Hydrometers measure the amount of sugar dissolved in water based upon water density. Most hydrometers are calibrated to read 1.000 at 60F (15C) in pure water. The presence of dissolved sugar increases the density of the water, which makes the hydrometer float higher.
Temperature is another variable that will affect water density. Temperatures higher than 60F, for example, will thin the density of the water and cause the measurements to be low.
This calculator compensates for temperature deviations from 59F. The correction formula is from Lyons (1992), who used the following formula to fit data from the Handbook of Chemistry and Physics (CRC): 1.313454 - 0.132674*T + 2.057793e-3*T**2 - 2.627634e-6*T**3. T stands for temperature in degrees Fahrenheit.
Lyons, C. (1992). Hydrometer temperature corrections. Home Brew Digest #963, Retrieved from http://hbd.org/brewery/library/HydromCorr0992.html
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