Surface Smoothness in Thermal Transpiration at Very Low Pressures

Abstract

The thermal transpiration ratio for helium has been measured by the absolute method with a leached pyrex tube joining the cold and hot regions. Cold temperature was $T_1\text{=77.4\hspace{0.17em}°}\mathrm{K}$ , hot temperature $T_2\text{=295\hspace{0.17em}°}\mathrm{K}$ , pressure range ${10}^{\text{−7}}$ to 100 Torr. The results agreed closely with those obtained when an aperture joined the cold and hot regions. In particular, the ideal limiting law $P_1{\mathrm{/P}}_2{\mathrm{=(T}}_1{\mathrm{/T}}_2\text{)1/2}$ was found at very low pressures, $P_1$ and $P_2$ being the pressures at $T_1$ and $T_2$ respectively. This result is in contrast to that found with smooth Pyrex tubes where unpredictable deviations from the ideal law were found. It is concluded that leaching the surface produces atomic roughness which forces all reflections to be cosine reflections, leading to the ideal law. The use of a leached tube permits precise thermal transpiration corrections in simple glass apparatus. The glass was leached for 100 h at 100 °C in 0.05 N HCl.