3. EQUIPMENT AND INSTRUMENTS 



107 



regenerates a sound pulse whose repetition 

 rate, or frequency, depends on the time it 

 takes for the pulse to move through the wa- 

 ter. The time required for the signal to pass 

 through the internal circuit is fixed and can 

 be determined by laboratory calibration. 

 Therefore, variations in time required to 

 make the complete cycle will be a function 

 of the speed of sound in the liquid. 



The frequency of completed cycles is shown 

 on an indicator aboard ship. The observed 

 frequency is used to enter a table or graph 

 from which velocity of sound at that point 

 is extracted. Close approximation of velocity 

 may be obtained by multiplying the fre- 

 quency reading by a constant for the ve- 

 locimeter. For Model TR-2, this constant is 

 0.20613. 



The fish is lowered by the electrical cable 

 which must be marked or run over a regis- 

 tering sheave so that readings can be ob- 

 tained at known depths. More complicated 

 systems contain depth measuring elements 

 and record velocity and depth on a tape. 

 Velocity measurements can be made as rap- 

 idly as the instrument can be lowered and 

 frequency readings recorded. 



Although the instrument has been tested 

 for a pressure of 16,000 pounds per square 

 inch, it is not used in depths where such 

 high pressure will be encountered. Its most 

 useful range is about to 50 fathoms. 



3-136 Improvements in oceanographic 

 instruments. — Bathythermographs record 

 temperature versus depth but to a limited 

 depth and with a temperature sensing ele- 

 ment which is not sensitive enough to record 

 the variations frequently encountered. The 

 scale is also very small. Various devices are 

 undergoing development to provide more ac- 

 curate and detailed temperature profiles and 

 to much greater depths. A thermistor is used 

 as the temperature sensing element in some 

 cases, thermocouples, and transducers in oth- 

 ers. Some instruments combine temperature 

 measuring and depth measuring units. 



Designs of oceanographic winches are be- 

 ing studied and improved. Methods of sam- 

 pling water and the bottom, underwater 

 photographic equipment, and instruments 



for measuring deep ocean currents are all 

 undergoing study. 



3—137 Drafting instruments. — Small 

 drafting instruments are an essential part 

 of the equipment of every hydrographic 

 party. Rolling of the vessel, exposure to spray 

 and salt air, and the variety of circumstances 

 under which they are used all contribute to 

 the likelihood of damage to or deterioration 

 of such instruments. Launch parties are fur- 

 nished a sturdy box for storing small In- 

 struments, pens, pencils, and ink when not 

 in use. Aboard ship such instruments when 

 not in use should be stored in appropriate 

 places and not left on the plotting tables. 

 They should always be kept clean, free from 

 rust, and in good repair. 



The supply of small instruments for a hy- 

 drographic party should include the follow- 

 ing: 



1 — Ordinary divider, 1 spacing divider, 



1 drop-bow pen, 1 pricker. 



1 — Ruling pen, 1 opisometer, 1 quarter 



meter scale and 2 or 3 small plastic 



triangles. 



3-138 Meter bar. — Scales used for con- 

 structing projections and for plotting sta- 

 tions or distances on hydrographic sheets are 

 graduated in the metric system. The stand- 

 ard scale is the meter bar (Fig. 46) so called 

 because its graduated scale is 1 meter in 

 length. It is made of German silver and 

 graduated with great precision. It is grad- 

 uated on one side at a scale of 1 : 10,000 and 

 on the reverse side at a scale of 1 :20,000. 

 All meter bars acquired in recent years have 

 been tested for accuracy and calibrated by 

 the National Bureau of Standards. A copy 

 of the calibration test will usually be found 

 inside the cover of the box in which the 

 meter bar is furnished. 



In addition to the meter bar, quarter- 

 meter scales, and eighth-meter scales are 

 available. These are not tested for accuracy, 

 and should be compared with a meter bar of 

 known accuracy before use. 



3-139 Beam compass. — The beam com- 

 pass consists of a light, inflexible bar of wood 

 or metal with a T cross-section, and two com- 



