In a previous post on making hard cider, I used a hydrometer to calculate the alcohol content of the finished cider, but I didn’t really explain the hows and whys behind it. In this post I’d like to go deeper into how to use a hydrometer, and give you all the details (and the math…) you need to do your own calculations.
Why Do You Need A Hydrometer?
You don’t absolutely need a hydrometer, but it’s a great tool to have for brewing. By using it, you can:
- Accurately determine how much sugar is in your must or wort (the mixture that you start your fermentation with)
- Check to see if your brew is still fermenting, or if it has stopped or stalled
- Calculate the final amount of alcohol in your brew
Think of it like your car’s gas gauge–you can get around without it, but it will definitely make your life easier!
What Exactly Is A Hydrometer?
A hydrometer is a glass tube with a weighted bulb at one end. It looks a lot like a thermometer, but there is no liquid (or other moving parts) inside. It measures the weight (or gravity) of a liquid in relation to the weight of water. Because the relation of the gravity to water is specified, the resulting measure is called a specific gravity. A hydrometer will float higher in a heavier liquid (one that has some sugar dissolved in it), and lower in a lighter liquid (like water or alcohol). By knowing how much sugar you started with and ended up with, you can easily calculate the resulting Alcohol By Volume percentage (ABV%).
There are many different varieties of hydrometers, but most will have at least two scales on them–a Balling or Brix scale that measures the percentage of sugar by weight, and a Potential Alcohol scale that measure percentage of alcohol.
The Potential Alcohol scale, which is what most home brewers will use, normally ranges from 0 to 20 percent alcohol. You can’t determine the amount of alcohol with just a single reading, but if you take both a before and after reading and apply a little math, you can calculate the ABV%.
The Brix scale is mainly used by grape growers and commercial wineries. It will give you the percentage of sugar that is in the liquid by weight. If you have a grape juice that reads 24 on the Brix scale, that means that the juice is made up of 24% sugar by weight.
When buying your hydrometer, make sure you purchase a test jar if it doesn’t come with the hydrometer already. The hydrometer will probably come in a storage tube, but it won’t have a wide base, and could tip over easily when filled.
How Do You Use A Hydrometer?
1. Take a sample and insert the hydrometer
You’ll take your first measurement after you’ve added all your ingredients, but before you’ve added the yeast. This reading should be taken as close to room temperature as possible.
Use a wine thief to get a sample of your liquid, and fill the testing jar. Be careful not to overfill–the liquid will rise when you insert the hydrometer. Add the hydrometer, and make sure there is enough liquid that it is floating off the bottom. Try to wait until all the air bubbles have left the tube, and make sure the hydrometer is centered in the tube, not touching the sides.
In this picture I’m just measuring plain water, and you can see it’s right around 1.000. If you’re using plain tap water this could be off by a little bit, due to minerals or other impurities in the water.
2. Record the Original Gravity reading
The increments of the hydrometer represent specific gravity points. Record the number on hydrometer where the liquid crosses it.
I used another of the Spike Your Juice kits to make a quick bottle of wine from some store bought grape juice (check out my previous post on how to do this), and took a reading of the juice with nothing added.
In this picture, you can see we have a reading of 1.064.
Once you have your reading, you may want to add additional sugar to raise the specific gravity, depending on what you are brewing, and how sweet you want the finished product to be. For example, a dry wine or mead might start with a gravity between 1.085 and 1.100, while a sweeter variety might start between 1.140 and 1.160. So this tells me that this wine will be relatively dry when finished.
3. Adjust for temperature
Hydrometer readings assume that you have a standard temperature of 59°F (15°C). Odds are your room isn’t exactly this temperature, so you’ll need to adjust your reading based on the actual temperature. You can calculate the difference by using the table below–just add the “G Change” to your Original Gravity reading to obtain the correct specific gravity.
The temperature in my kitchen was 78°F, so for my reading above, I would add .0022, giving me a corrected Original Gravity of 1.0662.
4. Wait, then record the Final Gravity reading
Once your brew has finished fermenting, you’ll take the Final Gravity (FG) reading (adjusting for the temperature once again). Typical FGs for various brews arefrom about 0.992 up to 1.010, depending on what you’re brewing.
As you can see, my Final Gravity reading is 1.002, right around where it should be. I still need to correct for the temperature, so I would again add .0022, give me a corrected FG of 1.0042. Actually, since I took both of my readings at exactly the same time, the temperature was bound to be the same for both of them, so I didn’t really need to add the .0022. You would just need to calculate it when you’re taking reading on different days, at (possibly) different temperatures.
Now the math comes in–don’t worry, it’s easy! The formula you’ll use is:
(OG – FG) x 131.25 = ABV%
In other words, subtract your Final Gravity from the Original Gravity, multiply that result by 131.25, and you have your Alcohol By Volume percentage.
So for the wine I brewed, my calculations are:
(1.0662 – 1.0042) X 131.25 = 8.1% ABV
Just a tiny bit over 8% alcohol by volume, about the same as a nice German Riesling.
I used two different hydrometers here, but you’ll want to use the same one for both of your gravity readings, so that you can be sure a slightly different one doesn’t throw your readings off.
Once you’ve taken your reading(s), don’t pour the liquid back in with the rest of your brew! You could wind up contaminating it.
Also, be sure not to overdo your testing! Every time you open up your brew to test it, you risk exposing it to harmful bacteria or wild yeasts. I recommend waiting to do the final test until you’re relatively certain that the fermentation is finished.