Take a closer look at your hydrometer. Most have three scales showing specific gravity, degrees Balling, and potential alcohol. Typically, homebrewers use the specific gravity scale, which measures the density of wort relative to that of pure water. So, a wort with an original gravity of 1.048 is 4.8 percent heavier than the same volume of water. Since the scale is read out to three decimal places, specific gravity is one of the most precise measurements you’ll make as a homebrewer, but precision is not the same as accuracy.
Most brewers just assume that their hydrometer is close enough to correct, but it could be off by enough to affect the quality of the beer or make it harder to reproduce recipes. Skipping calibration is not worth it, especially because you can easily calibrate this useful tool.
Picking Known Points
Just like you’d calibrate a thermometer against known temperatures such as freezing and boiling water, you’ll use a pair of known data points to assess your hydrometer’s accuracy. The first point is easy: distilled water should read as 1.000 at the hydrometer’s reference temperature (usually 60°F/15°C or 68°/20°C).
For a second point, we’ll take advantage of how Plato/Brix measurements are defined. A solution of 15° Plato is 15 percent sucrose, and 85 percent water. We can make such a solution by taking ½ oz (15 g) of table sugar and mixing it with 3 fl oz (89 ml) of distilled water. According to standard conversion tables, this should correspond to a specific gravity reading of 1.061.
It’s worth pointing out that I came across some conflicting information while researching this. Many resources treat Balling, Brix, and Plato as equivalent, but reputable sources show a significant difference between 15° Balling and 15° Plato (1.059 vs. 1.061). For this exercise, I would go with the Plato/Brix conversion.
Taking Measure
We’ll start by measuring the specific gravity of distilled water. This should register as 1.000 on the hydrometer. Remember that the instrument is temperature sensitive, so try to have your sample at the reference temperature identified on the hydrometer itself. If the temperature is off, you can look up the appropriate offset online and use that figure to adjust your reading.
After you’ve taken the temperature into account, compare your measurement against the expected result of 1.000. If they’re not the same, then your hydrometer readings will need to incorporate the difference.
But you’re not done; while the first measurement sets a base skew for the hydrometer, the markings themselves might be too close or too far apart. You can determine this with a second measurement. To get enough volume to float your hydrometer, make a 15° Plato solution by doubling the ratio defined earlier, using 30 g sugar and 170 g water. Once again, make sure the temperature matches the reference temperature or make the appropriate adjustment.
Reconciliation
Ideally, the temperature-adjusted measurement of the 15° Plato solution will be off from the target of 1.061 by the same amount that the pure water sample was. So, if the water registered as 1.002, then the solution should read 1.063. This would mean that you only need to subtract that offset in to have the accurate value. Most of the time, this should be the case.
On the other hand, if the sugar mix is off by a different amount, you’ll need to do some math to convert your readings. Assume SG0T is the temperature-adjusted reading for distilled water and SG15T is the temperature-adjusted reading for the 15° Plato solution, and SGxT is the temperature-adjusted reading you want to correct, you can apply the following formula:
Corrected SG = 1.000 + (((1.061 – 1.000) * (SGxT – SG0T))/(SG15T – SG0T))
To pick a relatively extreme example, if your hydrometer measured distilled water at SG0T = 0.998 (down 0.002) and the sugar solution at SG15T = 1.067 (up 0.006), then a reading of 1.059 yields an actual specific gravity of 1.054. Honestly, though, if my hydrometer were that far off on both ends without being a simple offset, I’d get a new one.
Once you know where you stand with your hydrometer, you can count on it to figure out the alcohol level of your beer to see how closely you matched a recipe or to assess your brewing efficiency.