Counting down to the SI redefinition: kelvin and degree Celsius

kelvin mountainIn case you missed it, the redefinition of the International System of Units (SI) is going into effect this World Metrology Day, 20 May 2019. Each month we are bringing you a blog post featuring one of the units of the SI. This month we are focusing on the kelvin, the SI base unit for thermodynamic temperature. It’s winter in the Northern hemisphere and outdoor temperatures have dropped, so let’s jump in!

Unit of the month – kelvin

Accurate temperature measurement is essential in a wide range of everyday processes, from controlling chemical reactions and food production, to the assessment of weather and climate change. And almost every engineering process depends on temperature – sometimes critically. Knowing the correct temperature is also essential, but much more difficult, in more extreme conditions, like the intensely hot temperatures required to produce steel or the very low temperatures required to use superconductors.

Measuring temperature has a long history. About 2,000 years ago, the ancient Greek engineer Philo of Byzantium came up with what may be the earliest design for a thermometer: a hollow sphere filled with air and water, connected by tube to an open-air pitcher. The idea was that air inside the sphere would expand or contract as it was heated or cooled, pushing or pulling water into the tube. Later, people noticed that the air contracted in volume by about one third as the sphere was cooled from the boiling temperature of water to the ice point. This caused people to speculate on what would happen if one could keep cooling the sphere. In the middle of the 19th century, British physicist William Thomson – later Lord Kelvin – also became interested in the idea of ‘infinite cold’ a state we now call the absolute zero of temperature. In 1848, he published a paper, On an Absolute Thermometric Scale’ in which he estimated that absolute zero was approximately, -273 °C. In honour of his investigations, we now name the unit of temperature, the kelvin, after him.

kelvin imageWhen Lord Kelvin carried out his investigations, it was not yet universally accepted that all substances were made out of molecules in ceaseless motion. We now know that temperature is a measure of the average energy of motion of these particles, and absolute zero – zero kelvin – corresponds to the lowest possible temperature, a state where the thermal motion of molecules has ceased.

In 1960, when the SI was established, the temperature of the triple point of water was defined to be 273.16 K exactly. This is the temperature at which (in the absence of air) liquid water, solid water (ice) and water vapour can all co-exist in equilibrium. This temperature was chosen as a standard temperature because it was convenient and highly reproducible. Accordingly, the kelvin was defined to be the fraction 1/273.16 of the temperature of the ‘triple point’ of water. We then measured the temperature of an object by comparing it against the standard temperature. Unusually in the SI, we also defined another unit of temperature, called the degree Celsius (°C). This is related to the kelvin by subtracting 273.15 from the numerical value of the temperature expressed in kelvin.

t(in °C) = T(in K) – 273.15

The reason for this is to make it easier to use in a wide variety of applications that had previously used the ‘centigrade’ scale. In our everyday life we are used to expressing temperature in degrees Celsius. On this scale water freezes at about 0 oC and boils at approximately 100 oC. Notice the conversion from kelvin to degrees Celsius subtracts 273.15, so the triple point of water is 0.01 °C.

With the redefinition, the kelvin will no longer be defined in terms of an arbitrarily-chosen reference temperature. Instead, we will define temperatures in terms of the energy of molecular motion. We will do this by taking the value of the Boltzmann constant k to be 1.380 649 × 10−23 exactly when expressed in units of joules per kelvin (J K−1). One joule per kelvin is equal to one kg m2s−2 K−1, where the kilogram, metre and second are defined in terms of hc and ∆ν. So after this redefinition, we will be effectively measuring the temperature in terms of the energy of molecular motion. The degree Celsius will be related to kelvin in the same way as it was before May 2019.

Why is redefinition of Kelvin important?

For almost all users, the redefinition will pass unnoticed; water will still freeze at 0 °C, and thermometers calibrated before the change will continue to indicate the correct temperature. However, the redefinition opens up the possibility of using new technologies to measure temperature, something that is likely to be of benefit first at extremely high or low temperatures.

Range of temperatures

Coldest natural air temperature measured on Earth (Antarctic) -89.2 °C
Mercury Freezes -38.8 °C
Water freezes (1) 0 °C
Earth surface, average over year, land and ocean 1978 (2) 14.1 °C
Earth surface, average over year, land and ocean 2017 (2) 14.9 °C
Hottest natural air temperature measured on Earth, Furnace Creek, USA 56.7 °C
Water boils (1) 100 °C (actually 99.974 °C)
Molten Steel About 1 600 °C
Surface of the Sun About 5 500 °C
Centre of the Earth (estimate) About 7 000 °C
Centre of the Sun (estimate) About 15 million °C

(1) changes with altitude, this value at sea level.
(2) This year’s value and represents the general trend at the time.

Most people use the degree Celsius (°C) where °C = K – 273.15.

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