Final gravity hydrometer
A hydrometer is used to measure the density of liquids. The denser the liquid, the higher it floats.
Sugar is denser than water or alcohol so in brewing we use a hydrometer to measure the amount of sugar in wort and beer.
The standard measure of density we use is called 'specific gravity' (SG). Specific gravity is measured at several points during the brewing process: During sparging, before the wort is boiled, after the wort is boiled, before fermentation, and after fermentation (to name a few).
The gravity of the wort before fermentation is called the 'original gravity' (OG) and the gravity of the beer after fermentation is called the 'final gravity' (FG). The difference between the two is used to calculate the alcohol content of the beer.
Distilled water has a specific gravity of 1.000. A typical 5% ABV (alcohol by volume) beer usually starts as wort with an original gravity of around 1.045 - 1.050 and finishes fermentation at around 1.007 - 1.012. The larger the difference between the starting and ending gravities, the higher the alcohol content of the beer and the lower the final gravity, the dryer the beer.
For greater accuracy, smaller range hydrometers are available such as a final gravity hydrometer (shown in picture), meant for taking readings in only the 0.980 to 1.020 range. A hydrometer test jar is also needed in order to float the hydrometer in the liquid sample.
Make sure to convert for temperature when taking readings and always calibrate any hydrometer the first time it is used (more information).
Interested in real time / automated gravity measurement while fermenting? Consider the Tilt Digital Wireless Hydrometer and Thermometer.
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