In the U.S. hydrometers measure specific gravity to the thousandths place; the scale is based on the specific gravity of water, which is 1.000. Hence, if you fill a vessel with water and place the hydrometer in it, the surface of the water will be level with the hydrometer's 1.000 mark. But dissolving sugar in the water will increase the density, causing the hydrometer to float higher. For example, one pound of sugar dissolved in one gallon of water will float the hydrometer to the 1.046 level. Hydrometers are standardized at 59 degrees Fahrenheit, but temperature correction tables are available online to ensure an accurate reading.
Hydrometers are essential for making wine. They serve a number of purposes, including assessing the progress of fermentation, gauging dryness and determining whether sugar must be added to reach the required alcohol content and desired sweetness level. At the beginning of fermentation the hydrometer will float at its highest while at the end of fermentation it will sink to its lowest point. Most standard dry table wines will have a post-fermentation specific gravity between 0.990 and 1.000.
The use of a hydrometer in brewing beer serves a similar function to that of wine-making. Beer recipes take into consideration the original gravity (OG) and final gravity (FG) of the product. The FG of most beers lies between 1.000 and 1.015. In both brewing and wine-making, test jars are often used in conjunction with a hydrometer. Instead of placing the hydrometer into the fermentation vessel, a sample of the must or wort is siphoned into a test jar; this decreases the risk of contaminating the entire batch.
Hydrometers are used in saltwater aquariums to ensure that salinity remains at the proper level to support healthy marine life. When water evaporates from aquariums, the salt does not, so the salinity of the water must be monitored and adjusted. Depending on the type of aquarium and life it is supporting, the specific gravity of the saltwater could safely range from 1.020 to 1.025.