The science of alcohol
The science of alcohol

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The science of alcohol

4.1.1 Using a hydrometer

In order to calculate the strength of your homebrew you will need to take two readings – one at the start of the brewing process, and one when it has completed. In Week 8 you will learn how to use these two readings to calculate the strength of your homebrewed beer.

Now watch the following video showing the use of a hydrometer to take your first homebrew reading at the start of the brewing process.

Download this video clip.Video player: soa_alcohol_boc_vid003.mp4
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This is an example of a commercially available hydrometer that you can purchase readily online. If you would like to measure the strength of your homebrew at the end of your fermentation experiment, you're going to need to get yourself one of these little pieces of kit. Now, a hydrometer measures a particular property of a substance known as specific gravity. Specific gravity is simply the ratio of the density of the substance you're interested in to the density of a reference substance, which is nominally taken to be water. And water we say has a specific gravity of 1.00. Now, to use a hydrometer, it's very, very simple. All we do is we take our hydrometer, keeping it upright, and we basically put it into the liquid that we want to measure the specific gravity of. So here, we have water. We need to leave our hydrometer to settle for a moment so it stops bobbing up and down in the liquid. And we should be able to take a reading now from our hydrometer. And you can see that it corresponds to approximately 1.0. This isn't a highly calibrated scientific instrument, but it's going to be accurate enough for our purposes. We're now going to need to do the same thing on our homebrew before we've started the fermentation process. So you're going to need to take a sample of your homebrew before you begin to take the starting specific gravity reading. And we refer to this as the original gravity. You're going to need to take this reading and make a careful note of it, because you're going to need it in a few weeks' time when you work out how strong your homebrew actually is. But we're going to do exactly the same thing. We're going to simply add it carefully into our starting mixture, and again let it settle. And do try and ensure, as well, when you take your reading that the surface of the liquid that you're putting the hydrometer into is as smooth as possible. So if there's any air bubbles, for example, try and remove some of those, or try and let the liquid settle, because you're going to want to take an accurate a reading as possible from the hydrometer. And we can see here that now our hydrometer has settled. We've got a reading of approximately 1.0, between 1.03 and 1.04, which is exactly what we would expect for our homebrew at the start of the fermentation process. The reason for this being a higher value than our water is because this liquid here has a higher density. Before we start the fermentation, we have quite a lot of sugar present within our starting mixture. The sugar's needed by the yeast in order to ferment into carbon dioxide and alcohol. This sugar adds mass to the liquid. And because density is calculated from mass divided by volume, a higher mass equals a higher density. And therefore, when we take our specific gravity relative to water, we have a high specific gravity reading. This is your original gravity. Keep a note of this, and we'll return to our homebrew in a few weeks' time to take another reading, which we'll need then to calculate our final strength of our beer.
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A hydrometer works on the principle that water has a specific gravity of 1.000. Prior to fermentation your beer will contain sugars which make the liquid denser and so the hydrometer will float higher in the liquid than in water and will therefore give a higher specific gravity reading. For example, the starting specific gravity for an average ale or lager will be in the range 1.038–1.050. This reading is termed the original gravity, or OG.

Following the instructions in the video, you should now take a reading of your OG for your homebrew. Make a careful note of this, as you will need it in Week 8.


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