ITS-90: The International Temperature Scale Explained
Source: BIPM. Published: 2025-03-01.
The Challenge of Precise Temperature Measurement
Temperature is among the most commonly measured physical quantities in science, industry, and medicine, yet it is also one of the most difficult to measure consistently across different locations and laboratories. Unlike mass, which can in principle be compared by placing objects on a balance, temperature must be inferred from some physical property that changes predictably with heat — the expansion of a liquid, the electrical resistance of a metal, the pressure of a gas. Each of these methods introduces its own uncertainties and systematic errors. For centuries, the existence of multiple incompatible temperature scales (Fahrenheit, Réaumur, Celsius, and others) compounded these practical difficulties. The development of a single internationally agreed temperature scale — one that could be realised consistently by any competent laboratory — was a long-standing goal of metrology.
What ITS-90 Is
The International Temperature Scale of 1990, universally known as ITS-90, is the practical temperature scale adopted by the BIPM's Consultative Committee for Thermometry in 1989 and in force since January 1, 1990. It replaced its predecessor, IPTS-68, and was designed to be a closer approximation to true thermodynamic temperature — the physically fundamental quantity defined through the Boltzmann constant — than any previous scale. ITS-90 covers a range from 0.65 K (just above absolute zero) to temperatures that can be measured with optical pyrometry (around 1300 K and above). It defines temperature not by a single thermometer type but by a series of fixed points and interpolating instruments, each optimised for its temperature range. The result is a scale that is both highly reproducible and more accurately aligned with thermodynamic temperature than any simpler system could achieve.
Fixed Points: From Hydrogen to Copper
The backbone of ITS-90 is a set of defining fixed points — temperatures at which specified pure substances undergo phase transitions (melting, freezing, or the triple point where solid, liquid, and vapour coexist). These transitions occur at highly reproducible temperatures that are largely independent of pressure at standard conditions, making them ideal calibration references. The scale's fixed points range from the triple point of equilibrium hydrogen (13.8033 K, about −259 °C) through the triple point of water (273.16 K, exactly 0.01 °C by definition) to the freezing points of tin (505.078 K), zinc (692.677 K), aluminium (933.473 K), silver (1234.93 K), gold (1337.33 K), and copper (1357.77 K). Between these fixed points, temperature is determined by interpolation using approved thermometric instruments — platinum resistance thermometers at lower temperatures, thermocouples and radiation thermometers at higher ones.
How ITS-90 Is Implemented in Practice
No single thermometer covers the full range of ITS-90; the scale is divided into overlapping sub-ranges, each with its own prescribed instrument and interpolation equations. At the lowest temperatures (0.65 K to 5 K), helium vapour pressure thermometers are used. From 3 K to 24.5561 K, a helium gas thermometer defines the scale. From 13.8 K up to the freezing point of silver (961.78 °C), standard platinum resistance thermometers (SPRTs) are the primary instruments. Above silver's freezing point, radiation thermometers calibrated against the silver, gold, or copper fixed points take over. National metrology institutes such as NIST, NPL (UK), PTB (Germany), and NMIJ (Japan) maintain ITS-90 realisations and provide calibration services against which industrial and scientific thermometers worldwide are ultimately traced. This traceability chain ensures that a temperature reading in a pharmaceutical cleanroom in Singapore corresponds to the same thermodynamic quantity as one made in a steel foundry in Germany.
PLTS-2000 for Ultra-Low Temperatures
ITS-90 does not extend below 0.65 K, but temperatures significantly below this are routinely achieved in condensed matter physics, quantum computing research, and cryogenic engineering. To provide an agreed scale in this regime, the BIPM adopted the Provisional Low Temperature Scale of 2000, known as PLTS-2000, which covers the range from 0.0009 K (0.9 millikelvin) to 1 K using the melting pressure of helium-3. PLTS-2000 is described as provisional because thermodynamic measurements in this extreme range carry larger uncertainties than at higher temperatures, and future improvements in measurement technology are expected to refine the scale. Research in this area is particularly active because quantum computers and certain detector technologies operate at temperatures just a few millikelvin above absolute zero, making precise low-temperature thermometry increasingly important.
ITS-90 and the Kelvin Redefinition of 2019
The 2019 revision of the SI redefined the kelvin by fixing the value of the Boltzmann constant k to exactly 1.380 649 × 10⁻²³ joules per kelvin. This change means that the kelvin is now defined independently of any material property or fixed point — it is a ratio of energy to dimensionless temperature. ITS-90 was not immediately replaced by this redefinition; practical temperature measurement still depends on agreed fixed points and interpolating instruments. However, the 2019 kelvin definition opened the door to a future revision of the temperature scale, potentially based on primary thermometric methods (such as acoustic gas thermometry or noise thermometry) that measure thermodynamic temperature directly without reference to fixed points. The BIPM's Consultative Committee for Thermometry is actively working on this next generation of temperature measurement, which will further reduce the small but non-zero differences that exist between ITS-90 and true thermodynamic temperature.
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