What is a hydrometer? According to Wikipedia:
hydrometer is an instrument used for determining the specific gravity of liquids. It is usually made of glass and consists of a cylindrical stem and a bulb weighted with mercury or shot to make it float upright. The liquid is poured into a tall jar, and the hydrometer is gently lowered into the liquid until it floats freely.
The point where the surface of the liquid touches the stem of the hydrometer is noted. Hydrometers usually contain a paper scale inside the stem, so that the specific gravity (or density) can be read directly in grams per cubic centimeter.
1. Fill the glass cylinder with a wort sample.
2. Put the hydrometer with the bulb end down. It will bob up and down in the sample. Note that the sample may overflow from the cylinder.
3. Assure that the hydrometer is not in contact with the sides of the cylinder and take the reading. Note temperature of the wort sample.
4. We need to correct for temperature by using the chart below.
The original gravity will be higher because of the sugar contained in it.
The final gravity will be lower because most of the sugars have fermented out and been converted to alcohol!
Hydrometers are calibrated at 60° F or 40° C so it’s important that you adjust your readings according to the table to the left to adjust for temperature variations.
Why do I need a hydrometer anyway?
The hydrometer will tell you what the alcohol level of your beer is. You compare the starting specific gravity (right after you pitch the yeast) to the final specific gravity (just before to bottle or keg).
After you pitch you yeast take a reading as shown above and write this number down.
This is your Original Gravity or O.G.
When the wort has finished fermenting (or you think it has) take another reading and write this number down.
This is your Final Gravity or F.G.
If you are satisfied that you’ve reached your final gravity based on your recipe, it’s time to calculate the alcohol level.
How to Calculate
the Percentage of Alcohol by Weight and by Volume
First we need to find the weight of the CO2 in the container. To do this we use this formula:
Weight of CO2 = O.G. – F.G.
Now we need to find the weight of the alcohol in the container. To do this we take the weight of the CO2 we just calculated and multiply by 1.05.
Alcohol in container = Weight of alcohol x 1.05
Now to find the percent alcohol in the brew we simply divide the Weight of alcohol by the amount of alcohol in the container.
Percent alcohol by weight (%AVW) = Weight of alcohol / Alcohol in container
The percentage of alcohol by weight (%ABW) is higher than the percentage of alcohol by volume (%ABV) because an equal mass of alcohol occupies more space than water would. So to convert from percent alcohol by weight to percent alcohol by volume you just divide by the density of alcohol which = 0.79.
Percent alcohol by volume (%ABV) = %AVW / 0.79
Another Method for calculating Percentages
To calculate the percentage of alcohol by volume we take our original gravity and subtract our final gravity, multiple that by 1.333, then multiply that by 100.
To calculate the percentage of alcohol by weight we take our original gravity and subtract our final gravity, multiply that by 105.
Calculating % Alcohol by Weight (%ABW)
(O.G. – F.G.) x 105
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