:: How do you measure the specific gravity and alcohol content of beer?


How do you measure the specific gravity and alcohol content of beer?

Today I am going to show you how to test the gravity of your beer, and why you should test it. Along with how to test your beer, I will also tell you when to test the gravity throughout the brewing process. Then we'll go over what to do with the measurements that you find, like finding out how much alcohol you made, and if your beer is done fermenting. You will need a hydrometer, which is how you will measure the gravity of your beer. Along with that, you will need a hydrometer test jar, which is used to hold the beer to be tested.

Lets start off with what I mean by gravity. Measuring gravity is just a way of measuring density. This is simply how much ''stuff'' is in solution. Sugar is more dense than water, so a solution such as wort with lots of sugars will float your hydrometer higher. Alcohol, on the other hand, is less dense than water, so your hydrometer will float lower. So essentially, when measuring gravity, you are measuring the amount of sugar in your beer. We will be using the specific gravity and potential alcohol scales on your hydrometer.

Now lets move on to how to measure your gravity.

You will need to first gently pour a sample of your sweet wort or fermented beer into your hydrometer test jar. There are several ways to do this, the simplest of which is with a wine thief. If you don't have a wine thief, a piece of sanitized siphon hose can be used to siphon your sample into the test jar.

Next, carefully lower your hydrometer into the test jar. Be careful here. If you drop the hydrometer in too fast, it may strike the bottom of the jar and break your hydrometer.

Because of some of water's wonderful qualities, it forms what is called a meniscus around the hydrometer. Always use the measurement at the top of this meniscus.

There are several scales on a hydrometer, including specific gravity, and potential alcohol. I'll show you how to use these numbers later. For now, just keep track of your readings.

If you find yourself in a pickle, and have only a hydrometer but no test jar, you can sanitize your hydrometer and drop it carefully into your fermenting bucket. Then simply read the gravity from there. This is not very accurate and shouldn't be practiced often.

Finally, the sample you just measured provides you with a great opportunity to taste your beer. I recommend tasting your beer throughout the process to make sure everything is going as planned.

Now that you have a measurement of gravity, we have one more step before we can use that measurement. We need to correct your measurement for temperature. The temperature will change the gravity reading of your sample, and your hydrometer should have come with a correction diagram. You can leave your sample out for a little while to bring it to room temperature. I'll give you an example of how to correct your readings for temperature in a few minutes.

Please go ahead and discard whats left of your sample.

Next, lets talk about when to measure.

The first measurement you need to make is your starting gravity. This is the gravity of the unfermented sweet wort that you will add yeast to. This is a starting point for you to compare to later in the beer making process, so it is very important to measure this and write it down.

The next time that you will want to test your gravity will be near the end of your fermentation. I recommend at least 10 days after you added your yeast. If you plan on using a secondary fermenter, this reading can be used to signal when to transfer your beer.

If your beer is done fermenting, and you think that you're ready to bottle, you now need to find your final gravity. This is the end-point that we will use to find out how much alcohol you have made in your beer. It is important to make one measurement, then wait a day and take a second measurement. If the two are the same, you are OK to continue. If the second reading is lower than the first, your yeast is not done turning sugar into alcohol. Wait another day or two, and test again.

Before I give you an example, lets talk about why its important to take these measurements. The most common reason to use these measurements is to find out how much alcohol you have in your beer. But they also will tell you if your beer is done fermenting. If you try to bottle a beer that is still fermenting, you run the risk of over-carbonating your beer as it finishes fermenting inside the bottles. This will make your beer gush into a foamy fountain.

So how do we figure out how much alcohol we made? You will use some simple equations to do this, along with the starting and finishing gravities that you recorded.

The first equation is simple. Its the difference of the starting potential alcohol and the final potential alcohol. That is your % alcohol. The second equation uses gravity. It is also the difference of starting and finishing gravity readings, times 105. This is a much more accurate value of how much alcohol you have made.

Now lets take a step by step walk-through of taking all the measurements you need, and all the calculations you'll make.

Use a wine thief or siphon out your sample into your test jar. Now carefully drop your hydrometer into the jar. Give the hydrometer a little spin to remove any bubbles. Remember to always read the top of the meniscus. Measure your starting gravity from the wort you made before adding your yeast. In this example, we'll use a reading of 1.050 at 78 degrees F. Now we correct for the temperature, and get 1.051. This is about 6.5% on the potential alcohol scale. This is what you will want to record Measure your beer after 10 days in the fermenter. Now we'll use an example reading of 1.010. Measure again one day later. We should see the same reading of 1.010 Our temperature is still 78 F, so our corrected measurement is now 1.011, which is about 1.5% on our potential alcohol scale. Don't forget to taste your sample. Now that we have our two numbers, lets do some calculations. Using the potential alcohol scale, 6.5-1.5=4. So your beer is about 5% alcohol. Using the gravity scale, we have (1.051-1.011)X105= 4.2%. As I mentioned earlier, the gravity readings are more accurate, and that is why we see 4.2% alcohol, instead of just 5%. You now know how to use your hydrometer. You also know when and why you should use it.

If you have any more questions, feel free to call us at 1-800-450-9535, or e-mail us at outpost@homebrewers.com.

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