Influence of gas-surface interaction on gaseous transmission probability through conical and spherical ducts

Gas flows through ducts are used in the metrology of vacuum in order to develop a continuous flow standard. Usually, the diffuse scattering is assumed for those particles which undergo collisions with the duct surface. However, many experiments pointed out a significant deviation from the diffuse scattering especially for light gases like helium. As a consequence, the conductance calculated on the basis of the diffuse scattering can be different from the real one leading to an additional uncertainty which is usually neglected. In the present work, the conductance of conical and spherical ducts applying the Cercignani-Lampis model of the gas-surface interaction is calculated. It is shown that in contrast to previous works, the energy accommodation can significantly affect isothermal gas flows. Moreover, a decrease of the accommodation coefficients not always leads to an increase of the transmission probability, but in some situations a smaller energy accommodation coefficient corresponds to a smaller conductance.