A primary temperature scale requires realising a unit in terms of its definition. For highudtemperature radiation thermometry in terms of the International Temperature Scale of 1990udthis means extrapolating from the signal measured at the freezing temperature of gold, silverudor copper using Planck’s radiation law. The difficulty in doing this means that primary scalesudabove 1000 °C require specialist equipment and careful characterisation in order to achieve theudextrapolation with sufficient accuracy. As such, maintenance of the scale at high temperaturesudis usually only practicable for National Metrology Institutes, and calibration laboratories haveudto rely on a scale calibrated against transfer standards. At lower temperatures it is practicableudfor an industrial calibration laboratory to have its own primary temperature scale, whichudreduces the number of steps between the primary scale and end user. Proposed changes to theudSI that will introduce internationally accepted high temperature reference standards mightudmake it practicable to have a primary high temperature scale in a calibration laboratory. Inudthis study such a scale was established by calibrating radiation thermometers directly to highudtemperature reference standards. The possible reduction in uncertainty to an end user as audresult of the reduced calibration chain was evaluated.
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