Our API scale hydrometers measure the scale which 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.

• Safety blue thermometer liquid fill

The thermometer scale (where applicable) for these thermo-hydrometers is located in the body. All instruments are suitable for certification for use as a primary reference with NIST traceability.

The API scale is based on the following formula: °API at 60 °F = 141.5/(specific gravity at 60 °F) - 131.5.