A hydrometer is a device used to measure the specific gravity of unfermented wort and fermented beer during the brewing process. The most common form of brewing hydrometer is a hollow glass device with a metal weight at the bottom. A paper scale is included inside the tube, and usually marked in one or more scales. The hydrometer floats freely in the liquid you are measuring, and is read at the point where the surface of the liquid touches the stem. The most frequently used scale on a hydrometer is the specific gravity scale. The specific gravity scale provides a relative measure of sugar as compared to water. Water would have a specific gravity of 1.000, while most finished beers have a starting gravity before fermentation of 1.030-1.060 and around 1.010 after fermentation. Sometimes specific gravity is shown as a 4 digit number without the decimal point - for example 1040. Fermentation lowers the specific gravity of the beer because alcohol is less dense than water.
Using a Hydrometer
The proper way to use a hydrometer is to draw a small quantity of wort or beer into a sampling tube or test jar, and then insert the hydrometer into the sample. Do not use the plastic tube the hydrometer came in for measuring a sample as it is too small for a proper measurement. Take your reading on the appropriate scale by reading where the surface of the sample crosses the scale on the hydrometer.
Adjusting for Temperature
The most common type of hydrometer sold for brewing is calibrated for use on water at temperature of 60 F. If the beer you are measuring is at a different temperature, you need to make an adjustment