Artefact free measurement: the redefinition of the international system of units

Wednesday 29 May 2019, 4.00PM

Speaker(s): Prof Graham Machin, NPL


A brief introduction to the National Physical Laboratory ( will be given followed by a discussion of the evolution of measurement practices from ancient times to the present.

The international system of units (the SI); namely the kilogram, the ampere, the kelvin, the metre, the mole, the second and the candela have served the measurement requirements of the world for many years. Since the inception of the SI a number of the individual unit definitions have evolved to meet advancing measurement needs. However this is all set to change. In November 2018 the General Congress of Weights and Measures (CGPM) met in Paris to usher in the most fundamental change the SI has ever had. At that meeting the CGPM agreed that the current semi-classical and partially artefact based unit definitions would give way to new definitions based on a set of fundamental, atomic and conventional constants with defined values. This came into force on World Metrology Day 20 May 2019.

The objective of the redefinition was to ensure that the SI remains enduringly fit for purpose into the 21st Century and beyond. This talk will introduce the SI, explain the changes that have been made and their impact on measurement. Emphasis will be given to the changes affecting the kelvin and how the redefinition will open up the possibility of no-drift always-right sensors through the development of practical primary thermometry in the 2020s.


Professor Graham Machin BSc (Hons), DPhil (Oxon), DSc, CPhys, CEng, FInstP, FInstMC GM is the science leader of the NPL Temperature and Humidity Group and an NPL Fellow. He has more than 28 years’ experience in thermometry research, published more than 210 technical papers and given numerous invited/keynote addresses. He has been an invited guest researcher at institutes in Japan and the USA, currently holds a visiting senior researcher position at the National Institute of Metrology, China. He is visiting Professor of Thermometry in Harsh Environments (University of Strathclyde), visiting Professor of Clinical Thermal Imaging (University of South Wales), Distinguished Visiting Fellow (colaborador honorífico) (University of Valladolid, Spain) and Honorary Professor of the Chinese Academy of Sciences. He represents the UK on the Consultative Committee of Thermometry (CCT) and IMEKO TC12, chairs the CCT working group for Noncontact thermometry and was chair of the Euramet Technical Committee for Thermometry (TC-T) from 2014 to 2018. In addition he has served on the EPSRC Physical Sciences Strategic Advisory Team, and is an international invited expert on the Chinese Academy of Sciences “very low temperature thermometry” project (2017-2022). GM was awarded the Institute of Measurement and Control (InstMC) Callendar medal in 2012 for “outstanding contributions to the art of temperature measurement” and a visiting fellowship of the Chinese Academy of Sciences (2017). He is currently the President of the InstMC (2018-2019). Current research interests are primary thermometry (acoustic, radiometric and especially all aspects of implementing and realizing the redefined kelvin), radiation thermometry and thermal imaging, new thermocouples, sensor self-validation methods, clinical thermometry (contact, non-contact and internal), reliable temperature (and other) measurements in hostile environments (especially aerospace and nuclear decommissioning) and metrology for wound prevention. He is director of “Implementing the new kelvin” and nominated director of “Realising the redefined kelvin” (to start Sep 2019) projects for EURAMET.
Research details can be accessed via Research Gate

Location: P/L/002