Page 411 equipment and instruments 4751 



cannot be used on board ship, since it involves a balance; neither can the method of 

 hydrostatic weighing, a modification of Mohr's balance, for the same reason. The 

 Nansen total immersion hydrometer gives very accm-ate results but the method is 

 laborious, and requires considerable time for a determination and relative stability of 

 the vessel. The Pettersson chain hydrometer also requires excessive time but is 

 otherwise well adapted for field measurements. These instruments have another 

 serious disadvantage in that large water samples are required. 



4751. Hydrometers 



A glass hydrometer has a small graduated stem, and a bulb counterweighted for the range of 

 graduations on the stem. In order to reduce the length of the hydrometer stem for use in a water 

 sample of small quantity, hydrometers are furnished 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. This total range 

 is sufficient to include the specific gravity of any sample from fresh water to the most saline likely to 

 be encountered in sea water. The scales of the hydrometers are graduated to 0.0002 of specific gravity 

 and can be estimated to 0.00005. Hvdrometers are tested for accuracv bv the National Bureau of 



^■^ i H H IK 



Figure 89.— Hydrometer set. 



Standards and a calibration table is furnished. A copy of the calibration table is usually sent to the 

 field party when hydrometers are shipped, but if not, it should be requested from the Washington 

 Office. All specific gravity measurements must be corrected for errors of graduation before use in 

 the salinity tables. 



The hydrometers used by the Coast and Geodetic Survey are usually graduated to indicate 

 specific gravity at a standard temperature of 15° centigrade, referred to fresh water as unity at a 

 temperature of 4° centigrade. This basis is indicated on the hydrometer by the following inscription : 

 D. 15°/4° C. The salinity tables have been computed on this basis. 



Hydrometers may be graduated on a different basis and there are some in use at present that are 

 graduated on a basis of D. 60°/60° F. If these are used for specific gravity measurements, the readings 

 must first be corrected to refer them to the basis of D. 15°/4° C. before the salinity tables are entered 

 (see 6331). A statement must be made on Form 717, Record of Temperatures, Salinities, and Theo- 

 retical Velocities, that the specific gravities recorded have been so corrected. These corrections 

 can be obtained from table 28 of the "Standard Density and Volumetric Tables" (National Bureau 

 of Standards Circular No. 19). For the specific gravity range of sea water encountered in the surveys 

 of the Bureau the correction can be taken as — 0.0010. Only hydrometers with manufacturer's 

 numbers from 652 to 976 inclusive are graduated on the basis of D. 60°/60° F. 



Another type of correction must be applied to observed hydrometer readings before the salinity 

 tables are entered, namely, the correction for the variation of the water sample temperature from the 

 standard of 15° C. for which the tables are computed. This is described in 6331. 



