Sensitivity of high vacuum standard parameters to the share of molecule specular reflections in the gas scattering

The molecular gas flow parameters were evaluated using cosine form of the scattering function of the molecules incident on the surface. This gas scattering model is commonly used for calculating the vacuum system parameters, but it is theoretically valid only if the vacuum system is in equilibrium. This also means that there is no flow in the system. We applied the Maxwell's model of gas scattering with the contribution of specular reflections to study how these reflections influence the flow parameters. In typical calibration systems with dynamic expansion of gases, a 10% contribution of the specular reflections results in the 0.18% decrease of the computed value of the gas density inside the gauge to be calibrated as compared with that when the cosine law is employed.