106 



HYDROGRAPHIC MANUAL 



Copenhagen water and other carefully pre- 

 pared standard solutions whose salinities 

 have been determined by repeated titrations. 

 The final calibration may be expressed in 

 graphic form at a large scale, or a salinity- 

 versus-resistance table may be computed. How 

 often the calibration must be renewed is un- 

 certain but should probably be done annually. 



When in use to measure salinities, one 

 cell, called the reference cell, is filled with 

 substandard sea water whose salinity has 

 been determined by careful titration and the 

 six measuring cells are filled with water from 

 the samples. About 20 minutes are required 

 to bring them to temperature equilibrium 

 before measurement can begin. One of the 

 measuring cells is then balanced on the 

 bridge with the reference cell, the resistance 

 reading is recorded and the procedure re- 

 peated with each of the six cells. Each sam- 

 ple is measured twice but in different cells. 

 Salinities are obtained from the graph or 

 tables using the resistance reading as an en- 

 tering argument. 



The reference cell and water are compared 



with a standard daily or more frequently if 

 the salinity drift requires it. 



When properly used the results are far 

 more accurate than titrations at sea, salinity 

 determinations can be made more rapidly, 

 and the equipment can be satisfactorily op- 

 erated with less-skilled personnel. 



3-135 Velocimeter. — The velocimeter is 

 an instrument devised by the National Bu- 

 reau of Standards for measuring the velocity 

 of sound in a liquid. Following this design 

 the Coast and Geodetic Survey has built an 

 experimental velocimeter, Model TR-2 (Fig. 

 45). 



The velocimeter consists essentially of a 

 sounding head or "fish," a conductor cable 

 to suspend the instrument, and a ship-board 

 indicator. Two small transducers and two 

 reflectors are mounted on one end of the 

 "fish" to form a sound path of fixed length. 

 A signal from the sending transducer trav- 

 els through the water to the receiving trans- 

 ducer where it is amplified to retrigger the 

 pulse generator and start the cycle again. 

 The system, called "sing around," continually 



Figure 45. — Velocimeter. 



