All hydrometers are calibrated to be accurate at a given temperature. Most newer hydrometers are calibrated for 68 degrees (F). Because the density of fluids changes as their temperature changes if you don’t measure your specific gravity at your hydrometers calibration temperature you’re going to get an inaccurate reading.
While the correction may be small for a single reading if you don’t correct it it becomes increasingly difficult to compare readings over time. For instance, if you take an original specific gravity reading at 75 degrees (F) and a final reading at 65 degrees (F) you can’t compare these readings because the fluid densities are different at the different temperatures. You must correct each of your readings for temperature differences before you can compare them.
Specific Gravity Temperature Correction Calculator
This calculator makes temperature corrections easy to figure out. Be sure to check and see what your hydrometer is calibrated for, while most hydrometers are calibrated to be accurate at 68 degrees (F) / 20 degrees (C) not all of them are.
The calibration temperature for your hydrometer should be printed on the scale. If it’s not printed there you can calibrate it yourself by putting your hydrometer in water samples at difference temperatures and seeing when it reads 1.000. That will be your calibration temperature.
The formulas used to make the temperature corrections are based on water density data presented in the Handbook of Chemistry and Physics 53rd Edition [http://www2.volstate.edu/CHEM/Density_of_Water.htm].