Introduction
The International Temperature Scale of 1990 (ITS-90) is the current international standard for temperature measurement. It provides a practical means of realizing thermodynamic temperature as closely as possible throughout the range from 0.65 K to the highest temperature practically measurable in terms of the Planck radiation law using monochromatic radiation.
ITS-90 superseded the International Practical Temperature Scale of 1968 (IPTS-68) and its 1975 revision (IPTS-68/75), incorporating significant improvements in accuracy and extending the lower temperature range.
Why ITS-90 Matters
🌍 Global Consistency
Ensures temperature measurements are consistent and comparable worldwide, enabling international trade and scientific collaboration.
🔬 Scientific Research
Provides the foundation for accurate temperature measurements in research, from materials science to climate studies.
🏭 Industrial Applications
Enables precise process control in manufacturing, pharmaceuticals, and other temperature-critical industries.
📏 Measurement Traceability
Establishes the chain of traceability from everyday thermometers to fundamental physical constants.
Key Features of ITS-90
Temperature Range
ITS-90 covers an extensive temperature range from 0.65 K (-272.5°C) to approximately 1,358 K (1,085°C) using defined fixed points, with extension to higher temperatures using radiation thermometry based on the Planck radiation law.
The Provisional Low Temperature Scale of 2000 (PLTS-2000) extends the scale below 0.65 K down to 0.9 mK, providing temperature measurements at the lowest achievable temperatures.
Fixed Points
The scale is defined by 17 fixed points - specific phase transitions of pure substances that occur at highly reproducible temperatures. These range from the triple point of hydrogen (13.8033 K) to the freezing point of copper (1,357.77 K).
Interpolation Methods
Between fixed points, temperatures are determined using specific interpolation instruments and equations:
- Helium vapor pressure thermometry (0.65 K to 5.0 K)
- Helium gas thermometry (3.0 K to 24.5561 K)
- Platinum resistance thermometry (13.8033 K to 1,234.93 K)
- Radiation thermometry (above 1,234.93 K)
Historical Context
First International Temperature Scale
The International Temperature Scale of 1927 (ITS-27) was adopted as the first international standard.
IPTS-68 Adopted
The International Practical Temperature Scale of 1968 significantly improved accuracy and extended the range.
ITS-90 Implementation
The current scale was adopted, incorporating advances in thermometry and extending to lower temperatures.
Implementation in Practice
National metrology institutes worldwide maintain and disseminate ITS-90 through:
- Primary standard apparatus for realizing fixed points
- Calibration services for secondary standards
- Research to improve the scale's accuracy and consistency
- Training and education for the measurement community
Getting Started with ITS-90
Whether you're a researcher, calibration professional, or student, understanding ITS-90 is essential for accurate temperature measurement. Explore our detailed sections on definitions, fixed points, and realization guides to learn more.
Current Status and Future
While ITS-90 remains the current international standard, the temperature metrology community continues to refine and improve it. Ongoing research includes:
- Improved fixed point realizations with lower uncertainties
- Development of new thermometry techniques
- Extension to lower temperatures using quantum-based methods
- Better understanding of non-uniqueness effects
The Consultative Committee for Thermometry (CCT) of the BIPM regularly reviews the scale and provides guidance on best practices and potential future revisions.