ITS-90 (International Temperature Scale of 1990)

Range and defining points

ITS-90 covers an extensive range from cryogenic to high temperatures. It uses a combination of fixed-temperature realizations, including low-temperature vapor-pressure points, triple points of substances, and melting or freezing points of pure metals. Two widely cited bounds are the nominal lower end near 0.65 K and an upper defining fixed point at the freezing point of copper (about 1357.77 K). The triple point of water (273.16 K) is one of the central, widely used calibration anchors.

How the scale is realized

Rather than prescribing a single instrument, ITS-90 specifies methods and interpolation instruments for different subranges so that users can reproduce the scale in practice. Typical realizations include:

• Helium vapor pressure or similar techniques for the lowest cryogenic temperatures.
• Standard platinum resistance thermometers (PRTs) for much of the laboratory range; PRT resistance is mapped to temperature by ITS-90 polynomial equations and fixed-point checks.
• Thermocouples and fixed-point metal cells for the highest temperatures, with procedures for melting/freezing point realizations.

Each method is accompanied by defined procedures, uncertainty budgets, and interpolation formulas to ensure traceability and comparability among national metrology institutes and accredited laboratories. For general information about metrological practice, see reference.

History and development

ITS-90 was prepared by international committees of metrologists and adopted to replace earlier practical scales. It consolidated improved knowledge of fixed-point temperatures, more accurate measurement techniques, and refined interpolation functions. Although ITS-90 is a practical scale, its development was informed by primary thermometry results from gas- and acoustic-based determinations of thermodynamic temperature.

Uses, significance and limitations

ITS-90 is the basis for calibration, quality control, and legal metrology worldwide. Calibration laboratories use ITS-90 to provide temperature calibration certificates for sensors, industrial processes, and scientific experiments. Its strengths are reproducibility and well-characterized uncertainties across laboratories. Limits include small systematic differences from true thermodynamic temperature that become relevant only at the highest precision; metrologists account for these differences when necessary using primary thermometry.

Related information and further reading

For technical details, including tables of fixed points, interpolation formulae, and recommended procedures, consult technical guides and official metrology publications. National metrology institutes and international organizations publish implementation notes and uncertainty guidance; examples can be found via technical resources and specialized calibration manuals linked at further reading.

Notable fact: ITS-90 remains the standard practical realization used after the modern redefinition of the kelvin, which changed the formal definition of temperature but left ITS-90 as the practical method for realizing and disseminating temperature measurements.