The API scale was selected in 1921 by the American Petroleum Institute (API), the U.S. Bureau of Mines, and the National Institute of Standards and Technology (NIST) as the standard hydrometer scale for petroleum products. Our API ASTM Specification Hydrometers are manufactured in strict accordance to specifications of ASTM (American Society for Testing and Materials) for accuracy and design. On select API Hydrometer Hydrometers with a built-in thermometer, the thermometer scale is located in the body of the hydrometer. A correction scale for conversion of the reading to 60°F is printed opposite the temperature scale.
