Specific gravity is the standard scale for what a hydrometer measures, choose either a plain form hydrometer or a combined form hydrometer. Plain form hydrometers do not have a built-in thermometer, use a separate thermometer to measure the temperature. Combined form hydrometers have a thermometer built into the body or stem to measure the temperature of the sample. API scale hydrometers are selected by the American Petroleum Institute (API), the United States Bureau of Mines, and the National Institute of Standards and Technology (NIST) as the standard scale for petroleum products. Baumé hydrometers are used for liquids heavier than water, such as syrups. Brix Scale hydrometers are used by the sugar industry. Salt hydrometers are graduated in % saturation or % by weight of salt in solution. Alcohol (Tralle and Proof Scales) hydrometers measure directly in percentage ethyl alcohol by weight in water from 0 to 100%. The Proof scale indicates the proof of alcohol from 0 to 200 proof. Proof is twice the percentage of alcohol by volume in solution. Soil hydrometers determine the specific gravity of a suspension, enabling the percentage of particles of a certain equivalent particle diameter to be calculated.