4752 HYDROGRAPHIC "MANUAL PaGE 412 



4752. Hydrometer Jar 



The hydrometer jar is a cylindrical glass beaker of sufficient size to contain the water sample and 

 permit immersion of the entire scale of the hydrometer without allowing the hydrometer to touch the 

 sides or bottom of the jar. For use on board ship, a wooden base should be attached to the flanged 

 bottom of the hydrometer jar to decrease the likelihood of its capsizing during rough weather. A 

 6-inch square piece of heavy wood may be used, to which the hydrometer jar is fastened by means of 

 two straps that fit over opposite sides of the flanged bottom. A disk of sponge rubber, cut slightly 

 larger than the inside diameter of the hydrometer jar and inserted in the bottom, will protect the 

 hydrometer from damage and possible breakage when it is placed in the jar. When the jar is cleaned 

 after use, the sponge rubber should be removed and washed with fresh water and squeezed dry before 

 being replaced in the bottom of the jar. A little fresh water left in the sponge will cause errors in 

 subsequent measurements of specific gravities. 



4753. Specific Gravity Measurements 



Specific gravity measurements may be made with the hydrometer jar placed on a 

 level bench or shelf, which should be provided on shipboard near the sounding machine, 

 at an appropriate height; or they may be made with the hydrometer jar held in the 

 hand, if there is excessive vibration or rolling. In either case the details of procedure 

 are the same. 



A sufficient quantity of the water specimen should be poured into the hydrometer 

 jar to float the hydrometer without touching the bottom, but without overflowing. 

 The hydrometer is placed in the jar and allowed to come to rest. If held in the hand, 

 the jar should be held near the top between the thumb and first or second finger and 

 allowed to swing freely, while the other hand is used to steady the floating hydrometer 

 until just before the reading is made when it must be allowed to float freely. The 

 readmg is made at the undersurface of the water (tlu-ough the glass container and the 

 water), after the outside of the jar has been wiped free of all condensed moisture. 

 With the eye brought nearly level with the water, the sm-face of the water will be seen 

 as a straight line intersecting the graduations on the hydrometer stem at the specific 

 gravity. These observations should be repeated after disturbing the hydrometer 

 and again allowing it to come to rest. 



The specific gravity of a water specimen varies with the temperatm-e of the spec- 

 imen, and to reduce the apparent (observed) specific gravity to the standard for which 

 the salinity tables are computed (see 6331) the temperature of the water specimen must 

 also be measured. A laboratory centigrade thermometer with a range from 0° to 50°, 

 by which the temperature may be read to tenths of degrees, is provided for this pm*- 

 pose. The thermometer is inserted in th^ hydrometer jar and temperature readings 

 are taken simultaneously with the hydrometer readings without lifting the thermometer 

 bulb from the water. 



Several precautions are necessary dm-ing specific gravity measurements. The 

 hydrometer should float freely near the center of the water column, the bulb touching 

 neither the side of the hydrometer jar nor the thermometer. Any small air bubbles 

 clinging to the hydrometer bulb should be detached before the specific gravity is read 

 by rotating the hydrometer or by moving it up and down in the water. If the tempera- 

 ture of the air differs materially from that of the water specimen, the water should be 

 stirred and the temperature of the specimen read before and after the specific gravity 

 measurement, the mean of the two temperatures being recorded. 



Some difficulty may be experienced in reading a hydrometer on shipboard, and 

 accurate readings are often impossible when the weather is rough and the ship rolls 



