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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Countdown to the SI redefinition

metr_bck_3Throughout history, measurement has been a fundamental part of human advancement. The oldest systems of weights and measures discovered date back over 4000 years. Early systems were tied to physical objects, like hands, feet, stones and seeds, and were used, as we still do now, for agriculture, construction, and trade. Yet, with new inventions and global trade more ever more accurate and unified systems were needed. In Europe, it wasn’t until the 19th Century that a universally agreed measurement system began to be adopted and the International System of Units (SI units) was born.

Now, after years of hard work and scientific progress, we are ready once again to update and improve the SI units. The redefinition of the International System of Units enacted on the 16 November 2018 during the General Conference for Weights and Measures will mean that the SI units will no longer be based on any physical objects, but instead derived through fundamental properties of nature. Creating a system centred on stable and universal natural laws will ensure the long-term stability and reliability of measurements, and act as a springboard for the future of science and innovation.

The redefinition of the SI units will come into force on the 20th of May 2019, the anniversary of the signing of the Metre Convention in 1875, an international treaty for the international cooperation in metrology.  To celebrate, we’ll be counting down each of the SI units – the metre, second, kilogram, kelvin, mole, candela, and ampere. Join us on the 20th of every month to find out where units are commonly used, how they’re defined, and the changes that will take place!

UNIT OF THE MONTH: METRE

“You’ve never heard of the Millennium Falcon? … It’s the ship that made the Kessel run in less than 12 parsecs!” Han  Solo’s description of the Millennium Falcon in Star Wars is impressive, but something’s not quite right. Do you know why? The unit he uses to illustrate the prowess of the Falcon – a parsec – isn’t actually a measure of time, but length! It probably won’t surprise anyone Han Solo isn’t very precise when it comes to the physics of his ship, but in fact he isn’t too far from the truth. This is because we use time to define length.

metre facts

What does this mean? Well, in the case of Han Solo, one parsec is about 3.26 light-years, and a light-year is the distance light travels in one year. Back down on Earth, we have the same method for defining length. In the International System of Units (SI), the base unit of length is the metre, and it can be understood as:

A metre is the distance travelled by light in 1/299792458 of a second.

The reason we use the distance travelled by light in a certain amount of time is because light is the fastest thing in the universe (that we know of) and it always travels at exactly the same speed in a vacuum. This means that if you measure how far light has travelled in a vacuum in 1/299792458 of a second in France, Canada, Brazil or India, you will always get exactly the same answer no matter where you are!

On 20 May next year the official definition of the metre will change to:

The metre is defined by taking the fixed numerical value of the speed of light in vacuum c to be 299 792 458 when expressed in the unit m s−1, where the second is defined in terms of the caesium frequency, ∆ν.

We’ll be returning to the definition of the second on 20 March, so join us again then to find out more.

So, what’s the difference? Actually, there’s no big change coming for the metre. Although the word order has been rephrased, the physical concepts remain the same.