A hydrometer is a tool that every winemaker should own. It is a small graduated tube which usually has three (3) scales, Specific Gravity, Brix Scale, and Alcohol Potential on it. Hydrometers are also available containing a thermometer. If you are not planning on purchasing a separate thermometer, then I recommend that you purchase a hydrometer that has one. The temperature is important when it comes to ensuring your must is within the proper range for healthy fermentation.
One can get extremely technical with a hydrometer and the readings it can provide. For the majority of home winemakers, however, there are just some basics that you need to understand to make sure your wine is progressing properly. For that we need to focus on the specific gravity scale. For those of you that are already asking, “specific what?” The specific gravity is the density of a liquid compared to water. I know, that definition may not have helped, but it really is not complicated if you’ve ever been swimming in the ocean and a pool. When you go swimming in the ocean, it is much easier to float than when you are in a pool. This is because the salt in the ocean makes the water more dense, therefore it has a higher specific gravity.
So carrying this analogy over to winemaking, when you float the hydrometer in an unfermented juice the sugar content allows the hydrometer to float high. As the wine ferments, the amount of sugar present decreases and the hydrometer floats lower giving a lower specific gravity reading. Now some may be saying that the temperature of the liquid affects your specific gravity readings and adjustments should be made. Technically, this is true since the Specific Gravity scale on the hydrometer is based upon water at 60°F. However, the adjustments for temperature are minimal and for the basic home winemaker, you simply want to know that your wine is fermenting properly and when it is time for the next step. As such, I don’t feel it is necessary to be that worried about it.
Finally, the other information you may want to know is roughly what is the alcohol content of your final wine. This can be determined through a simple calculation. If you take your starting specific gravity and subtract the specific gravity after you clear/stabilize your wine and divide that number by 0.736 and multiply by 100 you will have a good estimate of your alcohol percentage.
[SG (begin) - SG (final)] / 0.736 * 100 = % Alcohol
As I said, we have only brushed the surface of the hydrometer, but if you understand this much you be well on your way.
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