Simply put, a hydrometer is a home wine maker’s best friend. The hydrometer should not be confused, which occasionally it is, with a hygrometer. (This specifically is used to measure actually humidity levels.) Typically, a hydrometer is made from glass in the shape of a tube and small ball, and is filled with either lead or mercury. Believe it or not, both of these substances allow the hydrometer to float upright in liquids.
I’m sure many of you are asking at this point, “Well what do you use the hydrometer for?” The hydrometer is used to measure the actual density of any given liquid relative to the constant density of water. The liquid that needs to be tested is usually poured into a bucket and then the hydrometer is placed in the liquid. After a few moments, the hydrometer begins to float on its very own. There is usually a light paper scale that is attached to the very top of the device that give’s a reading as to what the actual density of the material is relative to waters density.
“Why is this important?” The hydrometers readings can be used to determine how “heavy” an alcoholic beverage is compared to a common substance which is water. The readings can also be used to calculate out the approximate amount of alcohol content that is in your beverage without using an actual alcoholometer. (The alcoholometer is the industry approved standard however for determining the actual alcohol content of a liquid.) Not only the important factor of alcohol content is able to be calculated, but by using a hydrometer, you will be able to determine if your batch is completely fermented.
As explained before, to use the hydrometer, you simply fill up a bucket with the liquid you wish tested and place your hydrometer inside. Let it set for a few moments and begin to float, at this time write the number down that appears on the paper scale. (Most accurate readings are achieved when keeping the substances temperature at 68 degrees). To determine if your alcohol is completely done fermenting, simply take a reading 2 days apart, if the numbers are unchanged then you’re done with the fermenting cycle.
However, if the numbers change significantly, then keep your newest number and allow another two days of fermenting. After fermentation is complete you can take your final number and using this equation, calculate your alcohol content. Using this example the reading you took was 1.08.      1.08 - 1 = .08 X 125 = 10    this final answer of ten is you’re actual content of alcohol. If this is your first batch, then congratulations, you’ve just made an alcoholic beverage with a 10% concentration.