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Using a Hydrometer

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The process of taking a hydrometer measurement at home usually involves removing a portion of the fermenting liquid from the fermentation vessel, placing it in a graduated cylinder, and dropping in an analog device to take a measurement by eye. The brewer then must discard the sample so as to prevent contamination with the ongoing brew. This process introduces a seam for human error while realistically generating only up to a dozen or so measurements throughout a fermentation’s lifecycle.

Hydrometers measure the relative density of a liquid based on the principle of buoyancy. The lower the density of the liquid, the further part B will sink. A scale is placed on part A to show depth that the instrument has sunk. The heavier a liquid, the higher the hydrometer will float.

Different hydrometers are calibrated for different liquids and ranges, and different scales are use for different industries. In brewing, a hydrometer is used to measure the amount of sugar in a brew both before and after fermentation. Before fermentation, the sugar content can be determined by a hydrometer to calculate the potential alcohol level of the final brew. After fermentation, another measurement is taken to determine the net loss of sugar, which can be used to calculate the amount of alcohol present in the final beverage. (Any sugar lost during brewing has been consumed by yeast to produce a proportional amount of alcohol).