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Previously average ocean water and melted snow were used as reference points. These were further refined in the 1960s by the standardized definition of Standard Mean Ocean Water (SMOW). The U.S. National Bureau Of Standards created physical water standards for global use. But the physical integrity of the US standards soon came into question. VSMOW was a recalibration of the original SMOW definition created by mixing distilled ocean water with other waters and remains one of the major benchmarks in use today. VSMOW water is important in the manufacture of high accuracy Temperature measurement reference standards. Both the Kelvin and Celsius scales are defined by the Triple Point of water (273.16 K and 0.01 °C respectively). The trouble is that for high accuracy measurements, not all water is the same so VSMOW water is used as the “standard” water. The reason for this is water molecules comprised of different isotopes of hydrogen and/or oxygen evaporate at different temperatures and at different rates. Consequently, snow, river water, and rain water (all of which are recently evaporated ocean water) tend to be enriched in those isotopes that evaporate faster. Triple point-based temperature reference cells filled with water of improper isotopic composition can cause errors of several hundred µK in the measured triple point. The isotopic composition of VSMOW water is specified as ratios of the rare isotope in question divided by its most common isotope and is expressed as parts per million (PPM). For instance 16O (the most common isotope of oxygen with eight protons and eight neutrons) is roughly 2,632 times more prevalent in sea water than is 17O (with an additional neutron). The isotopic ratio of VSMOW water is defined as follows: 2H / 1H = 155.76 ±0.1 3H / 1H = 1.85 × 10-11 (ignored for physical properties-related work) 18O / 16O = 2005.20 ±0.43 17O / 16/O = 379.9 ±1.6 Two effects of defining the triple point of VSMOW water as both 0.01 °C and 273.16 K are that the freezing and boiling points of water under one standard atmosphere (101.325 kPa) are no longer the defining points for the Celsius scale. The triple point of water was so close to being 0.01 K greater than water’s known melting point, it was simply defined as precisely 0.01 °C. However, current measurements show that the melting point of VSMOW water is very slightly (<0.001 K) less than 0 °C. Similarly, defining water’s triple point at 273.16 K precisely defines the magnitude of each 1 °C increment in terms of the absolute thermodynamic temperature scale (referencing Absolute Zero ). Now decoupled from the boiling point of water, the value “100 °C” is strictly, by definition, 373.15 / 273.15ths the temperature of 0 °C. The effect of this is that current measurements show that the boiling point of VSMOW water under one standard atmosphere of pressure is about 99.9839 °C on the absolute thermodynamic temperature scale. On the ITS-90 international temperature scale (the scale used for most high-precision instrumentation), the boiling point of VSMOW water is very slightly less, about 99.974 °C. The maximum density of VSMOW water is 999.97495 kg/m3 at 3.984 °C. External links |