Vienna Standard Mean Ocean Water

Vienna Standard Mean Ocean Water (VSMOW) is a water standard defining the isotopic composition of fresh water. It was promulgated by the International Atomic Energy Agency (based in Vienna) in 1968, and, since 1993, continues to be evaluated and studied by the IAEA along with the European Institute for Reference Materials and Measurements and the American National Institute of Standards and Technology. The standard includes both the established values of stable isotopes found in waters and calibration materials provided for standardization and interlaboratory comparisons of instruments used to measure these values in experimental materials.

The designation ocean water refers only to the evaporation of ocean waters in the hydrologic cycle as the original source of fresh surface and ground waters and precipitation, but VSMOW is not a standard for seawater. Fresh distilled VSMOW water is also used for making high accuracy measurement of water's physical properties and for defining laboratory standards since it is considered to be representative of average ocean water, in effect representing all water on Earth.

Before VSMOW was defined, average ocean water and melted snow were used as references. These conventions were refined in the 1960s by the standardized definition of Standard Mean Ocean Water (SMOW). The U.S. National Bureau of Standards (now the National Institute of Standards and Technology, NIST) created physical water standards for global use. However, the physical integrity of the U.S. standards came into question. The use of the SMOW standard was discontinued.

VSMOW is a recalibration of the original SMOW definition and was created in 1967 by Harmon Craig and other researchers from Scripps Institution of Oceanography at the University of California, San Diego who mixed distilled ocean waters collected from different spots around the globe. VSMOW remains one of the major isotopic water benchmarks in use today.

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