How to Use a Hydrometer For Homebrewing Beer

When embarking on your first adventure into creating a home brewed beer, parts of the fermenting process can seem a bit daunting, particularly if you have no background in science. In fact, when it is done properly, the whole process looks quite a bit like a science experiment, with vats and tubes and bubbling liquids – and even real, honest to goodness science equipment like the hydrometer.

Ah, the hydrometer. How many questions it poses in the mind of the first time brewer: What is it? Do I really need it? Why do I need it? How does it work? How can math help me make beer? Will it make my beer taste better?

Technically speaking, a hydrometer is a gauge used for measuring attenuation of a liquid. In English, that means it measures how much of the sugar has been rendered into alcohol (ethanol) by the yeast in your brew. Is the hydrometer absolutely necessary for home brewing? Probably it is not, but the meter is a magnificent tool for determining if everything is working properly within the confines of your fermenter.

The hydrometer works by measuring the Original Gravity (OG), taken when you add the yeast, and comparing it against the Final Gravity (FG), taken once your airlock has ceased bubbling. With this measurement, the hydrometer can tell you whether your yeast has turned the sugar to alcohol to the degree it should have, during that first period of fermentation. Typically the FG reading should be ¼ (or less) than the OG. If it is much higher than that, the yeast was not set up properly and the primary fermentation process did not complete as it should have in your wort. The hydrometer does not make your beer taste better, as such, but using it to gauge attenuation can save you from bottling up a batch of foul-tasting, not fully fermented “not-beer”.

The actual use of the hydrometer is fairly straight forward, though it will require some math and some temperature conversion via charts, unless you are able to keep your wort at a steady 59° F (or 15° C), since nearly all hydrometers are graduated to this temperature. You will need the hydrometer itself, a tall thin jar to float it in, a thermometer, the conversion chart included with your hydrometer, and at least one sterilized Wine Thief (WT) to draw off samples (it may be more convenient to have 2 sterilized and ready). A pair of turkey basters will do just fine in place of the WTs, of course, as long as they are also sterilized. You will also need pen and paper (or the electronic gadget of your choice) to record the readings.

Just before you plan to pitch your yeast, use a WT to draw up a sample from the fermenter. Inject the sample into the jar with the hydrometer and record the measurement. Immediately take the temperature and record that, as well. Empty and rinse the jar, and rinse the thermometer and the hydrometer. If you only have one WT, sterilize it again.

Once the air lock is no longer bubbling, you will repeat the process again, drawing up a sample, placing it in the jar and recording the measurements for gravity and temperature. This second time, you may wish to remove the hydrometer from the jar (after you take the measurements) and taste the sample. Since fermentation is not entirely complete it will not taste exactly like the finished product, but the taste should be very beer-like, with perhaps an extra dose of yeasty-ness.

Now it is time to use the conversion charts to determine whether the FG is about ¼ (or less) than the OG. If it is not, you will most likely need to scrap the batch and begin again. If the answer is yes, the FG is within 2 points of being ¼ the gravity of the OG, then you are done with the hydrometer and you may continue with brewing your beer!