National Physical Laboratory

High temperature research

Use of M-C high-temperature fixed-points within the context of redefinition of the base SI unit the kelvin as a route to improved temperature measurement.

Proposed changes to the SI will allow the full benefit of M-C high-temperature fixed-points as internationally accepted reference standards.

The SI unit of thermodynamic temperature is defined as 1/273.16 the temperature of the triple point of water of a specified isotopic ratio. Just as the unit of length is no longer based on a physical artefact there is a move to define all the SI base units in terms of phenomena rather than things. To have a definition of the kelvin consistent with this philosophy it is proposed that it will be defined in terms of the Boltzmann constant, kB. In other words the kelvin, and hence the °C, will be defined as the value such that a determination of kBT yields a value 1.380 65XX x 10-23 JK-1 for kB. Note that kelvin (K) is usually used when speaking of thermodynamic temperature while °C is used when a temperature on a scale such as ITS-90 is being discussed.

NPL has just completed an experiment to determine the value of kB to the necessary uncertainty by using speed of sound measurements. The impact of this is described in NPL's news story: Race towards a new temperature definition.

Once this change takes place there will be many ways to realise the kelvin. However such realisations are still unlikely to be practical for calibration laboratories, or even many NMIs, certainly over the full temperature range. What is intended is that there will be particular specified routes, or mises-en-pratique, to temperature realisation; agreed methods to establish practical temperature scales that are traceable to national standards. One option being pursued by NPL is to use M-C fixed-points with universally agreed melt temperatures. There are several ways these could be used:

  1. By analogy to the present ITS-90 the fixed-points could have defined values with zero uncertainty. Temperature scales could be set up anywhere with confidence that they all agree to within a couple of tenths of a degree.
  2. The fixed-points have agreed values with uncertainties. This would give a truly thermodynamic scale with uncertainties that match the limits of measurement capability at a fraction of the cost. The work of the optical and photonics team at NPL aims to establish the melt temperature of Pt-C to 0.2 °C and Re-C to 0.4 °C.
  3. Fixed-points made at NPL could be certified traceable to the kelvin with an associated uncertainty and disseminated to industry. This approach could make it cost effective to reduce the length of the calibration change and make much lower uncertainties available to industry.

The actual use in all cases would be by a fit of the output of a narrow band radiation thermometer to Plank's radiation law. This is a very robust approach in that simple measurements with intrinsic self checking can give low uncertainties without specialist equipment or knowledge.

Last Updated: 31 Oct 2014
Created: 6 Mar 2012

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