Investigation of the Accuracy of the ITS-90 with Reference to Thermodynamic Temperature in the Range from 400 °C up to 600 °C

Abstract

For several years Physikalisch‐Technische Bundesanstalt (PTB) has performed thermodynamic temperature measurements on a large area blackbody applying filter radiometers (FR) based on silicon photodiodes and interference filters with center wavelengths at 676 nm, 800 nm, 900 nm, and 1000 nm. These FRs were used for the determination of possible systematic deviations of temperatures measured according to the International Temperature Scale of 1990 (ITS‐90), T90, and the thermodynamic temperatures T. The measurements in 1995 with the 676‐nm‐FR and the 800‐nm‐FR revealed a difference of 50 mK of T−T90 at temperatures around the freezing point of silver (961.78 °C). The observed difference decreases with decreasing temperatures, indicating that it may be attributed to a systematic deviation in the ITS‐90 from T due to the gas thermometric temperature measurement at 457 °C, which has been used as a reference temperature for extrapolation of the ITS‐90 to higher temperatures. For a further, more detailed investigation it was necessary to measure the difference T−T90 down to temperatures of 420 °C, the temperature of freezing zinc, which serves as one of the temperature fixed points of the ITS‐90. However, the spectral responsivity of silicon photodiodes does not allow their application in filter radiometers with center wavelengths beyond 1000 nm, which are needed for accurate thermodynamic temperature measurements below 450 °C. Therefore a filter radiometer with center wavelength arround 1600 nm based on an Indium‐Gallium‐Arsenide photodiode has been developed. The design of this radiometer, the assessment of its spectral responsivity and its temperature dependence is shown. T−T90 results are presented for the temperature range from 400 °C to 600 °C obtained with filter radiometers with their center wavelengths at 800 nm, 900 nm, 1000 nm, and 1600 nm.For several years Physikalisch‐Technische Bundesanstalt (PTB) has performed thermodynamic temperature measurements on a large area blackbody applying filter radiometers (FR) based on silicon photodiodes and interference filters with center wavelengths at 676 nm, 800 nm, 900 nm, and 1000 nm. These FRs were used for the determination of possible systematic deviations of temperatures measured according to the International Temperature Scale of 1990 (ITS‐90), T90, and the thermodynamic temperatures T. The measurements in 1995 with the 676‐nm‐FR and the 800‐nm‐FR revealed a difference of 50 mK of T−T90 at temperatures around the freezing point of silver (961.78 °C). The observed difference decreases with decreasing temperatures, indicating that it may be attributed to a systematic deviation in the ITS‐90 from T due to the gas thermometric temperature measurement at 457 °C, which has been used as a reference temperature for extrapolation of the ITS‐90 to higher temperatures. For a further, more detailed investi...