A hydrometer is an instrument used to measure the density of liquids.
More specifically it measures the specific gravity of a liquid. The specific gravity is the relative density of a wine must in comparison to water.
A hydrometer floats higher in a heavier dense liquid than it does in plain water. This fact is the basis of hydrometer readings.
Once you add sugar and fruit juices to water you alter its specific Gravity.
Sugar is the catalyst that activates and feeds the yeast that ferments the must that makes the wine that tastes so fine by itself or when we dine.
For the purpose of wine making, a hydrometer measures the quantity of sugar in a solution that is capable of being converted to alcohol.
The Original Gravity is the specific gravity of a wine must prior to fermentation. The Final Gravity is the specific gravity measured upon completion of fermentation.
By knowing the amount of sugar dissolved in a liquid, how much sugar you began with [Original Gravity] and ended with, you can calculate the resulting alcohol or proof of your wine [Final Gravity].
The specific gravity of water is 1.000. Float your hydrometer in a test jar of water and that is the reading you should get. Dissolve sugar in that water and the hydrometer will float higher. A Pound of Sugar dissolved in a Gallon of water should give you a reading of 1.046. The sugar must be dissolved - not suspended in solution or laying on the bottom, but dissolved.
You can't measure the actual alcohol content of a beverage with a hydrometer , but you can measure the potential alcohol [PA] before you add in yeast [Original Gravity] and then measure it again once fermentation is complete [Final Gravity].
As the yeast converts sugar into alcohol and carbon dioxide, the weight of the liquid decreases because alcohol weighs less by volume. Hence - the hydrometer will not float as high as it did when you measured the Original Gravity. Fermented wine is lighter than water.
This synopsis is highly simplified, there are many other variables that are taken into consideration. Temperature being a major one. Wine making is not Rocket Science, but it can be pretty darn close.
How to use Your Hydrometer
You will need
A. Hydrometer designed for Beer or Wine
B . Test Jar. Sorry but an old mason jar will not suffice. It has to be long necked so the hydrometer can float unrestrained. The Jar should also be filled with an adequate amount of liquid so as to allow the hydrometer to float without touching the sides.
C. Item C can not be purchased - its called common sense.
Test samples should be at the correct temperature that the hydrometer is calibrated to.
Calibration temp. should be listed either on the instrument itself or the package it came with. The most common temp. is 600, but sometimes upwards of 700 F.
1. Fill Test Jar with sample of liquid to be tested.
2. Place Hydrometer into the liquid with the bulbous end down and slowly rotate it.
3. The hydrometer should not be in contact with the jars sides. You should take your reading a few times and be able to arrive at the same results each time. As per the old saying - measure twice - cut once.
Your reading is the Original Gravity. Repeat this process after Fermentation to arrive at your Final Gravity.
Subtract the Final Gravity for the Original Gravity multiply it by 131 and your result will be an fairly accurate approximation of alcohol by volume.