One piece of gear that makes your Northern Brewer Starter Kit feel a little like a chemistry
set for grownups is the hydrometer.
In "Homebrewing 101" we used the hydrometer to measure the specific gravity of our beer,
before and after fermentation.
Let's dig a little deeper into how this thing works.
A physicist would tell us that the hydrometer measures the gravity of our wort and beer.
Another way to say that would be that it measures the density of our beer, relative to the density
of water.
A brewer might say instead, that the hydrometer tells us how much dissolved sugar is in our
wort or in our beer, thanks to the malt extract, grains, and other sugars added on brew day.
These measurements tell brewers a couple important things.
The first important thing hydrometers tell us is the beer's specific gravity.
The hydrometer in your Northern Brewer Starter Kit reads on three different scales.
Two of these scales measure wort gravity.
Since we're in the U.S. we are going to talk just about specific gravity.
Specific gravity readings tell us about the progress of fermentation.
Before fermentation starts, the amount of sugar in the wort is at its highest.
The specific gravity reading at this point, the original gravity, or OG, will also be
at its highest.
If we were to measure the specific gravity of our wort during primary fermentation, the
readings would steadily drop, relative to the OG number.
This is because the yeast is using up the dissolved sugar, converting it to CO2 and
alcohol.
Because of contamination concerns, and because of losing lots of volume to sampling, we normally
don't take readings during a healthy fermentation.
When fermentation ends, the specific gravity will stop dropping and stay the same.
This number is the beer's final gravity, or FG.
This reading tells a brewer that it's safe to bottle the beer or transfer it to a secondary
fermenter.
The second important thing that hydrometers tell us is an approximate alcohol content
for our finished beer.
The third scale on your hydrometer is potential alcohol.
This scale gives us an idea of how much alcohol could be produced if all of the dissolved
sugar was fermented by the yeast.
Just like specific gravity, potential alcohol will be highest before fermentation, when
there's lots of sugar, and at its lowest at the end of fermentation, when most of the
sugar has been used by the yeast.
To estimate the alcohol content by volume of your beer, you need to know the potential
alcohol reading before fermentation, and the PA reading after fermentation ends, corresponding
to the OG and the FG.
Subtract the after fermentation reading from the before fermentation reading.
For example, our Caribou Slobber, with OG 1.052 had an original PA of 7% before fermentation,
and a final PA of 2% after fermentation.
2 subtracted from seven gives us an approximate abv of 5% in the finished beer.
Phew!
That's a lot of theory!
How do you actually take a hydrometer reading?
First, you need a wort sample.
Sanitize the beer thief that came with your starter kit and use it to fill a hydrometer
test jar, or the plastic tube the hydrometer comes in.
Second, float the hydrometer in the sample.
Give it a spin to make sure it's not sticking to the sides of the jar.
Finally, read the specific gravity, or potential alcohol, where the surface of the beer intersects
the scale.
Then discard the sample.
Don't put it back into the fermenter.
A couple tips for accurate measurements: The hydrometer is most accurate when the temperature
of the beer or wort sample is 60F.
Consult the temperature correction chart that came with your hydrometer if the sample is
significantly warmer or cooler.
CO2 gas from fermentation can skew the reading.
If the sample of fermented beer seems fizzy, degas it by pouring it back and forth between
two glasses.
Then take your measurement.
Hydrometers: they don't just look nerdy...they ARE nerdy.
But if you're a home brewer, you are, by definition, a beer nerd.
Congratulations, fellow beer nerd.
Brew on!