There are several instruments designed to measure specific gravity (density), including, specific gravity bottles (pyncnometers), hydrometers and refractometers. Hydrometers are long glass tubes which must have large volumes of sample in which to float. Refractometers rely on the refractive index of the sample liquid and require much smaller volumes.
Specific gravity and density or relative density is the ratio of the mass of a substance of a specific volume divided by the mass of the same volume of water at a specified temperature – usually 20°C. Specific gravity is expressed as relative to water where distilled water at 20°C is defined as 1.00; for example ethanol has a specific gravity of 0.78 and gold has a specific gravity of 19.3. Metric Relative Density are units of kg/m3 or g/cm3.
Specific gravity units are particular to an industry. Brewers (zymurgists) and the fruit juice industry uses degrees Brix, or °Bx, which is directly related to the concentration of sugar. The distilled beverage industry uses proof (in the US proof is equal to 2 x the % of alcohol by volume measure at 20°C). The petroleum industry uses °API (American Petroleum Institute). Petroleum products with a specific gravity greater than 10 °API are lighter than water and those lower than 10 °API are heavier than water. For °API the temperature must be specified, usually 60°F. Specific gravity can also be used to measure salinity or salt concentration.