Realization of the Kelvin

Holding glass thermometer

Until 2019, the kelvin (K) was defined as “the fraction 1/273.16 of the thermodynamic temperature of the triple point of water.” The temperature measurement world changed with the redefinition of the SI (International System of Units).

The Redefinition of the Kelvin

In May of 2019, the kelvin was redefined in terms of a fundamental quantity in nature known as the Bolzmann constant. Today, the kelvin is defined by taking the fixed numerical value of the Boltzmann constant k to be 1.380 649 ×10−23 when expressed in the unit J K−1, which is equal to kg m2 s−2 K−1, where the kilogram, meter, and second are defined in terms of other fundamental constants.

How Has the Kelvin Redefinition Changed How Temperature Is Measured?

Although the kelvin is now defined in an entirely new way, this redefinition has little immediate practical consequence for how the unit is realized or how temperature is measured.

The triple point of water (TPW) remains a convenient and practical tool to calibrate thermometers.

Thermometer technologies have not yet advanced to the point at which they can significantly reduce the already-small uncertainty in measurements of the TPW. The Consultative Committee for Thermometry, which makes recommendations to the CIPM (International Committee of Weights and Measures), has noted that multiple independent TPW measurements, which strictly follow the “mise en pratique” (practical application), vary by less than 50 millionths of a kelvin.

Traceability of thermodynamic temperature to the triple point of water and several other fixed-point cells remains acceptable even though these are not considered fundamental constants but are regarded as intrinsic standards having unchanging physical properties.

Temperature values for the reference points in both ITS-90 (The International Temperature Scale of 1990) and PLTS-2000 (The Provisional Low Temperature Scale of 2000) are specified with no uncertainty and will remain fixed until the scales are replaced.

Learn More About the Kelvin

The National Institute of Standards and Technology has a great deal of information on the Kelvin at

This post first appeared as lab-related accreditation Heads Up issue 74. Realization of the Kelvin. ANAB uses Heads Up to communicate to customers and interested parties about issues and information of note, including changes to policies and procedures.

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