- Created on 02 December 2011
- Written by Brianna
First, an etymology lesson: hydrometer comes from the words hydro (meaning water), and meter (meaning a device for measuring). What the hydrometer is measuring is the gravity of your liquid, specifically. Gravity refers to the density of a liquid. So, for example, molasses, which is super thick and gooey, will have a much higher density than pure water. Mixing something into your liquid, like chocolate syrup in your milk, will obviously increase the gravity of that liquid. Similarly, the gravity of a liquid will change with temperature. Take ice and water: both the same substance, but one has a much higher density than the other.
So, knowing these two things, let's apply them to brewing. Beer is made when the sugars inside your wort are fermented by yeast bodies, which convert those sugars into alcohol and gas (CO2). As the sugars disappear in this way, the gravity of your beer will decrease. Kinda like taking the chocolate syrup back out of the milk, if you could do that (makes you see how much of a miracle brewing is, huh?). As well as dropping the density level of your beer, fermentation produces alcohol, of course. Since these two processes are correlated thus (as one is happening, so is the other), we can use a formula that takes both the starting gravity (or Original Gravity, in brewing terms), and the Final Gravity measurements, to calculate how much alcohol has been produced.
And let's not forget that second important variable: temperature. Since gravity measurements are going to be different at different temperatures, we need to account for that. Luckily, those who have come before have made correction charts, so all you've gotta do is make sure you know the temperature of your sample, and then use a handy chart like so:
The following corrections are for a Hydrometer rated at 20°C / 68°F (ie the type Mr.Beer sells):
10°C (50°F) -0.002
15°C (59°F) -0.001
20°C (68°F) -None
24°C (75°F) +0.001
28°C (82°F) +0.002
32°C (90°F) +0.003
Now that we've properly figured out how to measure the gravity of our beer/wort, let's apply what we've learned.
When brewing your next batch of Mr.Beer, you're going to want to remember one crucial step in between topping your keg off and adding your yeast: take a hydrometer reading. This number is your Original Gravity (OG). Water is the constant here, with a gravity of 1.000. Your worts gravity will vary, of course, depending on what and how many ingredients are in your keg, but should at least be at 1.034 or higher. Remember that pesky temperature part, too. An easy way to measure the temp of your sample is to put a Stick-on Thermometer on your sample tube. Give it a few minutes at most to comfortably settle, and read the temperature so that you can adjust the gravity reading if you need to. Then, as you know, you'll take another sample at the end. I won't totally rehash it, cause I know we've got the instructions up there on the page. (click here to see)
I will, however, as a conclusion, swipe the example equation that you want to plug your numbers into, just to illustrate the point:
Example: OG = 1.038 FG = 1.010
ABV = (1.038 - 1.010) x 131 ABV = (.028) x 131 ABV = 3.67%
See? Math IS fun. At the very least, when it's helping with your brewing. And what more could a budding brewer ask for?