Measure Specific Gravity without a Hydrometer

Not too long ago, I broke my hydrometer at probably the worst possible moment of the brew day: just after I had taken a test sample but before I had taken a specific gravity reading. In a panic, I weighed the sample and hydrometer test jar on a small digital scale and then marked the liquid level on the side of the test jar. Then I discarded the sample wort and then filled the test jar with water to the line I marked previously. Then I weighed the water and the sample jar. By comparing the weight (actually mass, since my scale measures in grams) of the wort sample with the weight of the same volume of water, I was able to get a fairly accurate specific gravity reading. There's no reason why a broken hydrometer has to ruin your beer brewing sessions.

The easiest way to prepare against a broken hydrometer is to have two of them on hand at all times (or to procure a refractometer). This isn't always practical or possible, so it pays to know the basic concepts of measuring specific gravity. In a nutshell, and for the purposes of measuring for homebrewing, specific gravity is the ratio of the mass of a volume of wort (or beer) to the mass of an equal volume of distilled water. For our purposes, it is perfectly fine to assume the known density of distilled water (1g/mL) for the calculations that follow.

To measure specific gravity of your homebrew without a hydrometer or refractometer, you'll need a scale calibrated to measure accurately (+/- 0.1g) at small weights/masses. A common electronic jeweler's scale will work fine. I ordered one from an online vendor a couple of years ago to measure small quantities of hops and spice additions (check www.saveonscales.com for several models less than $30). You'll also need a graduated cylinder (50mL or smaller is best), which should run about $5. For this project, I'll be working in metric units.

First, measure the mass of the empty graduated cylinder. If your scale has a tare feature, go ahead and zero out the mass of the cylinder at this point. If not, record the mass of the empty cylinder. Take a small sample of your beer using your normal method and fill the cylinder so that the bottom of the liquid curve (called the meniscus) is even with the 40mL mark. Put the cylinder back on the scale and record the mass. If you used the tare feature on your scale earlier, you can go straight to calculating specific gravity. If not, subtract the mass of the empty cylinder from the mass of the cylinder plus the sample and use the result in your calculations.

To get the specific gravity of your sample, divide its mass by the mass of an equal volume of water (40mL in this case). Let's assume our sample had a mass of 42.5g. Since we know (assuming the above listed assumptions) that water has a mass of 1g/mL, we can assume that 40mL of water has a mass of 40g. That gives us a specific gravity reading of 1.063 (that is, 42.5g/40g) after rounding up.

When putting together your own measuring system, consider the following things: A sample size in the 35mL to 50mL range is a nice tradeoff between sample size and accuracy. The larger the sample, the more fine-tuned the result will be (assuming all other variable are the same)...but it also means less beer in your bottles or kegs at the end of the brewing process. A sample size of 100mL is also easy to work with and accurate, but it can be difficult to find a graduated cylinder of that volume with a small enough base to fit on the measuring tray of smaller scales.