International Temperature Scale of 1990

International Temperature Scale of 1990. The International Temperature Scale of 1990 is an equipment calibration standard, defined by the International Committee for Weights and Measures to accord closely with the thermodynamic temperature scale. It provides a precise, reproducible framework for practical temperature measurement, superseding the International Practical Temperature Scale of 1968. The scale is maintained and disseminated globally by national metrology institutes like the National Institute of Standards and Technology and the National Physical Laboratory (United Kingdom).

Definition and purpose

The primary purpose of the ITS-90 is to provide a universally accepted, highly precise protocol for calibrating temperature-measuring instruments. It serves as a practical approximation of the thermodynamic temperature scale, which is based on the fundamental laws of thermodynamics and is theoretically defined by the Carnot cycle. By defining a series of reproducible fixed points and prescribed interpolation instruments, the ITS-90 allows laboratories worldwide to achieve consistent and comparable temperature measurements. This standardization is critical for scientific research, industrial processes, and international trade, ensuring uniformity from the cryogenic range to very high temperatures.

Historical context and development

The development of the ITS-90 was driven by advances in metrology and the need for greater accuracy than provided by its predecessor, the International Practical Temperature Scale of 1968. Its creation was overseen by the Consultative Committee for Thermometry under the auspices of the International Committee for Weights and Measures. The work built upon a long history of temperature standardization, including earlier scales like the International Temperature Scale of 1948 and the foundational work of scientists such as Lord Kelvin. The adoption of the ITS-90 was formally endorsed at the General Conference on Weights and Measures in 1987, with its official publication occurring in 1990.

Defining fixed points and interpolation

The ITS-90 is constructed using seventeen defining fixed points, which are highly reproducible equilibrium states of pure substances. These range from the triple point of hydrogen at approximately 13.8 K to the freezing point of copper at 1357.77 K. Between these fixed points, temperature is determined by specified interpolation formulas using designated standard instruments. For much of its range, the platinum resistance thermometer is the primary interpolating device, while above the freezing point of silver, a monochromatic radiation thermometer operating according to the Planck radiation law is used. The scale also incorporates the triple point of water as a fundamental defining point, assigned a value of 273.16 K exactly.

Comparison with previous scales

The ITS-90 introduced significant refinements over the International Practical Temperature Scale of 1968, leading to measurable differences in reported temperatures. Key changes included the extension of the lower limit from 13.81 K down to 0.65 K using the vapor pressure of helium-3 and helium-4, and the revision of many fixed-point values based on new thermodynamic temperature measurements. The scale also eliminated the use of the IPTS-68's platinum resistance thermometer reference functions, adopting newer, more accurate formulations. These adjustments brought practical measurements into closer agreement with thermodynamic reality, as determined by techniques like acoustic thermometry and gas thermometry.

Practical implementation and usage

Implementation of the ITS-90 is the responsibility of national metrology institutes, such as the National Institute of Standards and Technology in the United States and the Physikalisch-Technische Bundesanstalt in Germany. These laboratories maintain primary standard instruments calibrated according to the scale's definitions and provide calibration services to industry and secondary laboratories. The scale is used extensively in fields requiring precise temperature control, including pharmaceutical manufacturing, semiconductor fabrication, and aerospace engineering. Its adoption ensures that temperature data reported in scientific journals, such as those from CERN or the NASA, are consistent and traceable to an international standard.

Amendments and ITS-90 updates

Since its adoption, the ITS-90 has been subject to minor amendments and clarifications issued by the International Committee for Weights and Measures through the Mise en Pratique for the definition of the kelvin. These updates typically address technical details, such as refined isotopic compositions for fixed-point materials like water and hydrogen, or improved formulations for interpolation. The scale itself remains the official international standard, though ongoing research in primary thermometry, including work at institutions like the National Physical Laboratory (United Kingdom), informs the Consultative Committee for Thermometry and may influence the development of a future temperature scale. The current definitions are published in the official documentation of the International Bureau of Weights and Measures. Category:Temperature scales Category:Metrology Category:1990 introductions