Some Measurements of Thermal Transpiration

Abstract

Measurements have been made of the pressure ratio, R, due to thermal transpiration in hydrogen, helium, argon, and nitrogen between a warm temperature of 297°K and cold temperatures of 77.3 or 195°K. It is found that R depends on X, the product of the pressure and tube diameter according to the empirical relation, $${\mathrm{R=P}}_1{\mathrm{/P}}_2{\mathrm{=(AX}}^2{\mathrm{+BX+R}}_m{\mathrm{)/(AX}}^2\text{+BX+1)}$$ , where A and B are constants which depend on the gas and on the warm and cold temperatures, and R_m=(T₁/T₂)^½. The R against P₂, and threfore X, curves for different gases are found to be related by a ``pressure shifting factor'' f. The use of these results to correct measurements of adsorption equilibrium and vapor pressures at low pressures and low temperatures is discussed. Vapor pressures at 77.3°±0.5°K corrected for the thermomolecular pressure effect have been measured for krypton and xenon: $$\begin{array}{cc}\mathrm{Krypton}& \text{1.754±0.002\hspace{0.17em}}\mathrm{mm};\\ \mathrm{Xenon}& {\text{1.93\hspace{0.17em}±0.01×10}}^{\text{−3}}\hspace{0.17em}\mathrm{mm}.\end{array}$$