Development of Precision Rh–0.5 at%Fe Thermometers of Chinese Production: Further Tests

Abstract

Following the practical impossibility to obtain new precision Rh–0.5 at%Fe thermometers over the past years, re-starting the commercial production of such thermometers in Yunnan (China) was explored by INRIM in cooperation with NIM and with the help of INTiBS for prototype characterization. The present aim is to obtain a stability of the new thermometers at the level of ±1 mK at 4.2 K. In 2008, a new batch of eight prototypes was produced. This paper reports the results of the measurements on the full characteristics of these prototypes in the range from 2.5 K to 25 K, which confirms a similarity to the typical characteristics of previous commercial RhFe thermometers, and of the effect of thermal cycling, showing for six out of eight thermometers, a stability better than ±1 mK (limited by the measurement expanded uncertainty of ≈0.8 mK) at 4.2 K and up to ≈10 K, and better than ±0.01 % T in the range from 2.5 K to 25 K. These results indicate that production problems on these commercially available thermometers are basically resolved at the aimed level of stability. Further studies are foreseen to improve production uniformity and to check if stability may actually be better by using a test apparatus of sub-millikevin uncertainty.