Other than our eyes, nose and taste buds, the hydrometer is probably the most important tool the brewer has at his disposal. The hydrometer can tell us many important things about the beer: how efficient our mash was (or whether we managed to get all the malt extract into the fermenter), whether or not the beer is done fermenting and how much alcohol is in the beer. When there is a suspected problem with a fermentation, nine times out of ten the hydrometer can give us an answer.
The Basics about Hydrometers
During fermentation, beer is a living thing. We can control the conditions and try to coax it into behaving as we would like, but we really aren't in control. You can’t reason with your yeast. So the best we can hope to do is measure what the beer is doing.
Beer starts out as sweet wort — sweet because it contains a large amount of sugars. The yeast eats these sugars and produces carbon dioxide and alcohol. At some point, either the sugar is all eaten up or the yeast has produced so much alcohol that it kills itself. In either case the fermentation will have stopped. But other conditions can cause the fermentation to stop prematurely — or not even start. And the way we can tell what’s happening is by using our hydrometer.
A hydrometer measures the specific gravity of a solution, typically water. The name says it all — “hydro” (water) and “meter” (a measuring device). The specific gravity of pure water is 1.000 (at 60° F). When sugar is dissolved in the water, the specific gravity goes up. As the yeast eats the sugar, the specific gravity goes down.
The hydrometer is made of glass. The thin end contains one or more scales where we take a reading. The fat end is weighted (usually with lead shot) so the hydrometer floats upright. The amount of weight is adjusted so the hydrometer floats at the right level (also known as being calibrated). The typical home brewing hydrometer is calibrated to read a specific gravity of 1.000 in pure water at 60° F.
The hydrometer will typically contain three scales or systems of measurement, and you’ll commonly see them referred to as “triple scale” hydrometers. The three scales are specific gravity, Balling (or Brix) and potential alcohol. We've discussed specific gravity. Balling and Brix are the same scale with a different name and express the sugar content in terms of percent sugar by weight. (You’ll sometimes hear brewers refer to degrees Plato. The Plato scale is almost identical to the Balling scale.)
When the hydrometer floats in the liquid, the thin end sticks up out of the liquid. The point where the liquid level crosses the scale is where we take our reading.
Reading Your Hydrometer
The first thing you need to do is get a sample of the wort. There are several ways to do this, but the most convenient is to use a device known as a “wine thief.” It’s called this because it’s used to steal a sample of wine, typically from the bung hole of a wine barrel. They work the same way a straw does when you play with your drink at the restaurant: When you immerse the straw in the liquid and put your finger over the upper end, you can lift the straw out of the drink and liquid remains in the straw.
If you don’t have a wine thief, a ladle or clean turkey baster will also work (get a new one and never use it for basting). Just make sure you squeeze the bulb before putting it in the wort so you don’t inject unwanted air into the wort. Be sure to sanitize whatever you use to get the sample, but it’s not necessary to sanitize the hydrometer or the flask. You’ll not be returning the sample to the wort.
One thing you should be aware of: Sugar can stratify in the wort, meaning there can be more sugar at the bottom than at the top. This is usually only a problem during an all-grain run-off into the kettle, where the first runnings are very sweet and the last runnings have almost no sugar. So it’s always a good idea to stir the wort to make sure you get a representative sample, but only prior to fermentation.
Fill the hydrometer test jar to about 1 inch below the top. This will vary depending on the test jar — you can float your hydrometer in water and make a mark on the flask to know how far to fill it. Just make sure there is enough liquid so the hydrometer doesn't touch the bottom. Now take your thermometer and measure the temperature of the liquid. Record the temperature. It should be between 50° and 105° Fahrenheit. (Do not try to measure hot or boiling wort with a hydrometer — the heat stress will break it.)Do not stir the wort if taking a sample near the end of fermentation — the sugar is no longer stratified and this could oxidize the beer.
Now we need to correct for temperature. Hydrometers are typically calibrated to read accurately at 60° F. If the temperature is different than that, the reading will be off. Your hydrometer should have come with a sheet that lists the temperature correction factors. If you no longer have this sheet, the chart at the bottom of the page will probably be close, if not identical. If your reading was 1.054 and the temperature of the sample was 77° F, for example, the actual corrected reading would be 1.056.Now gently lower the hydrometer into the liquid. (Don’t drop it in! The hydrometer could break when it hits bottom.) Make sure that the hydrometer is not touching the bottom or sides of the flask and that the flask is level. Also make sure that there are no bubbles adhering to the sides of the hydrometer, which would cause it to float higher than it should. Spinning the hydrometer to dislodge the bubbles is a good trick. Now position yourself so that you can sight along the level of the liquid. Record the reading that’s even with the lowest level of the liquid. (Due to surface tension the liquid tends to climb up the side of the flask and hydrometer, creating a “meniscus.” Take your reading at the bottom of the meniscus.)
What a Hydrometer Can Tell You
Use the hydrometer at two during your brewing cycle — at the beginning and again at the end.
Taking a reading at the beginning serves several purposes. For the all-grain brewer, it will tell you how efficient your mash was. Did you get a good conversion and rinse all the sugars out of the grain? For the extract brewer, did all the sugars you put in (in the form of extract) make it into the fermenter? To see how good a job you did, check the gravity reading against your recipe. If you are in the ballpark, you are okay. If it’s way high or low, something is wrong (check for sugar stratification — stir the wort and take another sample). Having a reading at the beginning is also necessary if we want to estimate the amount of alcohol in the beer. Make sure you keep track of the reading — you’ll need it.
Taking a reading at the end of fermentation tells us if the beer is really done fermenting. If you bottle the beer, this will ensure consistent carbonation levels and prevent exploding bottles! To tell if your beer is done fermenting, wait until you think the beer is done. Estimate this by looking at the beer, not by some arbitrary amount of time. There should be no signs of fermentation — the bubbling has stopped, the foam has subsided. There is no such thing as “the beer will be done in X days.” The only accurate way to tell if the beer is done fermenting is by taking hydrometer readings.
If you think the beer is done, take a reading (being sure to correct for temperature). Accuracy of the reading is important at this stage. Write this reading down! Now wait three days. Take another reading. If the readings are the same and in the right range (more on this in a moment) then the beer is done.
If the second reading is lower than the first, the beer is still fermenting. (If the second reading is higher than the first, then one of your two readings is wrong! Start again!) Write this reading down, too. Wait three days and take another reading. If this reading is the same as the previous reading, the beer is done. If it is still lower, then keep taking readings at three-day (or longer) intervals until you get a pair of readings that are the same.
What we’re actually doing is reading the sugar levels. Hopefully, the yeast will have eaten all it can and the two readings will be the same. If the second reading is lower, it means that the yeast was still eating sugar, because there is less now than before.
Now that we have readings from the beginning and end, we can calculate the alcohol content of the beer. Subtract the ending reading from the beginning reading and divide by 8. For example, if the starting gravity was 1.056 and the finishing gravity was 1.012, subtract 1.012 from 1.056 to get 0.044. Drop the zeros and decimal point (44) and divide by 8. That equals 5.5 or 5.5% alcohol by volume.
You can also use the potential alcohol scale built into the hydrometer. If the first reading is 8% potential alcohol and the ending reading is 2% potential alcohol, you subtract the second from the first to give us 6% alcohol.