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The Hydrometer

by Alberta Rager

Tips & Tidbits - The Hydrometer

The simplest and most important piece of test equipment you can get is a hydrometer. It allows you to measure the density of your wine, beer, mead or cider as compared to the density of fresh water. The density is referred to as Specific Gravity (SG). If sugar is dissolved into water, the sugar will make it denser. If alcohol is dissolved into water, the resulting liquid will be lighter. The first thing a hydrometer will tell you is Original Gravity (OG). If the OG is known, the potential alcohol can be determined. The next thing the hydrometer will tell you is how quickly the fermentation is progressing. If you take readings on successive days and the specific gravity reading has dropped by 10 points, you know that a strong and vigorous fermentation is going on. If it continues to stay the same on successive days, you know that fermentation has come to a stop.


A hydrometer works this way: Put a sample of your product in the test jar. A good way to do this is by using a sanitized wine thief and take only what you need for the test. Then float the hydrometer in the sample and carefully read the gravity level from the scale inside the hydrometer. Immediately write down the date and specific gravity reading in your handy notebook. If using "The Thief" rather than a test jar, put your hydrometer in "The Thief" after sanitizing both. Insert "The Thief" in your product and watch it fill. Follow the reading and recording procedures above, then hold the pin against the side of the carboy or bucket allowing the sample to return before removing "The Thief".


Using Your Hydrometer

A hydrometer consists of a hollow cylindrical glass tube weighted at the bottom with steel shot, and attached to a long narrow stem containing a tube of white paper marked with a graduated scale. The hydrometer will sink into a liquid until the weight of the displaced liquid equals the weight of the hydrometer. This means that it will sink deeper into a liquid of low density than it will sink into a liquid of high density. The density is described as the liquids Specific Gravity (SG). Pure water at 60 degrees F has a SG of 1.000. Sugar solutions are denser meaning that they have a greater SG while alcohol solutions are less dense meaning that they have a lower SG.

Here's how to read a hydrometer: put a sample of the beer, wine, mead or cider to be tested in a test jar. Try not to let sample foam while filling the test jar - this makes it impossible to read the hydrometer scale. Insert the hydrometer into the jar and twirl it gently with your fingers to dislodge any bubbles that might be clinging to it. When the hydrometer comes to rest, read the SG from the bottom of the meniscus. The meniscus is the surface of the liquid that climbs up the stem of the hydrometer because of surface tension. In other words, sight across the general surface of the liquid with your eye.

Interpreting Hydrometer Readings

To calculate the potential alcohol from a hydrometer reading, take the OG and subtract the SG of the finished product and multiply by 131. This will give you the percent alcohol by volume. Or even easier, our hydrometers are triple scale, so subtract the potential alcohol of the finished product from the original potential alcohol. The potential alcohol scale is next to the specific gravity scale on the hydrometer, so just turn it slightly for the potential alcohol reading. Most wine, mead and cider will have a final gravity reading of .998 +/- .002. Most beers will ferment down by approximately 75%.

Hydrometers are calibrated at 60 degrees F. Readings can be thrown off by a higher or lower temperature. A temperature correction scale was included with your hydrometer at the time of purchase. If you've misplaced yours and would like a replacement, let us know.


The last thing to remember about using your hydrometer is that the reading will only do you good if you write them down. When you can look up a series of SG reading you'll know immediately if your wine fermented quickly or slowly, or if your beer finished with a higher SG than intended. You'll be able to use this information to make better, beer, wine, mead and cider.


This article was published on Thursday May 14, 2009.