The kelvin redefinition and its mise en pratique

Abstract

In 2018, it is expected that there will be a major revision of the International System of Units (SI) which will result in all of the seven base units being defined by fixing the values of certain atomic or fundamental constants. As part of this revision, the kelvin, unit of thermodynamic temperature, will be redefined by assigning a value to the Boltzmann constant k. This explicit-constant definition will define the kelvin in terms of the SI derived unit of energy, the joule. It is sufficiently wide to encompass any form of thermometry. The planned redefinition has motivated the creation of an extended mise en pratique ('practical realization') of the definition of the kelvin (MeP-K), which describes how the new definition can be put into practice. The MeP-K incorporates both of the defined International Temperature Scales (ITS-90 and PLTS-2000) in current use and approved primary-thermometry methods for determining thermodynamic temperature values. The MeP-K is a guide that provides or makes reference to the information needed to perform measurements of temperature in accord with the SI at the highest level. In this article, the background and the content of the extended second version of the MeP-K are presented.

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“…Up to know, five thermodynamic measurable quantities are used to determine temperature in primary thermometry: the pressure of a gas in a constant volume, the speed of sound in a monatomic gas, the dielectric constant of a gas, the black‐body emission, and the power spectral density of Johnson‐noise in an electrical resistor . Moreover, although primary thermometers are generally complex and mostly employed for metrology purposes, they are currently engaged in the redefinition of the international temperature scale (1990s International Temperature Scale or ITS‐90) in terms of the Boltzmann constant ( k B ) …”

Thermodynamic properties of seawater, ice and humid air: TEOS-10, before and beyond

“…Along with it, a new thermodynamic temperature scale will be established in 2018 (Fellmuth et al, 2016;PTB, 2017;CCT, 2017;CGPM, 2018). This new temperature T and the ITS-90 temperature T 90 will be considered as two different physical quantities expressed in the same unit, namely the new kelvin (rather than being the same physical quantity expressed in two different units, K and a fictitious "K 90 ").…”

Linestrength ratio spectroscopy as a new primary thermometer for redefined Kelvin dissemination

“…On May 2019, the CIPM has anticipated the most radical revision of the definition of the SI base units in terms of exact fundamental constants, and in particular the Kelvin unit in terms of the assigned exact value of the Boltzmann constant, k [17,18]. The mise en pratique for the Kelvin redefinition (MeP-K-19) establishes, as first, that the accurate methods used to determine k [19][20][21][22][23], become PTs [24] because in their basic equations T always appears as thermal energy kT. Recently, we proposed linestrength ratio thermometry (LRT) [25] to determine Boltzmann constant with accuracy comparable or even better than other primary techniques.…”