ACCURACY OF OPTICAL PYROMETRY IN THE RANGE 800 °C TO 4000 °C

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Abstract
A pair of secondary standard tungsten strip lamps have had a luminance temperature – current calibration, in the range 800 °C to 2200 °C, at a number of national laboratories. An analysis of the calibration results confirms estimates of the accuracy of optical pyrometry in the range 800 °C to 2200 °C and supports the extension of these estimates to 4000 °C. The standard deviation uncertainty of optical pyrometry is shown to be about 1 °C at 800 °C rising to 2 °C at 2200 °C and 10 °C at 4000 °C, being about double this for the calibration of commercial pyrometers unless certain described precautions are taken.The reliability of the secondary standard lamps, when used under well-defined conditions, is confirmed and it is shown that they have a standard deviation calibration uncertainty of about 1 °C for the vacuum-type lamps in the range 800 °C to 1500 °C and 2 °C for the gas-filled lamps in the range 1500 °C to 2200 °C. Most of this uncertainty is due to primary standard optical pyrometer calibration errors. Attention is drawn to the fact that a carbon arc fulfills the requirements of a secondary luminance temperature standard at about 3514 °C.Recent determinations of the gold point and the second radiation constant indicate that the 1948 International Temperature Scale is lower than the thermodynamic scale by an amount varying from 0.8 °C at 800 °C to 12 °C at 4000 °C. This is already greater than the calibration errors of optical pyrometry and, in view of the still greater accuracies presaged by photomultipliers, a revision of the International Temperature Scale is suggested.

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