National Physical Laboratory

High temperature fixed points

Metal-carbon eutectic fixed points for radiation thermometry

NPL is making fixed-points based on metal-carbon eutectic alloys that can provide reference points with a certified uncertainty of 0.05% of the ITS-90 temperature. These are known as high-temperature fixed-points (HTFPs). NPL is coordinating a global research programme to introduce these reference standards for better temperature and optical radiation measurements.

Traceable temperature measurement above 1000 °C is subject to large uncertainties and difficulties of verification. Reference standards will allow significantly tighter temperature control. This will lead to reduced energy usage and development of novel materials. They will also lead to better calibration of optical radiation detectors and have pushed improvements to filter radiometer design.

NPL is at the forefront of developing fixed-points based on metal-carbon eutectic alloys. These have been shown to have unprecedented reproducibility, allowing checks to be made to 0.1 °C at 2500 °C. They have already been used for international comparisons and to help develop new materials for high efficiency jet turbine engines. NPL Temperature and Optical Technologies team is working as part of an international program of validation and after redefinition of the kelvin these reference sources will be introduced as a standardised method of realising temperature. Their worldwide acceptance will bring demonstrated equivalence for production and legislation.

The Problem

The International Temperature Scale of 1990 (ITS-90) at temperatures over ~1000 °C relies on extrapolation from a reference value; the melting temperature of silver (962 °C), gold (1064 °C) or copper (1084 °C). Even the best realisation at an NMI will typically have uncertainties of 1 °C (k = 2) at 2500 °C. Without a high-temperature reference the only way to validate such a scale is by comparison, typically uncertainties would be 2-3 °C at high temperature. If the compared scales do not agree there is no simple way to determine which is at fault. Because of the expense, time and expertise involved in a realisation at this level it is not a practical proposition for most calibration labs. Instead they rely on a standard calibrated at an NMI. Most industrial users will be getting their scale at third hand with at best uncertainties of about 7-8 °C.

The Solution

Normally alloying causes changes to melting behaviour depending on the alloy composition. For example salt lowers the melt temperature of water to keep roads ice free in winter. At high temperatures materials react vigorously and it is difficult to measure a material's melt temperature without the crucible dissolving and causing contamination. Eutectic alloys exhibit an 'invariant reaction', which defines a unique temperature for a given combination of metal and carbon (graphite). This is true regardless of the exact amount of graphite dissolved This has allowed metal-carbon (M-C) alloys to be used in graphite crucibles without contamination from the crucible itself. As a result M-C eutectic alloy based fixed-points have so far been the only solution to the need for high-temperature references that have the necessary accuracy and practicality.

Eutectic fixed-points for thermocouple thermometry

The development of metal-carbon eutectic fixed-points (HTFPs) described above is also being performed within the contact thermometry area at NPL. These temperature fixed-points provide useful reference points above the upper limit of ITS-90 for calibration and/or validation of high temperature thermocouples e.g. Pt/Rh (Type B, R, S) and Pt-Pd. This technology has been implemented by industry and has been applied to NPL's UKAS accredited calibration options. NPL is now able to offer calibrations of high temperature thermocouples using the Co-C (1324 °C) and Pd-C (1492 °C) fixed points. Furthermore, the contact thermometry HTFP cells are available for purchase.

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Last Updated: 2 Dec 2014
Created: 11 Sep 2007

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