Realization of ITS-90 Above the Silver Point at the NIM

The high-temperature primary standard system was gradually improved at the National Institute of Metrology (NIM) in China after 2004. A new primary standard pyrometer (PSP) was developed, one with a size-of-source effect of 1 × 10^?4, and regional thermostats for interference filters, photoelectric detectors, and I/V converters. The relative spectral responsivity of the entire PSP was calibrated by means of a new facility. A new LED-based measurement facility and novel systematic error correction model were utilized to characterize the PSP nonlinearity and extend the photocurrent to PSP temperature readings of about 2680 °C. As an improved scheme, the fixed-point blackbody pyrometer assembly was utilized to realize and disseminate the International Temperature Scale of 1990 above the silver point. This scheme can avoid the influences of instability and inhomogeneity of tungsten strip lamps and corrects pyrometer drifts, thereby improving the realization uncertainty and simplifying the transfer chain. The expanded uncertainties of the scale realization ranged from 0.04 °C at the silver point to 0.48 °C at 2474 °C.