Specific Gravity Measurements

Specific Gravity Measurements

When you brew a beer, you undoubtedly ask the question "how much alcohol will be in my finished beer?" In order to have a successful batch, as well as create beer that follows style guidelines, it is important to know the alcohol content of your homebrew. Fortunately there is an easy way of calculating the alcohol content of your homebrew as well as checking the fermentation progress using a simple device known as a hydrometer.

A hydrometer uses the physical process known as specific gravity (density differences) to gauge the amount of fermentable sugars available in wort. The conversion of "heavy" sugars into "light" alcohol (attenuation) and the resulting lower density is what allows us to know the alcohol percentage. This is why at least two gravity readings must be taken in order to get an accurate measurement of the alcohol content of a beer- preferably just before you pitch the yeast in the primary fermenter and prior to bottling or kegging your beer. It may also be helpful to take a specific gravity reading in between the primary and secondary fermentation to see if the yeast are doing their job (most of the fermentation will occur in the primary), but be aware that every time you open up your fermenter there is potential for contamination.

Another important aspect of a specific gravity reading is the temperature of the liquid. Since density changes with temperature (remember science class) it is also important to do temperature corrections to your specific gravity reading if your liquid is not at the calibrated level for your hydrometer (calibration temperature varies). Luckily, most hydrometers come with a chart or simply use this table to readjust your specific gravity calculations.

The Process

You will need a hydrometer and a clean cylindrical tube that can hold enough liquid (app. 1 inch in diameter, 7 inches tall) to allow the hydrometer to float. Prior to pitching the yeast, take some of your wort and fill your tube halfway full with your wort (or halfway empty if you’re more pessimistic). At this point, it will be helpful to take a quick temperature reading so that you can make any adjustments later. Now that you have your wort contained, take your hydrometer and place it in the wort. It should bob up and down a bit but will eventually settle out. You may see some bubbles form around the hydrometer- these are not wanted. For the most accurate reading spin your hydrometer in the container- this will eliminate the bubbles. Once your hydrometer stops spinning and bobbing, look at the number that is most in line with the level of the wort (you may have to read from the top or the bottom of the meniscus - check the instructions that came with your hydrometer). Most hydrometers have numbers that range from 1.000 to 1.080 or so. Record this number. This number is known as the Original Gravity (OG) and will give you an estimate of the alcohol percentage assuming fermentation goes as planned. Follow the same steps the next time you take a reading. The final reading, known as the Final Gravity (FG), should be less than your first reading- usually 1/4 to 1/5 of the OG. Remember to dispose of your hydrometer wort (drinking it is an acceptable method). You would not want to place it back with your homebrew because of contamination concerns. To find out the alcohol percentage:

1. OG - FG = TG (Terminal Gravity)

2. TG x 105 = % Alcohol by weight (a.b.w.)

3. a.b.w. x 1.25= % Alcohol by volume (a.b.v.)

Here is a table of specific gravities and the potential alcohol content