When the temperature of a liquid changes, its specific gravity also changes and the indication of a hydrometer in the liquid will, therefore, differ at different temperatures. All hydrometers should be marked with the temperature at which they are intended to be correct.

In actual practice, however, it is not always necessary that the hydrometer be used at its standard temperature, but if readings are to be compared, they must all be made at the same temperature. For example, it may be found by experience that a certain sirup has the proper consistency when cool, if the reading on the hydrometer is 1.36 at 80° F. The standard temperature of the instrument may be 60° F., but if experience has shown that a certain reading of the hydrometer at some other temperature gives satisfactory results, it is not necessary to wait for the liquid to cool to the standard temperature of the instrument.