The Importance of Establishing a Very High-Temperature Radiation Thermometry Measurement Capability at the National Metrology Institute of South Africa (NMISA)

Abstract

The ITS-90 scale above the Silver (Ag) point is realized using a standard linear radiation thermometer by measurement at a reference point which is an Ag, Gold (Au), or Copper (Cu) point. The fixed-point measurement at the Cu point coupled with relative responsivity, nonlinearity and range ratio measurement is used in the scale realization above the Cu point through extrapolation based on Planck’s Law. The uncertainty of the scale above the Cu point increases as the square of the ratio between measured and reference temperatures at very high temperatures. Since extrapolation above the copper point involves a significant source of uncertainty, it makes measurement in the high temperature range less accurate and less reproducible. At present, radiation thermometry deals with blackbody sources at working temperatures up to 3500 °C. In order to decrease the uncertainty, new types of fixed points are being investigated and found to be effective. The assigning of fixed-point temperatures to some of the suitable high-temperature fixed-point cells is completed. This development leads to the era of low uncertainty measurement at a very high temperature range. In this paper, the importance of developing such measurement capability at NIMSA is demonstrated.