Comment
Michael de Podesta principal research scientist, National
Physical Laboratory, UK
Redefining the kelvin T
of the kelvin is: ‘… the fraction 1/273.16 of the temperature of the triple point of water’. This deceptively simple statement defines the magnitude of ‘one kelvin’ and ‘one degree Celsius’, and from this definition scientists have devised procedures that allow us to measure temperatures from close to absolute zero to thousands of degrees Celsius. Although it is not usually obvious to users, every temperature measurement is fundamentally a statement of how much hotter or a colder a temperature is than the triple-point of water. However, in the next few years a new definition is likely to be adopted. The exact wording has not been decided but it is likely to be something like: ‘The kelvin has a value such that the Boltzmann constant, kB
emperature measurements are commonplace in chemical processes. So it may come as a surprise to realise that plans are under way to change the definition of the familiar units – the kelvin and the degree Celsius. But don’t worry, it won’t affect you immediately, and may help you in the future. Currently every temperature measurement on Earth is defined in terms of the temperature of the triple point of water, TTPW
, because the definition , has the value 1.380 6XYZ x 10-23 J K-1
exactly.’ The aim is to choose the last digits XYZ so that the magnitude of one degree is roughly the same as the current definition. If we get this right no one will find anomalous changes in their experimental results after the change. But if there won’t be any detectable effects, why do metrologists want to make this change? To understand why they are doing this, we need to consider a curious fact: temperature is the only commonly measured quantity that we learned to measure before we knew what it was we were measuring. If, 200 years ago, we had understood that
28 Chemistry&Industry • November 2013
‘If, 200 years ago, we had understood that temperature is a measure of the kinetic energy of molecular motion we would have defined temperature directly in terms of units of energy.’
After the change, TTPW
temperature is a measure of the kinetic energy of molecular motion we would have defined it directly in terms of units of energy. But this didn’t happen. For a number of fascinating cultural and historical reasons, it became accepted for temperature measurements to refer to other ‘standard temperatures’, eg ‘ice point’ and ‘boiling point’ of water. And that practice is still in operation. Currently we define TTPW
to be 273.16 K exactly.
but there will be an uncertainty associated with the value. In contrast, kB
will still have the same value , which currently has an
uncertainty of just over 1 part in a million, will be defined exactly with no associated measurement uncertainty. So, although the magnitude of one kelvin will remain unchanged, there will be a fundamental change in our conception of what we mean by one degree. Instead of temperatures being defined in terms of their relation to another standard temperature, they will be linked to a specific amount of molecular energy. There will also be some additional benefits.
First, because it makes no reference to any particular substance or temperature, the definition should never need to be changed or modified. In contrast, in 2005 the current definition had to be modified to specify the isotopic composition of the water in the triple-point cell. Secondly, because every temperature measurement is currently a
– say greater than 1000°C or less than 100K, the uncertainty of measurement could eventually fall as we exploit new physical principles based on a fixed value of kB. For example, high temperatures are commonly determined by measuring the amount of radiant energy emitted by a body. Already it is possible to achieve lower measurement uncertainties by measuring this emission directly in terms of energy units rather than as we currently do, by comparing the radiant brightness with the brightness at another known temperature. This change to the definition of the kelvin is part of a wider change to the International System of Units, which aims to replace the definitions of the seven SI base units with definitions in terms of fixed values of fundamental constants of nature. The second and the metre are already defined in this way: the second is defined in terms of the frequency of vibration of a particular isotope of a caesium atom, and the metre is defined in terms of the second and the natural constant, the speed of light in vacuum. In the forthcoming changes it is hoped to replace the current definitions of the kilogram, ampere and mole by definitions in terms of fixed values of the Planck constant, h, the elementary change, e, and the Avogadro constant, NA
ratio to TTPW TTPW
respectively. In support of these changes, with my
colleagues at NPL, I have just made the most accurate ever determination of the Boltzmann constant. The experiment involved a low- uncertainty determination of the speed of sound in argon gas which we achieved by developing the acoustic resonance technique pioneered in the UK by Mike Ewing at University College London and Martin Trusler at Imperial College, and in the US by Mike Moldover at the National Institute of Standards Technology (NIST).
, for temperatures very different from
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