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A hydrometer is used to track the fermentation progress by measuring the conversion of sugar to ethanol by the yeast which is known as attenuation. A hydrometer, as far as the brewer is concerned, determines the amount of alcohol content in a liquid by measuring the specific gravity or difference in gravity (density) between pure water and water with sugar dissolved in it. Density is the weight of an object divided by the volume it occupies.
Specific gravity is usually expressed to three decimal places or simply as "gravity points" or the last three decimals For example, ale with an original specific gravity of 1.059 can be described as having 59 "gravity points". Beers typically have a final gravity between 1.015 and 1.005. Before you use your hydrometer you should calibrate it in pure (I use distilled) water.  Your hydrometer should read 1.000 at the specified temperature when in pure water. If it reads either higher (1.001 or more) or lower (0.9999 or less), simply subtract or add the amount of the difference from your readings in wort or beer. For example, if your hydrometer reads 0.998 in pure water at 60°F (its calibration temperature) it is reading two "points" low and means that two "points" need to be added to any reading taken in wort or beer. In other words, if your wort reads 1.050, your corrected reading would be 1.052. Conversely if it read 1.002 in water, it is reading two "points" high and you would need to subtract two points from your reading, e.g. a wort or beer reading of 1.052 should be adjusted to 1.050.
Most hydrometer readings are standardized to around 60°F but you should check the documentation that came with your hydrometer to verify at what temperature it has been standardized. Liquid gravity (density) is dependent on temperature and temperature correction tables are usually included with the hydrometer. Its best to check the temperature and gravity at the same time. a great tool to do this is the thermohydrometer from PerfectBrewing.com for $12.99. If you checked the temperature of your sample with a thermometer then took it out and put in your hydrometer which could be warmer or colder than the liquid it could change the temperature just enough to give you an incorrect reading. Take a sample of wort from the kettle immediately after boiling. The wort is well mixed and nothing has been added, such as water for topping-off or yeast. Use a sanitized siphon or wine thief ($8.90) or even a turkey baster to collect a sample of wort/beer and put in a hydrometer tube ($4.99) or a tall narrow container. Check the temperature and place the hydrometer in the liquid giving it a quick spin to remove any bubbles stuck to its side. Make any adjustments necessary based on temperature or calibration and record your findings. The stated FG of a recipe is not the goal instead the goal should be to make a beer that taste good. The hydrometer is just a means to gauge the fermentation progress and you should only be concerned about a high hydrometer reading when primary fermentation has ended and the reading is about one half of the OG, instead of the normal one fourth. Don't make the mistake of checking the gravity too frequently as each time you open the fermenter, you are risking infection from airborne microbes. Check the gravity after the boil and leave it alone until the bubbling in the airlock stops. |
