Editing Thermodynamic temperature (section)

== Modern redefinition of the kelvin ==

For 65 years, between 1954 and the [[2019 revision of the SI]], a temperature interval of one kelvin was defined as {{sfrac|273.16}} the difference between the [[triple point]] of water and absolute zero. The 1954 resolution by the [[International Bureau of Weights and Measures]] (known by the French-language acronym BIPM), plus later resolutions and publications, defined the triple point of water as precisely 273.16&nbsp;K and acknowledged that it was "common practice" to accept that due to previous conventions (namely, that 0&nbsp;°C had long been defined as the melting point of water and that the triple point of water had long been experimentally determined to be indistinguishably close to 0.01&nbsp;°C), the difference between the Celsius scale and Kelvin scale is accepted as 273.15 kelvins; which is to say, 0&nbsp;°C corresponds to 273.15 kelvins.<ref name="BIPMbrocure">{{cite web|url=http://www1.bipm.org/en/si/si_brochure/chapter2/2-1/2-1-1/kelvin.html|title=SI brochure, section 2.1.1.5|access-date=9 May 2008|publisher=[[International Bureau of Weights and Measures]]|url-status=dead|archive-url=https://web.archive.org/web/20070926215600/http://www1.bipm.org/en/si/si_brochure/chapter2/2-1/2-1-1/kelvin.html|archive-date=26 September 2007}}</ref> The net effect of this as well as later resolutions was twofold: 1) they defined absolute zero as precisely 0&nbsp;K, and 2) they defined that the triple point of special isotopically controlled water called [[Vienna Standard Mean Ocean Water]] occurred at precisely 273.16&nbsp;K and 0.01&nbsp;°C. One effect of the aforementioned resolutions was that the melting point of water, while ''very'' close to 273.15&nbsp;K and 0&nbsp;°C, was not a defining value and was subject to refinement with more precise measurements.

The 1954 BIPM standard did a good job of establishing—within the uncertainties due to [[isotope|isotopic variations]] between water samples—temperatures around the freezing and triple points of water, but required that ''intermediate values'' between the triple point and absolute zero, as well as extrapolated values from room temperature and beyond, to be experimentally determined via apparatus and procedures in individual labs. This shortcoming was addressed by the [[International Temperature Scale of 1990]], or ITS{{nbh}}90, which defined 13 additional points, from 13.8033&nbsp;K, to 1,357.77&nbsp;K. While definitional, ITS{{nbh}}90 had—and still has—some challenges, partly because eight of its extrapolated values depend upon the melting or freezing points of metal samples, which must remain exceedingly pure lest their melting or freezing points be affected—usually depressed.

The 2019 revision of the SI was primarily for the purpose of decoupling much of the SI system's definitional underpinnings from the [[kilogram]], which was the last physical artifact defining an [[SI base unit]] (a platinum/iridium cylinder stored under three nested bell jars in a safe located in France) and which had highly questionable stability. The solution required that four physical constants, including the Boltzmann constant, be definitionally fixed.

Assigning the Boltzmann constant a precisely defined value had no practical effect on modern thermometry except for the most exquisitely precise measurements. Before the revision, the triple point of water was exactly 273.16&nbsp;K and 0.01&nbsp;°C and the Boltzmann constant was experimentally determined to be {{val|1.38064903|(51)|e=-23|u=J|up=K}}, where the "(51)" denotes the uncertainty in the two least significant digits (the 03) and equals a [[Standard deviation|relative standard uncertainty]] of 0.37&nbsp;ppm.<ref name=codata2017>
{{cite journal
 |title=The CODATA 2017 values of ''h'', ''e'', ''k'', and ''N''<sub>A</sub> for the revision of the SI
 |collaboration=Committee on Data for Science and Technology (CODATA) Task Group on Fundamental Constants
 |first1=D B |last1=Newell |first2=F |last2=Cabiati |first3=J |last3=Fischer |first4=K |last4=Fujii
 |first5=S G |last5=Karshenboim |first6=H S |last6=Margolis |first7=E |last7=de Mirandés
 |first8=P J |last8=Mohr |first9=F |last9=Nez |first10=K |last10=Pachucki |first11=T J |last11=Quinn
 |first12=B N |last12=Taylor |first13=M |last13=Wang |first14=B M |last14=Wood |first15=Z |last15=Zhang
 |journal=Metrologia |volume=55 |issue=1 |pages=L13–L16
 |date=29 January 2018
 |doi=10.1088/1681-7575/aa950a |bibcode=2018Metro..55L..13N
 |doi-access=free
 }}</ref> Afterwards, by defining the Boltzmann constant as exactly {{val|1.380649|e=-23|u=J|up=K}}, the 0.37&nbsp;ppm uncertainty was transferred to the triple point of water, which became an experimentally determined value of {{val|273.1600|0.0001|u=K}} ({{val|0.0100|0.0001|u=°C}}). That the triple point of water ended up being exceedingly close to 273.16&nbsp;K after the SI revision was no accident; the final value of the Boltzmann constant was determined, in part, through clever experiments with [[argon]] and helium that used the triple point of water for their key reference temperature.<ref>{{cite web|url=https://www.nist.gov/programs-projects/acoustic-thermometry|title=SI Redefinition – Kelvin: Boltzmann Constant|access-date=13 Dec 2020|publisher=[[National Institute of Standards and Technology]]|url-status=dead|archive-url=https://web.archive.org/web/20200701093442/https://www.nist.gov/si-redefinition/kelvin/kelvin-boltzmann-constant|archive-date=1 July 2020}}</ref><ref>{{cite web|url=https://www.nist.gov/programs-projects/acoustic-thermometry|title=Acoustic Thermometry|access-date=13 Dec 2020|publisher=[[National Institute of Standards and Technology]]|url-status=dead|archive-url=https://web.archive.org/web/20200923025023/https://www.nist.gov/programs-projects/acoustic-thermometry|archive-date=23 September 2020}}</ref>

Notwithstanding the 2019 revision, water triple-point cells continue to serve in modern thermometry as exceedingly precise calibration references at 273.16&nbsp;K and 0.01&nbsp;°C. Moreover, the triple point of water remains one of the 14 calibration points comprising ITS{{nbhyph}}90, which spans from the triple point of hydrogen (13.8033&nbsp;K) to the freezing point of copper (1,357.77&nbsp;K), which is a nearly hundredfold range of thermodynamic temperature.