Continuum hydrodynamics accurately describes the flow of fluids over a wide range of systems. However, continuum models are unable to adequately describe the flow of fluid under extreme confinement. In particular, the no-slip boundary condition invoked in continuum flow calculations is violated for low density gases flowing in microtubes or nanoporous materials. The calculation of gas mass transfer rate in microtubes using experiments and simulation is investigated in the case of small absolute pressures. Specifically, in the present work, Argon gas is considered in a 3D model of two metallic tanks connected through microchannels. The pressure difference between the inlet and the outlet of the structure is monitored as a function of time. Thus the gas transfer rate is evaluated and compared with experimental data, in order to verify the departure from the standard Navier - Stokes equations with the no-slip boundary condition.