The hydrometer is a simple instrument that measures the weight - or gravity - of a liquid in relation to the weight of water. Because the relation of the gravity to water is specified, the resulting measure is called a specific gravity. A hydrometer will float higher in a heavy liquid, such as one with a quantity of sugar dissolved in it, and lower in a light liquid, such as water or alcohol. In truth, the average winemaker has no interest in the specific gravity of a must per se, but has a very keen interest in the amount of sugar dissolved in it, for yeast converts sugar into carbon dioxide and alcohol. By knowing how much sugar one started with and ended with, one can easily calculate the resulting alcohol.
There are many variants of the hydrometer. Some have only one scale, some two and some three. The typical hydrometer measures three things: specific gravity (S.G.), potential alcohol (P.A.), and sugar.
Accuracy in Reading
The specific gravity scale will usually read from 0.990 to 1.120. The S.G. of water is 1.000. If you fill a test jar (a deep chimney-shaped vessel that holds from 1/2 to two cups of juice) with water and float your hydrometer in it, the water surface should rest at the 1.000 mark. As you dissolve sugar in the water, the hydrometer will float higher. One pound of sugar dissolved in one U.S. gallon* of water will float the hydrometer to the 1.045 level. ** Some vinters use a shorthand and simply call this level 45. I do not do that. I call it 1.045, as it actually reads. Shorthand can cause mistakes.
The most accurate readings can be obtained by ensuring that the hydrometer is thoroughly clean and, if using a narrow vessel like a Hydrometer Chimney, making sure there is enough clearance (about 1/2") between the stem and the sides of the chimney. This helps keep the hydrometer out of contact with the walls of the chimney and lessens the chance of inaccuracies due to surface tension.
Most hydrometers are calibrated to read liquids at 20 degrees Celcius. Any higher or lower and you reading won't be dead on.
A selection of various hydrometers for measuring specific gravity: