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, Helium, and Air 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.