New realization of the melting point of gallium at NIM and validation by a comparison with PTB

For more than 30 years, a PTFE-glass composite design has been used for gallium fixed-point cells at the National Institute of Metrology (NIM), China. Because the outer glass case is prone to breakage, it cannot guarantee the long-term stability of gallium fixed-point cells. In order to improve the long-term reliability and the robustness of the gallium fixed-point cells, in recent years, many efforts have been devoted to developing stainless steel-cased gallium cells. In order to improve the vertical temperature uniformity surrounding the gallium fixed-point cells, an annular copper-ethanol heat pipe was adopted to be operated as an isothermal liner. Due to the flattening ability of the heat pipe by means of phase transitions, the maximum temperature difference within 13 cm from the bottom of the thermometer well was less than 1.3 mK when the gallium fixed-point cell was controlled at a temperature of about 29 °C. An induced inner melt method was used to obtain an inner liquid-solid interface adjacent to the thermometer well. The melting temperatures of the plateau over 64 hours were in good agreement within 0.02 mK using the ethanol heat pipe technique. Additionally, in order to confirm the international equivalence of the new gallium cell of NIM, a bilateral comparison between NIM and PTB was carried out at PTB in 2019. The comparison results show that the average temperature difference between the NIM cell (s/n: Ga-M-002) and the PTB national reference cell (s/n: Ga648) amounts to 0.01 mK, which was ascribed to the impurities within the fixed-point materials.