Using your Hydrometer
Using Your HydrometerA 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. This density is described as the liquids' Specific Gravity. Pure water at 15°C (60°F) has a S.G. of 1.000. Sugar solutions are denser (meaning they have a greater S.G.) and alcohol solutions are less dense (meaning they have a lower S.G.).
Knowing how to use your hydrometer will let you calculate several things about your beer and wine: the potential alcohol, how quickly the fermentation is proceeding, and when fermentation is finished. You read a hydrometer like this: put a sample of the beer or wine to be tested in a hydrometer jar. Try not to let the sample foam up as you pour it in the jar—this makes it impossible to read the hydrometer scale. Insert the hydrometer into the jar and twirl it gently with your fingers to shake off any bubbles that might be clinging to it. When the hydrometer has come to rest, read the S.G. 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.
Hydrometer readings can be thrown off by temperatures higher or lower than the hydrometer has been calibrated for. All of the hydrometers that we sell are intended for use at 15°C, 60°F. If the temperature of your sample is different, use this table calculate the correct S.G.
°C °F Correction
10 50 -0.005
15.5 60 0.000
21 70 +0.001
25 77 +0.002
29 84 +0.003
35 95 +0.005
40.5 105 +0.007
Therefore, if your sample is 29°C and your gravity reading is 1.095, when you look at the table you'll see that you have to add 0.003 to correct your reading. The corrected S.G. is 1.098. The last thing to remember about using your hydrometer is that the readings will only do you good if you write them down somewhere. When you can look up a series of S.G. measurements, you'll know immediately whether a wine fermented quickly or slowly, or if your beer finished with a higher S.G. than intended, and you'll be able to use that information to make better beer and wine.
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