Learning How to Use a Hydrometer

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hydrometer homebrew

Determining Density, Gravity and Alcohol Content in Beer and Wine

So why should homebrewers, winemakers, cidermakers, and meadmakers learn how to use a hydrometer? The dual functions of a hydrometer are to determine the density of liquids and estimate the alcohol content in fermented beverages.

All About the Hydrometer

A hydrometer is a fragile measuring device that is long, cylindrical, narrow and weighted at the bottom. With the weighted end submerged in the liquid, the calibrated stem projects out of the liquid at a height determined by the density of that liquid. Readings from the hydrometer are taken at the liquid’s surface.

Triple scale refers to the three different measuring scales within the hydrometer. Two of them — the Specific Gravity scale and the Balling scale — measure liquid density, while the third scale measures potential alcohol (homebrewers seem to prefer the specific gravity scale). The first two scales measure the density of liquids relative to water. Ordinary water, at 60° F has a specific gravity of 1.000.

  • For comparison’s sake, at the same temperature, gasoline has a specific gravity around 0.66, whole milk is about 1.028, and mercury is 13.600!

The Balling scale performs the exact same function as the specific gravity scale, except that it reads in different incremental numbers. On the Balling scale, the gradations are called degrees Plato. A homebrew with a Specific Gravity of 1.048 has an equivalent density of 12.5 degrees Plato. (These two scales are relative to each other in the same sense that both Fahrenheit and Celsius scales can be used to measure air temperature)

Taking the Measurements

hydrometer homebrew

The purpose of measuring the density of your brew accomplishes two things: it tells you when your brew is done fermenting (thus, when it’s time to bottle your beer) and it allows you to calculate the alcohol potential of your brew. Of course, potential and actual are two different things, which is why you need to take two different hydrometer readings.

Taking a hydrometer reading before the wort is fermented gives you the original gravity; likewise, it tells you the alcohol potential. When you take a second hydrometer reading after fermentation is over (whether a few days or a few weeks later), you get a final gravity -also called a terminal gravity- and a final alcohol potential. Subtracting the final gravity from the original gravity or the final alcohol potential from the initial alcohol potential tells you, in simple mathematical terms, how much of the sugar was eaten by the yeast or how much alcohol is in your brew.

Example: initial alcohol potential is 6, final alcohol potential is 2; 6 – 2 = 4 % alcohol.

Remember: when yeast eats sugar it produces alcohol, so any decrease in gravity results in a reciprocal increase in alcohol.

A Few More Important Points

  • If the liquid temperature at the time you take your density reading is not around 60° F, the numbers will be skewed. Hot wort readings will be lower than they should be and cold bottling readings will be higher than they should be due to the temperature-sensitive density of the liquid. Therefore, it’s always best to measure your beer at or near 60° F.
  • Alcohol potential readings are measured in alcohol by volume (ABV), not alcohol by weight (ABW).
  • A hydrometer can also call attention to possible fermentation problems. Average healthy yeasts consume at least 65 percent of available sugars –and usually more; if the final gravity reading of your beer is not 35 percent or less of the original gravity; .048 (1.048) x .35 = .0168 (1.017), there may still be too much sugar left in the beer to bottle it yet.

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