Using Your Hydrometer

Using your Hydrometer

A hydrometer is a precision instrument used to measure the density of a liquid.  When placed in any liquid, the hydrometer will sink until the weight of the displaced liquid equals the weight of the hydrometer. This means that it will float 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, or SG. Sugar solutions are denser (meaning they have a greater SG) and alcohol solutions are less dense (meaning they have a lower SG).

Our hydrometers are calibrated at 60°F using pure water, which has an SG of 1.000.

Knowing how to use your hydrometer will allow you to calculate several things about your beer and wine: the potential alcohol, how quickly the fermentation is proceeding, and when fermentation is finished. To measure specific gravity:

  • Add a sample of the beer or wine to be tested in a test jar or cylinder. Try to avoid gathering foam as you pour the sample —bubbles make it difficult 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 cling to it.
  • When the hydrometer has come 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. Read the value across the flat liquid surface, before it touches the hydrometer.
  • Adjust the hydrometer reading based on temperature of the liquid. Use this table calculate the corrected SG.

Temperature Correction Table

Degrees Farenheight Degrees Celsius Correction Factor
50 10 -0.005
60 15.5 0.000
70 21 +0.001
77 25 +0.002
84 29 +0.003
95 35 +0.005
105 40.5 +0.007

If your sample is 84°F and your gravity reading is 1.095, the table shows that you have to add 0.003 to correct your reading. The corrected SG is 1.098.

What if your reading is between values?  Lets say your sample is 74o F.  The temperature value is between the range of 70-77, so we approximate using the range.  If you have a reading of 70-73o F, add .001 to your actual hydrometer reading.  If the reading is 74-77o F, add .002. 

Calculating Alcohol by Volume (ABV)

While our hydrometers may have a scale to calculate ABV, the best method is to use your calculated starting and finished gravity (SG and FG) to determine the actual ABV%.  The equation is:

(Corrected starting gravity - corrected finished gravity) X 133 = ABV%

Example: SG=1.100, FG=1.000   ABV = (1.100-1.000) X 133 = 13.3% ABV  

Keep your readings in a journal.  When you can look up a series of SG measurements, you'll know immediately whether a wine fermented quickly or slowly, or if your beer finished with a higher SG than intended, and you'll be able to use that information to make better beer and wine.