3. EQUIPMENT AND INSTRUMENTS 



99 



watercup (Fig. 36) is an efficient instru- 

 ment for collecting water samples at any 

 depth. It is similar to the Tanner-Sigsbee 

 reversing thermometer frame in the way it 

 is attached to the sounding line and in that 

 a propeller is employed to hold the valves 

 closed. There are two poppet valves on the 

 watercup, one at the top and one at the 

 bottom, which are connected by a rod through 

 the axis of the cylinder so that the valves 

 open and close simultaneously. The propeller 

 is mounted at the top and revolves freely 

 on descent allowing the valves to remain 

 open and water to pass through the cup. 

 The valves close when the downward motion 

 ceases, and, as the cup is drawn upward, the 

 propeller engages a clutch which clamps the 

 valves in the closed position. When drain- 

 ing the water into the hydrometer jar, the 

 locking device should be unscrewed all the 

 way and the valve lifted quickly with the 

 fingers, otherwise the water will spray in all 

 directions around the jar. 



3-125 Hydrometer sets. — The method 

 usually used by the Coast and Geodetic Sur- 

 vey to determine the salinity of seawater is 

 to measure the specific gravity of the water 

 sample with a hydrometer. If the specific 

 gravity is known, the salinity may be found 

 by the use of tables or a graph. The tem- 

 perature and specific gravity of a water 

 specimen are measured with a hydrometer 

 set consisting of a hydrometer jar, a labora- 

 tory centigrade thermometer, and a set of 

 three hydrometers graduated for different 



Figure 37. — Hydrometer set. 



ranges of specific gravity. These are illus- 

 trated in Fig. 37. 



A glass hydrometer has a small graduated 

 stem and a bulb counterweighted for the 

 range of graduations. They are very fragile 

 instruments and must be handled carefully 

 to prevent breakage. Hydrometers are fur- 

 nished in sets of three, graduated for the 

 following ranges of specific gravity: 0.9960 

 to 1.0110; 1.0100 to 1.0210; and 1.0200 to 

 1.0310. Hydrometers are compared with 

 standards maintained at the Washington 

 Office, and no hydrometer will be furnished 

 which is not correct to 0.0002 of specific 

 gravity. Observed specific gravity may be 

 used without correction for calibration 

 errors. 



Hydrometers are graduated to indicate 

 specific gravity at a standard temperature 

 of 15° C, referred to the specific gravity of 

 fresh water at a temperature of 4° C. as 

 unity. Salinity tables are computed on this 

 basis. 



3-126 Specific gravity measurements. — A 



sufficient quantity of the water specimen 

 should be poured in the jar to float the hy- 

 drometer without touching the bottom. The 

 jar may be placed on a level bench or shelf, 

 or held in the hand if there is excessive 

 rolling. If held in the hand, it should be 

 grasped near the top and held between the 

 thumb and one finger so that it can swing 

 freely. Float the hydrometer and rotate it 

 slowly, insert the thermometer and watch the 

 mercury column until it comes to rest. The 

 hydrometer should float freely without touch- 

 ing the sides of the jar. With the eye nearly 

 level with, but slightly below the water level 

 in the jar, the surface of the water will be 

 seen as a straight line intersecting the grad- 

 uations on the hydrometer stem. The water 

 temperature should be observed and recorded 

 at once. Disturb the hydrometer and repeat 

 the observations as soon as it comes to rest. 

 If the samples are to be preserved for 

 laboratory analysis, they should be stored in 

 citrate bottles properly labeled and identi- 

 fied. Observed densities and temperatures 

 may be converted to salinity by reference to 



