Hydrometers are calibrated glass floats that are used to compare the densities of liquids. They are used to measure the specific gravity (SG) or relative density of a material. Specific gravity is the density of a material divided by the density of water at standard temperatures. Specific gravity is expressed as a ratio. Specific gravity instruments include Baumé hydrometers for liquids, such as syrups, that denser than water; salt hydrometers, which show the percentage of salt in solution by weight; Brix hydrometers, devices used by the sugar industry to indicate the percentage of sucrose by weight; API hydrometers, which use a standard scale for petroleum products as defined by American Petroleum Institute (API); and Tralle and proof hydrometers, instruments which display the percentage of ethyl alcohol by weight in water. A thermohydrometer includes both a thermometer and a hydrometer. Thermohydrometers enable the accurate measurement of both temperature and specific gravity of a sample.