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Written by Web Master
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Sunday, 15 April 2007 |
Using a hydrometer is probably the most under utilized step in the homebrewing process. It is not necessary to use your hydrometer, but it will help to ease your mind if you understand what you are reading. Naturally, this page is all about using this strange piece of brewing equipment. Pronounced hy-drom-e-ter, this is an instrument used to determine specific gravity. I suppose we should talk about what these instruments are used for and how that can play an important role in your homebrewing.
Using a hydrometer is a quick and easy step in the homebrewing process. All you need to do is take a sample of your homebrew in a test jar along with the hydrometer. There are a few times that this will need to be done, and I say need only if you want to keep track of the fermentation process or to determine alcohol content or if you are formulating a new recipe... Well, I guess there are lots of times when hydrometer readings are helpful. The reading you obtain is called a specific gravity reading, sometimes seen as SG. Readings are also referred to as OG for Original gravity, taken before the yeast is added to the fermenter and also FG for final gravity which is taken at the end of fermentation.
Specific gravity is a measure of density of a liquid, in the case of homebrewing the reading refers to the amount of sugar in a the wort. This sugar is what your yeast will consume or ferment the wort into beer and as this happens, you will see the reading on your hydrometer change. Now that you have a basic idea of what your hydrometer does, lets put the numbers to use in some practical scenarios. The first thing that comes to my mind is that you can use the readings to track fermentation and determine when it is complete. This is accomplished by taking readings through out the fermentation process, so you will want to keep a log for the readings. The first one is the OG or original gravity reading taken just before you add the yeast to the fermenter. The next reading can come once the visible signs of fermentation have slowed or stopped. Here you will take one reading each day for 3 consecutive days, keeping track of each. If your reading remains unchanged each day over the 3 day period, your all done fermenting. At this time you would want to record that reading as the FG or final gravity. Thats the basic use of the hydrometer and with the information you have collected, you can figure out a few other things about your homebrew.
In an effort to make all this worthwhile, I would like to toss out some equations that use the hydrometer readings. You will find the equation and then a practical example.
First, you can determine what the alcohol percentage is in your homebrew.
Lets make some assumptions for all of our equations. Let's say we made a Pale Ale with an original gravity of 1.055 this is the OG. Now after fermentation is complete you have a final gravity of 1.012.
OG=1.050
FG=1.012
You can see that we have some numbers that may be difficult to work with and can be simplified to make the math easier. All you need to do is remove the 1.0 portion of the readings and you have what some people refer to as gravity points.
So now your readings look like this.
OG = 50 or 1.050
FG = 12 or 1.012
Y = OG-FG
Okay. Lets see that equation.
Alcohol Percent
OG-FG = Y*.125 = ABV
With our numbers you will see this:
50-12 = 38*.125 = 4.75 or 4.75%
If you want you can use the actual readings like this:
1.050-1.012 = .038*125 = 4.75 or 4.75%
Its all a matter of decimal points, in the first we have simplified the readings to the points of gravity but multiply by .125 where in the second one we use the actual reading but multiply by 125. Its the same but different. Use the one that makes the most sense to you and is easiest to remember.
Note: Typically hydrometers are calibrated to 60F which means that if your sample is warmer or cooler your readings will not be 100% correct. This is not all that important as long as your hydrometer readings are taken at the same temperature from beginning to end. IF YOU PLAN TO SUBMIT YOUR HOMEBREW TO COMPETITION, MAKE SURE YOU HAVE THE CORRECTED NUMBERS.
Most homebrewing recipe kits will contain target OG and FG numbers which makes guessing your alcohol content easy before making the batch of beer. It should also be noted that the readings that you get lend some insight into the flavor or profile of the beer. For example, water has a specific gravity of 1.000 which has no sugars in it. Most homebrewing recipes will have a FG of 1.010 or higher. Remember that he higher the number or closer to the OG, the sweeter the beer will be, likewise if you have a reading that is 1.010 or lower, more of the fermentable sugars are converted to alcohol and Co2 and the result is a dry beer.
Controlling the final gravity should only be done by your yeast choice and never by stopping fermentation before it is complete. We may be diving into a topic that is more advanced than this page is intended for but I will explain this a little here. If you do not allow your homebrew to complete the fermentation process, you will be bottling beer that has too much sugar left and the result is over carbonated beer or even exploding bottles. In the bottle the yeast will continue to eat the sugars and create Co2 which will not have an escape route, causing over carbonation first and eventually the bottle will not be able to handle the internal pressure and pop. So the better way to keep some sweetness in your homebrew, you should pick a yeast strain that leave some sugars unfermented.
This is getting way off topic for this page and will be discussed in more detail as soon as I can get the yeast page up and running. Anyway, you can estimate a final gravity with the use of a hydrometer and the percent of attenuation of the yeast strain you plan on using. You can also set the target OG and FG and then choose a yeast strain that has a matching attenuation. Attenuation is the measure of thoroughness of fermentation. Given as a percent describing how much available sugar has been converted to alcohol during the fermentation process. The higher the amount left over sugars the sweeter the beer, this would translate to a higher gravity reading. Please see the equations below, hopefully it will seem clear.
To figure out what your target FG will be use the following assumptions.
OG = 50 or 1.050
FG = Y
Attenuation of yeast = 72% or .72
Equation is this:
(OG - FG) / OG = Apparent Attenuation
With our numbers you would see this:
(50 - Y) / 50 = .72
(50 - Y) = (50 * .72)
So you end up with:
50 - Y = 36
Or
50 - 14 = 36 so Y = 14 or 1.014
The second equation would be used if you are formulating a recipe and have an idea of what your target OG and FG are. In this case you would need to determine the Attenuation target so you can make a yeast selection that will help you hit the FG number.
Here is the basics, we will use the same assumptions as in the last equation.
OG = 50 or 1.050
FG = 14 or 1.014
Attenuation of yeast = Y
Equation is this:
Apparent Attenuation = OG / (OG - FG)
With our numbers you would see this:
Y = (50 - 14) / 50>
So the math would be this:
Y = (36 / 50)
So
Y = .72 or 72% attenuation
This would lead you to pick a yeast strain that has an apparent attenuation of 72%. If you are formulating a recipe, this can help you determine residual sugar levels. Attenuation information is available from the yeast manufactures websites in most cases.
Wow thats a flash back to high school algebra!!! And I thought I would never use it again....
This has been a whole lot more technical that I had in mind for this page but I do think it is important to understand what all these numbers can do for you. Please do not worry about the numbers all that much. They are guides for you to strive for only. Your beer will not be ruined if you do not hit your targets. Just use that hydrometer and keep good records in case you need the information later in the brewing process.
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Last Updated ( Tuesday, 01 May 2007 )
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