The increased precision 

 resulting from the use of a synchronous motor controlled by a tuning fork, to drive the registering 

 device, is especially valuable in hydrographic surveying. However, such synchronous-motor systems 

 are more complicated than those that are governor-controlled and require the attention of trained 

 personnel. 



c. Tachometers. — A knowledge of the average motor speed is essential, especially where it is not 

 adequately regulated or where only partly controlled. Tachometers of various types are the devices 

 normally used to indicate motor speeds or motor-speed changes. 



