The International Temperature Scale of 1990 (ITS-90) is an equipment calibration standard specified by the
International Committee of Weights and Measures (CIPM) for making measurements on the
Kelvin
The kelvin, symbol K, is the primary unit of temperature in the International System of Units (SI), used alongside its prefixed forms and the degree Celsius. It is named after the Belfast-born and University of Glasgow-based engineer and phy ...
and
Celsius temperature scale
Scale of temperature is a methodology of calibrating the physical quantity temperature in metrology. Empirical scales measure temperature in relation to convenient and stable parameters, such as the freezing and boiling point of water. Absolute ...
s. It is an approximation of thermodynamic temperature that facilitates the comparability and compatibility of temperature measurements internationally.
It defines fourteen calibration points ranging from to ( to )
and is subdivided into multiple temperature ranges which overlap in some instances.
ITS-90 is the most recent of a series of International Temperature Scales adopted by the CIPM since 1927.
Adopted at the 1989 General Conference on Weights and Measures, it supersedes the International Practical Temperature Scale of 1968 (amended edition of 1975) and the 1976 "Provisional 0.5 K to 30 K Temperature Scale". The CCT has also published several online guidebooks to aid realisations of the ITS-90.
The lowest temperature covered by the ITS-90 is 0.65 K. In 2000, the temperature scale was extended further, to 0.9 mK, by the adoption of a supplemental scale, known as the
Provisional Low Temperature Scale of 2000 The Provisional Low Temperature Scale of 2000 (PLTS-2000) is an equipment calibration standard for making measurements of very low temperatures, in the range of 0.9 mK (millikelvin) to 1 K, adopted by the International Committee for Weights and Meas ...
(PLTS-2000).
In 2019, the kelvin was redefined. However, the alteration was very slight compared to the ITS-90 uncertainties, and so the ITS-90 remains the recommended practical temperature scale without any significant changes. It is anticipated that the redefinition, combined with improvements in primary thermometry methods, will phase out reliance on the ITS-90 and the PLTS-2000 in the future.
["''Mise en pratique'' for the definition of the kelvin in the SI"]
BIPM, May 2019.
Details
The ITS-90 is designed to represent the
thermodynamic (absolute) temperature scale (referencing
absolute zero) as closely as possible throughout its range. Many different thermometer designs are required to cover the entire range. These include helium vapor pressure thermometers, helium gas thermometers,
standard platinum resistance thermometers (known as SPRTs) and
monochromatic radiation thermometers.
Although the Kelvin and Celsius temperature scales were (until 2019) defined using the
triple point
In thermodynamics, the triple point of a substance is the temperature and pressure at which the three phases (gas, liquid, and solid) of that substance coexist in thermodynamic equilibrium.. It is that temperature and pressure at which the sub ...
of water ( or ), it is impractical to use this definition at temperatures that are very different from the triple point of water. Accordingly, ITS-90 uses numerous defined points, all of which are based on various
thermodynamic
Thermodynamics is a branch of physics that deals with heat, work, and temperature, and their relation to energy, entropy, and the physical properties of matter and radiation. The behavior of these quantities is governed by the four laws of the ...
equilibrium states of fourteen pure
chemical elements and one
compound
Compound may refer to:
Architecture and built environments
* Compound (enclosure), a cluster of buildings having a shared purpose, usually inside a fence or wall
** Compound (fortification), a version of the above fortified with defensive struc ...
(water). Most of the defined points are based on a
phase transition
In chemistry, thermodynamics, and other related fields, a phase transition (or phase change) is the physical process of transition between one state of a medium and another. Commonly the term is used to refer to changes among the basic states o ...
; specifically the
melting
Melting, or fusion, is a physical process that results in the phase transition of a substance from a solid to a liquid. This occurs when the internal energy of the solid increases, typically by the application of heat or pressure, which inc ...
/
freezing
Freezing is a phase transition where a liquid turns into a solid when its temperature is lowered below its freezing point. In accordance with the internationally established definition, freezing means the solidification phase change of a liquid ...
point of a pure chemical element. However, the deepest
cryogenic points are based exclusively on the
vapor pressure
Vapor pressure (or vapour pressure in English-speaking countries other than the US; see spelling differences) or equilibrium vapor pressure is defined as the pressure exerted by a vapor in thermodynamic equilibrium with its condensed phas ...
/temperature relationship of helium and its isotopes whereas the remainder of its cold points (those less than room temperature) are based on
triple point
In thermodynamics, the triple point of a substance is the temperature and pressure at which the three phases (gas, liquid, and solid) of that substance coexist in thermodynamic equilibrium.. It is that temperature and pressure at which the sub ...
s. Examples of other defining points are the triple point of
equilibrium hydrogen ( or ) and the freezing point of aluminium ( or ).
The defining fixed points of the ITS-90 refer to pure chemical samples with specific isotopic compositions. As a consequence of this, the ITS-90 contains several equations to correct for temperature variations due to impurities and isotopic composition.
Thermometers calibrated via the ITS-90 use complex mathematical formulas to interpolate between its defined points. The ITS-90 specifies rigorous control over variables to ensure reproducibility from lab to lab. For instance, the small effect that atmospheric pressure has upon the various melting points is compensated for (an effect that typically amounts to no more than half a
millikelvin
List of orders of magnitude for temperature
Detailed list for 100 K to 1000 K
Most ordinary human activity takes place at temperatures of this order of magnitude. Circumstances where water naturally occurs in liquid form are shown in light gr ...
across the different altitudes and barometric pressures likely to be encountered). The standard also compensates for the pressure effect due to how deeply the temperature probe is immersed into the sample. The ITS-90 also draws a distinction between “freezing” and “melting” points. The distinction depends on whether heat is going ''into'' (melting) or ''out of'' (freezing) the sample when the measurement is made. Only gallium is measured at its melting points; all other metals with defining fixed points on the ITS-90 are measured at their freezing points.
A practical effect of the ITS-90 is that the triple points and the freezing/melting points of its thirteen chemical elements are precisely known for all temperature measurements calibrated per the ITS-90 since these thirteen values are fixed by definition.
Limitations
There are often small differences between measurements calibrated per ITS-90 and
thermodynamic temperature
Thermodynamic temperature is a quantity defined in thermodynamics as distinct from kinetic theory or statistical mechanics.
Historically, thermodynamic temperature was defined by Kelvin in terms of a macroscopic relation between thermodynamic ...
. For instance, precise measurements show that the boiling point of VSMOW water under one standard atmosphere of pressure is actually 373.1339 K (99.9839 °C) when adhering ''strictly'' to the two-point definition of thermodynamic temperature. When calibrated to ITS-90, where one must interpolate between the defining points of gallium and indium, the boiling point of VSMOW water is about 10 mK less, about 99.974 °C. The virtue of ITS-90 is that another lab in another part of the world will measure the very same temperature with ease due to the advantages of a comprehensive international calibration standard featuring many conveniently spaced, reproducible, defining points spanning a wide range of temperatures.
Although “International Temperature Scale of 1990” has the word “scale” in its title, this is a misnomer that can be misleading. The ITS-90 is not a scale; it is an ''equipment calibration standard''. Temperatures measured with equipment calibrated per ITS-90 may be expressed using any temperature scale such as Celsius, Kelvin, Fahrenheit, or Rankine. For example, a temperature can be measured using equipment calibrated to the kelvin-based ITS-90 standard, and that value may then be converted to, and expressed as, a value on the Fahrenheit scale (e.g. 211.953 °F).
ITS-90 does not address the highly specialized equipment and procedures used for measuring temperatures extremely close to absolute zero. For instance, to measure temperatures in the nanokelvin range (billionths of a kelvin), scientists using
optical lattice
An optical lattice is formed by the interference of counter-propagating laser beams, creating a spatially periodic polarization pattern. The resulting periodic potential may trap neutral atoms via the Stark shift. Atoms are cooled and congrega ...
laser equipment to
adiabatically cool atoms, turn off the entrapment lasers and simply measure how far the atoms drift over time to measure their temperature. A cesium atom with a velocity of 7 mm/s is equivalent to a temperature of about 700 nK (which was a record cold temperature achieved by the
NIST in 1994).
Estimates of the differences between thermodynamic temperature and the ITS-90 () were published in 2010. It had become apparent that ITS-90 deviated considerably from PLTS-2000 in the overlapping range of 0.65 K to 2 K. To address this, a new
3He vapor pressure scale was adopted, known as .
For higher temperatures, expected values for are below 0.1 mK
for temperatures 4.2 K – 8 K,
up to 8 mK at temperatures close to 130 K,
to 0.1 mK
[ at the triple point of water (273.1600 K),
but rising again to 10 mK at temperatures close to 430 K, and reaching 46 mK at temperatures close to 1150 K.Estimates of the Differences between Thermodynamic Temperature and the ITS-90]
(2010)
Standard interpolating thermometers and their ranges
Defining points
The table below lists the defining fixed points of the ITS-90.
See also
*
Thermodynamic (absolute) temperature — the "true temperature" which ITS-90 is attempting to approximate.
*
Provisional Low Temperature Scale of 2000 The Provisional Low Temperature Scale of 2000 (PLTS-2000) is an equipment calibration standard for making measurements of very low temperatures, in the range of 0.9 mK (millikelvin) to 1 K, adopted by the International Committee for Weights and Meas ...
(PLTS-2000) — A newer temperature scale for the range of 0.0009 K to 1 K, based on the
melting pressure of
helium-3.
*
Kelvin
The kelvin, symbol K, is the primary unit of temperature in the International System of Units (SI), used alongside its prefixed forms and the degree Celsius. It is named after the Belfast-born and University of Glasgow-based engineer and phy ...
*
Triple point
In thermodynamics, the triple point of a substance is the temperature and pressure at which the three phases (gas, liquid, and solid) of that substance coexist in thermodynamic equilibrium.. It is that temperature and pressure at which the sub ...
*
Vienna Standard Mean Ocean Water
Vienna Standard Mean Ocean Water (VSMOW) is an isotopic standard for water. Despite the name, VSMOW is pure water with no salt or other chemicals found in the oceans. The VSMOW standard was promulgated by the International Atomic Energy Agency ( ...
(VSMOW)
*
Resistance thermometer
Resistance thermometers, also called resistance temperature detectors (RTDs), are sensors used to measure temperature. Many RTD elements consist of a length of fine wire wrapped around a heat-resistant ceramic or glass core but other constructio ...
*
Platinum resistance thermometer
Resistance thermometers, also called resistance temperature detectors (RTDs), are sensors used to measure temperature. Many RTD elements consist of a length of fine wire wrapped around a heat-resistant ceramic or glass core but other constructio ...
*
Planckian locus § International Temperature Scale – how successive revisions of the temperature scale have affected the relation between spectrum and temperature of a black body
References
* Preston-Thomas H.
Metrologia, 1990, 27(1), 3-10 (amended version).*
*
External links
The Internet ITS-90 Resource (by ISOTech Ltd)ITS-90 (by Swedish National Testing and Research Institute)NIST ITS-90 Thermocouple Database (by United States Department of Commerce, National Institute of Standards & Technology)Conversion among different international temperature scales; equations and algorithms.CIPM official publication of ITS-90 in 78th meeting in 1989
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Temperature