National Physical Laboratory

Extra points for thermometry

Jonathan Pearce explains the impact of the National Physical Laboratory's (NPL) work on temperature measurement in an invited article for Nature Physics.

A triple point of water cell, on which the current definition of the kelvin rests
A triple point of water cell, on which
the current definition of the kelvin rests

Jonathan's article covers more than 400 years of advances in temperature measurement – from one of the first temperature scales, traced back to 1612, based on the expansion of a gas; through the development of the centigrade (later Celsius) scale in the 18th century; to the first modern temperature scale, introduced in 1887, linked to absolute temperature.

Today, almost every technological process depends in some way on temperature measurement and control – from intercontinental flights to reliable electricity. They all depend on a sophisticated measurement infrastructure that allows temperature measurements to be traced back to the SI unit of temperature, the kelvin, via the International Temperature Scale of 1990 (ITS-90).

But for temperatures above around 1000 °C, ITS-90 relies on extrapolation from a reference value, the melting temperature of silver (962 °C), which makes temperature measurements in this range subject to large uncertainties. To overcome this problem, NPL is at the forefront of current efforts to develop high-temperature fixed points, based on the melting temperatures of metal–carbon eutectic alloys at temperatures up to almost 3,200 °C.

High-temperature fixed points have proven to be extremely reproducible and NPL is coordinating a global research programme to introduce these reference standards for better temperature measurements. The significant improvements in temperature control they bring have already been used in a wide range of situations, for example, to improve manufacturing processes for high-efficiency gas turbines, develop new temperature sensors and characterise nuclear accident conditions. After the upcoming redefinition of the kelvin, high-temperature fixed points will be introduced as a standardised method of realising temperature.

Read the full article in Nature Physics

Find out more about NPL's Temperature & Humidity research

For more information, contact Jonathan Pearce

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Last Updated: 12 Jan 2017
Created: 12 Jan 2017


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