A hydrometer is a fragile and sensitive glass laboratory instrument that works by floating in a liquid at a specific level based on the density of the liquid. In brewing, the increase in density is generally the result of the addition of sugar, so we can use the hydrometer to gauge the approximate amount of alcohol that we will create through fermentation. In the bottom of the meter is a weight to help pull the hydrometer down into the liquid, and this is balanced with the weight of the meter itself and the air in the meter to ensure that it is only able to sink a certain distance into the liquid based on how dense the liquid is. In the stem, there is a piece of paper with the scale(s) that give you the information you need. To take a hydrometer reading, place a sample of the wash into your test cylinder and carefully lower your hydrometer into the sample. Do not drop the hydrometer, as it can hit the bottom of your test cylinder and break. Give the hydrometer a slight spin to dislodge any bubbles that may be clinging to it, as these bubbles can lift your hydrometer slightly, resulting in an inaccurate reading. Your hydrometer reading is taken by reading the number where the surface of the liquid crosses the stem of the hydrometer.
You will now need to adjust your reading based on the temperature of the liquid. Hydrometers are calibrated for a certain temperature, most commonly 60° F (15.6° C). This is important to remember, as the density of the liquid will change with its temperature. Take honey as an example. As you heat it, it will become thinner in consistency, or less dense. The same is true with your wash, and although it is not a noticeable difference to you or me, it is very noticeable to a sensitive piece of equipment such as a hydrometer. For this reason, you will need to know the temperature of the wash when you take your hydrometer reading, as well as the temperature that your hydrometer is accurate at. Most hydrometers will have this information printed on the paper inside them. A good rule of thumb is that for every 10° F from the calibrated temperature, add 0.002 to 0.003 to the actual specific gravity reading. For example, if your hydrometer is accurate at 60° F, and your wash is at 90° F, then you will add 0.006 to 0.009 to your actual reading (a 30° difference at 0.002-0.003 for every 10°). Some hydrometers will provide a more accurate correction table specifically for the brand and style of meter, but a difference of even 0.002 or 0.003 is less than 1 percent difference in your final fermented alcohol percentage, so it is certainly nothing to become extremely concerned about for anyone other than commercial operations, where that translates directly to the bottom line.
A hydrometer reading is of help to you in several ways, depending on when it is taken. You will usually take a specific gravity reading once you have mixed your wash and before adding the yeast. This reading can be used to tell you how much alcohol you will potentially create if all the added density is from sugar and if it all ferments out. This may sound like a lot of “ifs,” but don’t worry, it will all make sense as we continue through the fermentation process and take more hydrometer readings. Here is where a second scale on your hydrometer—the potential alcohol scale—can be very helpful. After recording your initial specific gravity reading, known as the original gravity (OG), you can look at the potential alcohol scale to get an idea of how much alcohol this particular wash may create. The problem with having the potential alcohol scale is that it is commonly misinterpreted by novices as the actual amount of alcohol present, and when taking a reading after fermentation is complete, they will look at a potential alcohol reading that is often at or below 0 percent and they will think that there is no alcohol present, sending them into a panic. Just remember that this scale is potential alcohol, which is the amount of alcohol yet to be produced from when the reading is taken until fermentation is complete. Therefore, a reading of 0 percent means that there is no alcohol yet to be produced.
Similarly, those who do not understand the difference between a hydrometer and an alcoholmeter will sometimes use their hydrometer to test the alcohol content of a distilled spirit and immediately think that their meter is broken when it drops out of sight. This is because a hydrometer is calibrated to the density of water, which is much more dense than alcohol. To obtain the alcohol content of a distilled spirit, you need to use an alcoholmeter.
A hydrometer can also be used to confirm that fermentation is complete. If fermentation has slowed, or appears to have stopped, but you are not certain that it has completed, you can use your hydrometer to test for this. To do so, take a hydrometer reading and then wait at least 24 hours before taking another reading. If the second reading is lower than the first, then fermentation is not yet complete. If the reading remains stagnant, then fermentation is complete and you can move on to the next step in the process.
Read More from The Joy of Home Distilling
Excerpted with permission from The Joy of Home Distilling: The Ultimate Guide to Making Your Own Vodka, Whiskey, Rum, Brandy, Moonshine, and More by Rick Morris. Copyright 2014 by Skyhorse Publishing, Inc.