In micro-devices the equivalent mean free path of the gas molecules can be of the same order as the characteristic dimension of these devices. In this case the fluid can be considered to be under rarefied conditions and it cannot be treated as a continuum medium as it usually done by classic fluid mechanics. Depending on their level of rarefaction, micro gas flows often require modeling by means of molecular based approaches. Gas rarefaction induces macroscopic non-equilibrium effects at the interface between gas and solid surfaces for the gas macroscopic parameters, such as viscous and thermal slip and temperature jump at the wall, for the gas velocity and its temperature, respectively. These non-equilibrium phenomena are affected by the specific configuration of the surface of the micro-device, such as surface roughness and materials, and the molecular structure of the gas considered. At the current state of the art, experimental observation on rarefied gas flows is generally limited to stationary flow configurations. Very little experimental efforts on transient rarefied flows have been conducted so far. Our work focuses on the experimental and numerical analysis of time-dependent isothermal rarefied gas flows through a long metallic micro-tube. The experimental methodology was conceived on the basis of the constant volume technique and adapted to measure the evolution with time of a transient mass flow rate through a micro-tube. Furthermore, the characteristic time of each experiment, extracted from the pressure measurements in each reservoir, offered a clear indication on the dynamics of the transient flow as a function of the gas molecular mass and its rarefaction level. Moreover we present an original methodology to extract stationary mass flow rates by using the tube conductance, which can be associated to the characteristic time of the experiment, measured for different mean pressures between two tanks. The measured conductance of the tube and the stationary mass flow rate at the inlet and outlet of the micro-tube was compared to numerical results obtained with the BGK linearized kinetic equation model. The results were obtained in a wide range of rarefaction conditions for nitrogen N2.