A hydrometer measures the density of a liquid relative to that of water, also known as the liquid's specific gravity. Commercial hydrometers are available in stores that sell winemaking supplies, or you can make your own hydrometer as part of a school experiment. A homemade hydrometer generally consists of a straight drinking straw with a small amount of waterproof clay sealing one end. A liquid with a high specific gravity causes the straw to float higher in the liquid. You must calibrate a homemade hydrometer before you can measure the specific gravity of a liquid.
Fill a clear drinking glass with distilled water. The drinking glass must be tall enough to prevent the bottom of your homemade hydrometer from touching the bottom of the glass. Place the hydrometer in the glass, and release the hydrometer so that it floats in the water. Spin the hydrometer gently in place to dislodge any air bubbles.
Ensure your hydrometer is not touching the glass, and wait for it to stop bobbing in the water. Hold the hydrometer between your thumb and forefinger at the level of the water and pull it out of the water. Mark a horizontal line with a waterproof marker on your hydrometer where the water level was.
Label this line with the specific gravity of the liquid. Distilled water has a specific gravity of 1.000, so you should label this line as "1.000." Specific gravity is a ratio of values with equal units of measure, so specific gravity has no units.
Repeat steps 1 through 3 for at least one other liquid with a known specific gravity. For example, ethanol has a specific gravity of 0.789. Calibrate your hydrometer with additional liquids to increase its accuracy.
Place your hydrometer into a liquid with an unknown specific gravity. Compare the level of the liquid against the markings on your hydrometer. This will allow you to estimate the specific gravity of the unknown liquid. For example, if you place your hydrometer into a glass of olive oil, which has a specific gravity of 0.915, the level of the liquid on the hydrometer should be between the specific gravity of distilled water and that of ethanol.