The development and characterisation of high temperature fixed points at NMISA

Abstract

Many National Metrology Institutes (NMIs) realise the spectral irradiance scale by obtaining traceability from a cryogenic radiometer through the use of calibrated filter radiometers. The filter radiometers are used to determine the temperature of a high temperature black body which is then used as a reference source, which spectral radiance can be determined from Planck’s equation. The uncertainty of the temperature measurement makes the most significant contribution to the uncertainty of realising the spectral irradiance scale. High temperature fixed points (HTFPs), above the copper point, can be used to improve these uncertainties. After more than ten years of research, results obtained on metal-carbon eutectic fixed points by several NMIs, showed that these novel high temperature fixed points could lead to significant improvements in high temperature metrology and could be considered as potential fixed points in a future International Temperature Scale.This paper describes the development and characterisation of selected high temperature metal-carbon fixed points at NMISA. It is demonstrated that these fixed points can be utilised as reproducible, stable reference standards for temperatures above the copper point.

Keywords

• Eutectic cell
• high temperature fixed point
• phase transition
• melt temperature
• radiometry