title: Progress towards the determination of thermodynamic temperature with ultra-low uncertainty.
abstract: Previous research effort towards the determination of the Boltzmann constant has significantly improved the supporting theory and the experimental practice of several primary thermometry methods based on the measurement of a thermodynamic property of a macroscopic system at the temperature of the triple point of water. Presently, experiments are under way to demonstrate their accuracy in the determination of the thermodynamic temperature T over an extended range spanning the interval between a few kelvin and the copper freezing point (1358 K). We discuss how these activities will improve the link between thermodynamic temperature and the temperature as measured using the International Temperature Scale of 1990 (ITS-90) and report some preliminary results obtained by dielectric constant gas thermometry and acoustic gas thermometry. We also provide information on the status of other primary methods, such as Doppler broadening thermometry, Johnson noise thermometry and refractive index gas thermometry. Finally, we briefly consider the implications of these advancements for the dissemination of calibrated temperature standards.
authors.affiliation: Istituto Nazionale di Ricerca Metrologica (INRiM), 91 Strada delle Cacce, 10135 Torino, Italy r.gavioso@inrim.it.
authors.forename: Roberto M
authors.identifier: http://orcid.org/0000-0002-1631-5133
authors.initials: RM
authors.lastname: Gavioso
authors.affiliation: Istituto Nazionale di Ricerca Metrologica (INRiM), 91 Strada delle Cacce, 10135 Torino, Italy.
authors.forename: Daniele Madonna
authors.identifier: http://orcid.org/0000-0002-8760-8574
authors.initials: DM
authors.lastname: Ripa
authors.affiliation: Istituto Nazionale di Ricerca Metrologica (INRiM), 91 Strada delle Cacce, 10135 Torino, Italy.
authors.forename: Peter P M
authors.identifier: http://orcid.org/0000-0001-5161-8554
authors.initials: PP
authors.lastname: Steur
authors.affiliation: Physikalisch-Technische Bundesanstalt (PTB), Abbestrasse 2-12, 10587 Berlin, Germany.
authors.forename: Christof
authors.initials: C
authors.lastname: Gaiser
authors.affiliation: Physikalisch-Technische Bundesanstalt (PTB), Abbestrasse 2-12, 10587 Berlin, Germany.
authors.forename: Thorsten
authors.initials: T
authors.lastname: Zandt
authors.affiliation: Physikalisch-Technische Bundesanstalt (PTB), Abbestrasse 2-12, 10587 Berlin, Germany.
authors.forename: Bernd
authors.initials: B
authors.lastname: Fellmuth
authors.affiliation: National Physical Laboratory (NPL), Teddington TW11 0LW, UK.
authors.forename: Michael
authors.initials: M
authors.lastname: de Podesta
authors.affiliation: National Physical Laboratory (NPL), Teddington TW11 0LW, UK.
authors.forename: Robin
authors.initials: R
authors.lastname: Underwood
authors.affiliation: National Physical Laboratory (NPL), Teddington TW11 0LW, UK.
authors.forename: Gavin
authors.initials: G
authors.lastname: Sutton
authors.affiliation: Laboratoire Commun de Métrologie, LNE-CNAM, 61 Rue du Landy, 93210 La Plaine Saint-Denis, France.
authors.forename: Laurent
authors.initials: L
authors.lastname: Pitre
authors.affiliation: Laboratoire Commun de Métrologie, LNE-CNAM, 61 Rue du Landy, 93210 La Plaine Saint-Denis, France.
authors.forename: Fernando
authors.initials: F
authors.lastname: Sparasci
authors.affiliation: Laboratoire Commun de Métrologie, LNE-CNAM, 61 Rue du Landy, 93210 La Plaine Saint-Denis, France.
authors.forename: Lara
authors.initials: L
authors.lastname: Risegari
authors.affiliation: Dipartimento di Matematica e Fisica, Seconda Università di Napoli, Viale Lincoln 5, 81100 Caserta, Italy.
authors.forename: Livio
authors.identifier: http://orcid.org/0000-0002-2241-7603
authors.initials: L
authors.lastname: Gianfrani
authors.affiliation: Dipartimento di Matematica e Fisica, Seconda Università di Napoli, Viale Lincoln 5, 81100 Caserta, Italy.
authors.forename: Antonio
authors.identifier: http://orcid.org/0000-0002-1253-9172
authors.initials: A
authors.lastname: Castrillo
authors.affiliation: National Physical Laboratory (NPL), Teddington TW11 0LW, UK.
authors.forename: Graham
authors.identifier: http://orcid.org/0000-0002-8864-6951
authors.initials: G
authors.lastname: Machin
keywords: Boltzmann constant; International Temperature Scale; thermodynamic temperature