The hydrometer, invented in the early eighteenth century, is a simple floating device calibrated to measures the specific gravity or density of a liquid. It is readily available at your local home brew shop, is inexpensive and has several important vinegar making uses.
If you are making apple cider vinegar from apple juice, this test unit can be used to determine the amount of sugar in the juice and the potential amount of alcohol you will get by completely fermenting that apple juice to hard cider.
The hydrometer is a sealed, hand blown glass tube, which contains lead shot in the bottom and several calibrated scales in the upper stem. (One scale for specific gravity, one for potential alcohol and one for sugar content.) It is used with a tall cylindrical testing jar, which holds the liquid to be tested.
Once the testing jar is filled with juice and the glass tube inserted, it will sink to a level determined by the density of the sample, which is related to the amount of soluble solids (mostly sugar) contained in the juice.
The Hydrometer Test Procedure
Correction for Specific Gravity
For example, if on a hot summer day your apple juice reads 1.036 on the specific gravity scale and the temperature of the juice is 95 degrees F (35 C) then the correct specific gravity would be: 1.036 + 0.005 = 1.041.
Using the Hydrometer to Determine
Multiplying the Brix value by 0.56 will give the potential alcohol content (% by volume) that the juice would have if the sugar was completely fermented to alcohol.
If your hydrometer does not have a brix or potential alcohol scale, then you can use the following table to convert your juices specific gravity to degrees Brix and potential alcohol content.
Note: The table specific gravity values are taken at 20 degrees C.
Ref.: USDA Technical Inspection Procedures-Brix Measurement
@20 degrees C
|Potential Alcohol Content
% by volume
Click here to see a hydrometer being used to measure the sugar content of an apple juice in this short You Tube video.
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