ITS-90

ITS-90
Marián Hubinský · CC BY-SA 4.0 · source
Name	International Temperature Scale of 1990
Birth date	1990
Field	Metrology
Known for	Thermodynamic temperature approximation, fixed-point definitions, calibration

ITS-90

The International Temperature Scale of 1990 is an internationally agreed practical temperature scale established to provide uniform calibration and dissemination of temperature measurements across national metrology institutes such as Bureau International des Poids et Mesures, National Institute of Standards and Technology, Physikalisch-Technische Bundesanstalt, National Physical Laboratory (United Kingdom), and International Organization for Standardization. It replaced earlier scales and linked fixed-point realizations with interpolation instruments used by laboratories including National Research Council (Canada), Institut National de Métrologie (France), METAS, CSIR-NPL, and JILA. ITS-90 underpins comparisons and standards activities involving organizations such as Comité International des Poids et Mesures, International Electrotechnical Commission, International Bureau of Weights and Measures', and regional bodies like European Commission-funded metrology networks.

History and development

ITS-90 evolved from predecessor scales such as the International Practical Temperature Scale of 1968 and the Kelvin (unit) reform discussions that engaged scientists and institutions including Richard C. Jones (metrologist), William A. Macleod, Fritz Haber Institute-affiliated researchers, and committees of the Comité Consultatif de Thermométrie. Drafting and adoption involved interlaboratory comparisons coordinated by BIPM and technical working groups in the International Committee for Weights and Measures that consulted experts from National Institute of Metrology (China), Istituto Nazionale di Ricerca Metrologica, Laboratoire National de Métrologie et d'Essais, Centro Nacional de Metrología, and Japan National Metrology Institute. Conferences and symposia at venues like International Temperature Symposiums, GUM (Guide to the Expression of Uncertainty in Measurement) workshops, and meetings in Paris, Bipm Sèvres, Braunschweig, and Teddington informed the scale’s text and tables.

Definition and scope

The scale defines temperature by specifying sixteen principal fixed points and interpolation formulas employed with instruments such as platinum resistance thermometers, gas thermometers, and radiation thermometers. It ties realization to reproducible phenomena cited by laboratories including Argonne National Laboratory, Lawrence Berkeley National Laboratory, CERN, Max Planck Institute for Chemistry, National Metrology Institute of Japan, and NIST. ITS-90 covers the range from a few millikelvins (via low-temperature fixed points) to several thousand kelvins (via blackbody radiators and high-temperature fixed points) relevant to institutions like Oak Ridge National Laboratory, Los Alamos National Laboratory, and Sandia National Laboratories. The international scope addresses calibration chains between primary standards maintained at BIPM and national standards maintained at PTB, NPL, and NMIJ.

Fixed points and temperature scale realization

ITS-90 lists fixed points such as the triple point of water (H2O), the freezing points of tin, zinc, aluminium, and the thermodynamic equilibrium points of neon, argon, oxygen, and hydrogen isotopic variants referenced by cryogenic facilities at Cambridge University Cavendish Laboratory, MIT, and ETH Zurich. High-temperature fixed points use metal freezing plateaus and blackbody cavities developed by groups at Fraunhofer Society, Rutherford Appleton Laboratory, and Institut Laue-Langevin. Fixed-point cell design, material purity, and isotopic composition were subjects of studies published by teams at University of Oxford, University of Tokyo, University of California, Berkeley, University of Melbourne, and Peking University. Realization procedures reference apparatus from manufacturers and labs like Fluke Corporation, Isotech Ltd, Cryogenic Ltd, and Oxford Instruments.

Measurement methods and instrumentation

ITS-90 implementations employ platinum resistance thermometers (PRTs), standard platinum resistance thermometers (SPRTs), thermocouples, acoustic gas thermometry setups, and radiation thermometers. Instrumentation development drew on research at Bell Labs, IBM Research, Siemens, Hitachi, Toshiba, General Electric Research Laboratory, and academic groups at Princeton University, Harvard University, Caltech, University of Illinois Urbana–Champaign, and Seoul National University. Techniques include four-wire resistance measurement circuits used in laboratories such as NPL and PTB, self-heating correction protocols developed at NIST, and real-time data acquisition systems from companies like National Instruments. Calibration artefacts and comparison methods were refined through intercomparisons coordinated by APMP, EUROMET, and SIM.

Uncertainties, corrections, and calibration

Uncertainty budgets for ITS-90 realizations incorporate contributions from thermometer self-heating, immersion effects, thermal gradients, isotopic composition, and repeatability as analyzed under guidance from ISO/IEC committees and the JCGM. Correction algorithms for non-ideal behavior are based on studies by researchers at BIPM, NRC, CENAM, VSL, and INRIM; Monte Carlo and analytical uncertainty propagation methods echo work at CNAM, Université de Paris, and ETH Zurich. Calibration hierarchies and certificates reference traceability chains between national laboratories such as NPL, PTB, NIST, NIM, and KRISS.

Implementation and international standards

ITS-90 is incorporated into standards and calibrations by bodies like ISO, IEC, ASTM International, European Committee for Standardization, and national standards institutes including ANSI, Standards Australia, DIN (German Institute for Standardization), and AFNOR. International comparisons (e.g., CCT-K series) coordinated by CCT and BIPM assess equivalence among labs including VNIIM, INRIM, SP],], and Istituto Nazionale di Metrologia. Implementation affects legal metrology regimes in countries represented by WTO negotiation participants and regional metrology organizations such as APMP and SIM.

Applications and impact on metrology

ITS-90 underlies temperature calibration in industries and research sectors tied to Semiconductor Research and Manufacturing, Aerospace Corporation, Pharmaceuticals, Energy Research Centre, Climate Science Institutions, and laboratories such as Max Planck Institutes and Lawrence Livermore National Laboratory. Its adoption improved consistency across interlaboratory studies including thermal property measurements at CERN, cryogenic research at RIKEN, and thermophysical property databases maintained by NIST and IAPWS. The scale influenced advances in acoustic gas thermometry, dielectric-constant gas thermometry, and radiometric methods pursued at BNL, NPL, PTB, NIST, and Istituto Nazionale di Ricerca Metrologica, fostering improved links to the thermodynamic definition of the kelvin and ongoing research by the Comité Consultatif de Thermométrie.

Category:Metrology