The International Temperature Scale of 1990 (ITS-90) is an equipment calibration standard for making measurements on the kelvin and Celsius temperature scales. ITS–90 facilitates the comparability and compatibility of temperature measurements internationally. ITS–90 offers defined calibration points ranging from 0.65 K to approximately 1358 K (-272.5 °C to 1085 °C) and is subdivided into multiple temperature ranges which overlap in some instances. The kelvin (symbol: K) is the SI unit of temperature, and is one of the seven SI base units. ... The degree Celsius (symbol: °C) is an SI derived unit of temperature. ...

ITS-90 is designed to represent the absolute thermodynamic temperature scale (referencing absolute zero) as closely as possible throughout its range. Many different thermometer designs are required to cover the entire range. These include Helium Vapor Pressure Thermometers, Helium Gas Thermometers, Standard Platinum Resistance Thermometers and Monochromatic Radiation Thermometers Thermodynamic temperature is a measure, in kelvins (K) of temperature for thermodynamics, with a uniquely defined zero point at absolute zero. ... Absolute zero is a fundamental lower bound on the temperature of any macroscopic system. ...

Although the kelvin and Celsius scales are defined using absolute zero (0 K) and the triple point of water (273.16 K and 0.01 °C), it is impractical to use this definition at temperatures that are very different from the triple point of water. Accordingly, ITS–90 uses numerous defined points, all based on the thermal properties of phase changes in materials (melting or freezing) or their triple point values. Thermometers calibrated per ITS–90 use complex mathematical formulas to interpolate between these defined points. Other examples of defining points are the triple point of hydrogen (–259.3467 °C) and the freezing (melting) point of aluminum (660.323 °C). A practical effect of ITS–90 is the triple points and the freezing/melting points of its thirteen chemical elements are precisely known for all temperature measuring instruments calibrated per ITS–90 since these thirteen values are fixed by its definition. Only the triple point of Vienna Standard Mean Ocean Water (VSMOW) is known with absolute precison—regardless of the calibration standard employed—because the very definitions of both the kelvin and Celsius scales are fixed by international agreement based, in part, on this point. In physics, the triple point of a substance is the temperature and pressure at which three phases (gas, liquid, and solid) of that substance may coexist in thermodynamic equilibrium. ... // Overview of VSMOW VSMOW, or Vienna Standard Mean Ocean Water, is a standard defined in 1968 by the International Atomic Energy Agency. ...

There are often small differences between measurements calibrated per ITS–90 and absolute thermodynamic temperature. For instance, precise measurements show that the boiling point of VSMOW water under one standard atmosphere of pressure is actually 373.1339 K (99.9839 °C) when adhering strictly to the two-point definition of absolute thermodynamic temperature. When calibrated to ITS–90, where one must interpolate between the defining points of gallium and indium, the boiling point of VSMOW water is very slightly less, about 99.974 °C. The virtue of ITS–90 is that another lab in another part of the world will measure the very same temperature with ease due to the advantages of an international standard featuring many conveniently spaced, reproducible, defining points spanning a wide range of temperatures.

The below table shows defining fixed points of ITS-90.

In physics, the triple point of a substance is the temperature and pressure at which three phases (gas, liquid, and solid) of that substance may coexist in thermodynamic equilibrium. ... General Name, Symbol, Number hydrogen, H, 1 Chemical series nonmetals Group, Period, Block 1, 1, s Appearance colorless Atomic mass 1. ... General Name, Symbol, Number neon, Ne, 10 Chemical series noble gases Group, Period, Block 18, 2, p Appearance colorless Atomic mass 20. ... General Name, Symbol, Number oxygen, O, 8 Chemical series Nonmetals, chalcogens Group, Period, Block 16, 2, p Appearance colorless Atomic mass 15. ... General Name, Symbol, Number argon, Ar, 18 Chemical series noble gases Group, Period, Block 18, 3, p Appearance colorless Atomic mass 39. ... General Name, Symbol, Number mercury, Hg, 80 Chemical series transition metals Group, Period, Block 12, 6, d Appearance silvery white Atomic mass 200. ... // Overview of VSMOW VSMOW, or Vienna Standard Mean Ocean Water, is a standard defined in 1968 by the International Atomic Energy Agency. ... The melting point of a solid is the temperature at which it changes state from solid to liquid. ... General Name, Symbol, Number gallium, Ga, 31 Chemical series poor metals Group, Period, Block 13, 4, p Appearance silvery white   Atomic mass 69. ... The melting point of a solid is the temperature at which it changes state from solid to liquid. ... General Name, Symbol, Number indium, In, 49 Chemical series poor metals Group, Period, Block 13, 5, p Appearance silvery lustrous gray Atomic mass 114. ... General Name, Symbol, Number tin, Sn, 50 Chemical series poor metals Group, Period, Block 14, 5, p Appearance silvery lustrous gray Atomic mass 118. ... General Name, Symbol, Number zinc, Zn, 30 Chemical series transition metals Group, Period, Block 12, 4, d Appearance bluish pale gray Atomic mass 65. ... Aluminum is a soft and lightweight metal with a dull silvery appearance, due to a thin layer of oxidation that forms quickly when it is exposed to air. ... General Name, Symbol, Number silver, Ag, 47 Chemical series transition metals Group, Period, Block 11, 5, d Appearance lustrous white metal Atomic mass 107. ... General Name, Symbol, Number gold, Au, 79 Chemical series transition metals Group, Period, Block 11, 6, d Appearance metallic yellow Atomic mass 196. ... General Name, Symbol, Number copper, Cu, 29 Chemical series transition metals Group, Period, Block 11, 4, d Appearance metallic brown Atomic mass 63. ...

See also

• Thermodynamic (absolute) temperature
• Triple point
• Vienna Standard Mean Ocean Water (VSMOW)

Thermodynamic temperature is a measure, in kelvins (K) of temperature for thermodynamics, with a uniquely defined zero point at absolute zero. ... In physics, the triple point of a substance is the temperature and pressure at which three phases (gas, liquid, and solid) of that substance may coexist in thermodynamic equilibrium. ... // Overview of VSMOW VSMOW, or Vienna Standard Mean Ocean Water, is a standard defined in 1968 by the International Atomic Energy Agency. ...

External links

• ITS-90 (by Bureau International des Poids et Mesures which is the governing body)
• ITS-90 (by Swedish National Testing and Research Institute)
• ITS-90 (by Omega Engineering)
• Temperature seminars, products, and services
• Temperature Sensors (information repository)

References

• Preston-Thomas H., Metrologia, 1990, 27(1), 3-10 (amended version).