The Hydrometer
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 CO_{2} in the container. To
do this we use this formula:
Weight of CO_{2} =_{ }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 CO_{2 }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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