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The hydrometer is an instrument which is constructed on the Archimedes principle that a solid body displaces its own weight of the liquid in which it floats. Hydrometers can be divided into two general classes; namely for liquids heavier than water and for liquids lighter than water. The
base hydrometer scale is Specific Gravity, in which distilled water equals 1.000 as the initial point. Liquids lighter than water are scaled below 1.000 specific gravity and liquids heavier than water are scaled above 1.000 specific gravity. Many other scales are commonly used, such as API, Brix,
Baume, Plato, etc. All of which are convertible into specific gravity by formula. Hydrometers are usually calibrated at 60°F/60°F. To
determine the density of a liquid, the liquid should be at 60°F. If the temperature varies, the liquid will either contract or expand, depending upon the temperature. Therefore, the density fluctuates with the temperature. Where there is a variation from the standard 60°F, corrections must
be applied to the hydrometer reading. To assure proper corrections, a separate accurate thermometer should be used, or a hydrometer in combination with a thermometer, which is sometimes referred to as a “thermohydrometer.”