The International Practical Temperature Scale of 1968 in region 13.81 K to 90.188 K as maintained at the National Bureau of Standards

The reproducibility of the International Practical Temperature Scale of 1968 (IPTS-68) in the region of 13.81 K to 90.188 K using standard platinum resistance thermometers is discussed. The effect of errors above 0 °C on the calibration between 13 and 90 K is analyzed. Calibrations resulting from least squares and "fixed point" treatment of the data are compared. The results show that in general the NBS-IPTS-68 temperature scale maintained at the National Bureau of Standards (NBS) by reference standard platinum resistance thermometers is transferred to thermometers calibrated at the NBS within a few tenths of a mK between about 20 and 90.188 K and within about ±1 mK between 13.81 and 20 K. Agreements within a few mK are shown between the observed values of temperatures, employing platinum resistance thermometers calibrated in terms of the NBS-IPTS-68 scale, and the values of temperatures of the fixed points in the region 13.81 to 90.188 K given in the text of the IPTS-68.