Avoid getting your heart broken when brewing at home: Use data, don’t guess. How many times have you heard a story about the brewer who checked on his fermenter only to discover his batch was either syrup or water? Why did that happen? Because he didn’t take any measurement! Enter the hydrometer.
A hydrometer is used to measure density, which reveals how much sugar are in your liquid before the yeast turns it into alcohol. Use a hydrometer before pitching yeast into the liquid. Without one, you won’t know what you started with.
How to Use a Hydrometer
In this chart, we show how Original Gravity (OG) relates to Final Gravity (FG) with various type of beverages. For example, an average American IPA has an OG that lands somewhere around 1.060-ish. That OG give you a starting ABV range of 5. 7 percent. The lower OG is too watery; the higher OG risks stressing out yeast. When yeast get stressed, it makes undesired fusel alcohols, which taste like nail polish remover.
Knowing these targets early allow you to tweak either your water volume or your grain bill prior to boil. It transforms what was once a chaotic process into a controlled experiment.
The one caveat is that temperature do make a difference in this equation. The hydrometer is calibrated at 60 degrees Fahrenheit. More likely then not, your homebrew kit is somewhere around room temperature, maybe seventy-five or even eighty degrees. Because warm water is less dense (and expands), it will cause the hydrometer to read a little low. If you don’t account for this, your calculation of alcohol by volume will be incorret. Either cool down your sample with an ice bath or use a correction factor from the same reference tables to account for this. It is a small step, but it will ensure consistent result.
Proper technique is needed to read this instrument. When lowered into solution, the glass stem will bob up and down. Gently spinning the instrument will free any air bubbles stuck to glass stem. Air pockets increase buoyancy causing the hydrometer to float higher than it should. That leads to an incorrect gravity reading (too high). Let the device settle and observe the point where liquid intersects with the glass, at the bottom of the curve. Avoid using the top of the curve as your measurement; doing so introduce a parallax error that reduces accuracy by several points.
So, now that you have your two numbers, what’s the math? It’s simple: Subtract your final gravity from your original gravity. Then, multiply that number by 131.25. That result is your alcohol by volume percentage.
For those of you making distilled spirits, there’s a wrinkle: proof enters the equation here. In the US, we use proof; doubled over the actual alcohol content. So, an eighty-proof spirit has forty percent alcohol. It’s an old method that has stuck around because of tradition; but it’s still how things get labeled in this country.
Brewing becomes a craft if you maintain a logbook. Everything goes in there: the taste notes, the gravity readings, and temperature. It becomes a feedback loop that let you fine-tune your process over time. Maybe you realize your favorite yeast strain doesn’t reach the same final gravity as you thought. Perhaps you discover fermentation slows down during winter because of ambient temperatures. Those insights are only available once you start logging the information.
Control is key to brewing, and learning how to use the hydrometer let you replicate your success. The next time you pull out your fermenter, you’ll have a pretty good idea of what’s going on in there. You should of used it more often.