1. Be sure the Hydrometer is clean and dry.
2. Use a smooth clear cylinder or jar that is dry or well rinsed with portion of sample.
3. Sample must be thoroughly mixed before testing, preferably by a stirrer which reaches the bottom of the sample container.
4. Immerse the Hydrometer slowly in liquid to a point below which it naturally sinks (not over 1/8").
5. Do not make reading until the hydrometer and liquid are at rest and free from air bubbles.
6. The temperatures of the hydrometer and liquid tested should be equal.
7. More accurate readings result when the temperature of the sample equals the temperature at which the hydrometer is graduated and approaches that of the surrounding atmosphere. When differences of temperature are unavoidable and vary from the standard, readings can be adjusted by use of correction tables.
8. The surface tension of the liquid can affect the hydrometer indication. The liquid for which a hydrometer is intended must be specified since readings will vary in liquids having the same density but different surface tensions.
9. Hydrometers of equivalent dimensions may be compared with each other even if the liquid used differs in surface tension from the specified liquid, but comparisons of dissimilar instruments, in such a liquid, must be corrected for the effect of surface tension.
10. To avoid errors due to spontaneous changes in surface tension, skimming and formation of surface films of impurities from apparatus, liquid or air, overflow the cylinder immediately before taking the reading.
11. Observe a point below the plane of the liquid surface and then raise the line of vision until this surface, seen as an ellipse, becomes a straight line. The point where this line cuts the hydrometer is the reading of the instrument.
12. Errors of parallax when reading thermometer scale can be avoided by aligning the near end of the mercury column, the portions on either side of the stem and that seen through the capillary so they appear to be a straight line. When the eye is in this position the line of sight is normal to the stem.
*Condensed largely from NIST Circular No. 16.
