Reproducibility and Assignment of phenol temperature fixed point

Abstract

Negative temperature coefficient thermistor (NTC) is widely used in temperature measurement field, especially in the temperature range from 0 ℃ to 40℃. For its strong nonlinearity, NTC should be calibrated in this temperature range with three or more temperature fixed points. However, in this temperature range, there is only two defined temperature fixed point in ITS-90: Gallium point and water triple phase point. In this paper, a phenol temperature fixed point was developed to make up the deficiency. Melting and freezing temperature curves are studied for the single-component phenol in a conventional-size cell. The phenol melting plateau can last for more than 10 hours with melt range of less than of 100 mK. The freezing plateau can last approximately 2 hours with the observed temperature variation less than 5 mK, which showed a high degree of reproducibility. Different methods were employed to accurately determine the phase transition temperature. By repeatedly realizing phenol melting plateaus and freezing plateaus, the melting point temperature was determined to be 40.6105 ± 0.015 ℃ (k = 2) with a reproducibility of less than 7.4 mK, and the freezing point temperature was 40.6346 ± 0.0034 ℃ (k = 2), with a reproducibility of less than 1.5 mK. The results showed that the phenol freezing point temperature is the most suitable temperature reference standard near 40.6 ℃.