MEASUREMENT OF SALINITY OF SEA WATER 



By Jerry H. Service, United States Coast and Geodetic Survey 



INTRODUCTION 



This publication has to do with methods available for use on board 

 ship for measuring the proportion of total dissolved solids in sea 

 water. The adoption of echo sounding for hydrographic surveys 

 made necessary a study of the speed of sound in sea water and its 

 variation with the depth and with the temperature and the salinity 

 of the water. The effect of salinity upon the speed of sound is not 

 great; a change of salinity through the whole range encountered in 

 ocean waters produces scarcely more than 1 per cent change in speed. 

 The accuracy requirements of this bureau demand, however, that the 

 hydrographer shall determine the salinity of the water in which 

 soundings are being made to the nearest gram per kilogram (part per 

 thousand) ; that is, to two significant figures. 



It becomes necessary, then, to provide one or more standard 

 methods that are simple of execution and that will give on board a 

 moving ship and even in unskilled hands an accuracy such that there 

 will be no doubt of the second significant figure of the result, and 

 preferably a range of uncertainty in the third figure of not more than 

 two or three units. 



Two inexpensive methods have been found that are satisfactory. 

 When the motion of the ship is not great, hydrometers of good quality 

 that have been calibrated have been found to give satisfactory results 

 when properly used; and for use in any ordinary weather suitable 

 for hydrography a chemical titration method has been worked out 

 that requires little or no previous experience for its successful 

 manipulation. 



The writer has calibrated the dipping refractometer to give salinity 

 of sea water. Although the apparatus involved is rather expensive, 

 so that probably it will not be supplied generally to the ships of the 

 bureau, it is quite simple and easy to manipulate and will give good 

 results on board ship even in the roughest weather. 



It seems probable that an inexpensive electrical conductivity method 

 can be worked out that will be easy of manipulation and give results 

 adequate for the needs of the bureau. At the present time, however, 



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