Investigation into the influence of surface conditions on the tube conductance for the molecular flow regime

Abstract

The flow of different gases in a long, circular tube was investigated with a focus on the influence of the surface condition on the conductance in the molecular regime. The measurements of conductance were made with an uncertainty below 0.5%. A stainless-steel tube with a uniform diameter and roundness was used. The precisely known geometry of the tube allows an accurate calculation of the conductance in the molecular regime under the assumption of a perfectly diffuse scattering of molecules on the surface. Any deviation from diffuse scattering on the surface influences the molecular conductance of the tube. A comparison of the measured and calculated conductance in the molecular regime allows an estimation of the effective tangential momentum accommodation coefficient (ETMAC) of the tube's surface. The tube was first measured in the as-received condition (electropolished surface), and after that exposed to different treatments that altered the surface condition and influenced the ETMAC. In addition, the same tube was chemically etched to increase the surface roughness. Both surface types were treated by heating in vacuum, oxygen and hydrogen. A large variation in the conductance of 10%–20% was observed after etching, depending on the gas (He, CH4, Ne, N2, Ar, Kr).