A hydrometer is an instrument used to measure the specific gravity of liquids. In the brewing process, we use it to determine how much fermentable sugar the wort contains. This is important for a few reasons. The one most people think of first is using the OG (original gravity taken before fermentation begins), and comparing it with the FG (final gravity taken after fermentation) to determine how well the yeast attenuated. This tells us the beer's alcohol content. The first calculation I learned and probably the one I use the most today, gives a good approximation of ABV. OG - FG * 131 = ABV%.
Another important reason to know where your beer "finished" is that any sugar left behind will contribute to the flavor profile and may or may not be desirable depending on the style you are brewing. The best way I've found to insure it finishes where you want is to pitch the right amount of yeast.
There are a few different ways to take a hydrometer reading and they all involve floating the hydrometer in beer. This guide will focus on obtaining the FG and show you a nice trick using a beer/wine thief to do it. I should mention however that my preference is NOT to open the fermenter unless it is absolutely necessary which leaves two options that I know of. The first is to use a satellite container (usually a beer bottle) with a sample taken after the yeast was pitched. You can put a small paper towel in the top to keep bugs out and place it close to the main fermenter--just be sure to pour enough to fill your hydrometer container when it comes time to take a reading. Some people say satellites do not represent how the main beer will finish, but I have not found this to be the case. Our satellites are always within a point of the reading taken from the carboys.
My favorite way to take a final gravity reading is described at the end of our Rack Beer to a Keg guide. This sample comes from the the last liter of racked beer that will be quick-carbonated for early tasting.
Notice the green cap on the Star San? When the existing caps break like they often do, you can use a 2-liter cap as a replacement.
Pour a little Star San or your favorite sanitizer into the large plastic container and add water. I recommend getting one of these if you haven't already. They are great for sanitizing larger equipment like racking canes etc. and are available at any all-purpose store like Target or Walmart.
Tear off a small sheet of press n' seal and then again so you have two squares. Toss in all of the stuff and let it sit a minute.
I try to be as sanitary as possible and (knock on wood) have never had an infection of any kind. 43 batches and counting!
Run sanitizer through the thief a few times and push the tip on the side of the container to drain. This will insure all of the parts are sanitized.
The other square goes on the top of the thief until you're ready to take a reading.
Stage The Area
I like to get setup ahead of time by moving the hydrometer in place and unstick the carboy stopper so all I have to do is move it aside and take the reading.
You may not be able to fit the thief into all carboys and this is another reason why I like Better Bottles. Glass carboys typically have a small opening that is not suitable for this.
Next, I carefully move the thief up and down to retrieve a sample and be sure there is enough to float the hydrometer.
One of the reasons I'm taking a FG for this beer is that it's a barley wine and originally started at 1.104. If the FG is not as low as I hope, then we may need to change some things like raise the temp a little.
Next, slide the hydrometer down into the thief and settle out the bubbles.
Okay... 1.024 is not bad. I have the fermenter temperature up to 74 degrees from this point on, so I definitely think it should drop another few points in two weeks.
If it doesn't drop any further, the ABV% will be:
1.104 - 1.024 = .08 * 131 = 10.48%
If it drops to 1.020 then we'll be at an even 11%
Finally, touch the tip of the thief on the inside edge of the carboy to drain the sample and close her up!
The container and water back at the sink are a great way to wash everything off for next time.