National Physical Laboratory

Radiation Thermometry of High-Temperature Fixed-Points

Metal-carbon eutectic alloy fixed-points constructed at NPL provide temperature reference standards up to 2500 °C for radiation thermometry.

NPL has world-class capability for constructing and characterising high-temperature metal-carbon eutectic alloy fixed-points.

Measurements of M-C fixed-points constructed at different NMIs have shown excellent agreement. Comparing NPL cells with others has shown reproducibility between sources within the comparison uncertainties of:

  • Platinum-carbon 50 mK at 1738 °C.
  • Rhenium-carbon 100 mK at 2474 °C

In contrast comparisons based on transporting pyrometers and lamps can have uncertainties at the level of 2-3 °C at 2000 - 2500 °C.

Where absolute values are needed these fixed-points can be assigned a UKAS certified ITS-90 temperature with an uncertainty of 0.9 °C for Pt-C and 1.3 °C for Re-C. The fixed points can be used in any ceramic or graphite furnace, although the better the uniformity the better the results. The actual repeatability of the individual fixed-point blackbodies is typically 10-20 mK when used with a sufficiently uniform furnace.

NPL is involved in establishing the thermodynamic temperature of these fixed-points by optical measurement with filter radiometers by the Optical and Photonics team. Once this is done NPL will be able to disseminate reference sources with the lowest uncertainty that can be achieved by the world's best NMIs, but at a fraction of the cost of realisation.

The table shows the full range of M-C fixed points that NPL can make together with approximate transition temperatures and UKAS accredited uncertainties.

Metal Melt Temperature
/ °C
U(k=2)
/ °C
Iron 1153 0.6
Cobalt 1324 0.7
Palladium 1492 0.8
Platinum 1738 0.9
Ruthenium 1954 1.0
Iridium 2290 1.2
Rhenium 2474 1.3


New service

NPL is able to calibrate radiation thermometers using these fixed points to give UKAS-accredited traceability to national standards. For suitable thermometers this can give a substantial reduction in the calibration uncertainties achievable. NPL can also supply fixed point cells with certified reference temperatures 0.025% uncertainty (k = 1) up to 2750 °C, and radiation thermometers suited to exploit these recent developments.

Find out more

Download Reduced uncertainty radiation thermometer calibration service flyer Adobe Acrobat PDF file

Supply of NPL high temperature eutectic fixed points

Last Updated: 17 Dec 2013
Created: 11 Sep 2007

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