http://www.algone.com/salinity.php
"The hydrometer works on the principle that a solid body displaces its own weight of the liquid in which it floats. The hydrometer is calibrated at 60˚F (15.55˚C) in which distilled water equals 1.000 as the initial point. The readings will rise with increasing density of the water.
As the instrument is calibrated at a temperature of 60˚F (15.55˚C), it requires that the water to be tested will also have a temperature of 60˚F (15.55˚C) in order to get an accurate reading."