Probe- or mini-permeameters are used for a quick determination of permeability distributions both in the field and in the laboratory. Based on designs of portable equipment, a fully automated probe has been developed for laboratory use. It allows the measurement of 100 values in an hour while scanning a sample on a dense grid. Thus heterogeneity on a scale of several millimeters can be detected. In our set-up, one of three different gases, helium (He), nitrogen (N-2), and sulphurhexafluoride (SF6), is depleted from a vessel through a silicone-rubber seal, which is pressed against the surface of the sample. During depletion the pressure is measured as a function of time; this curve forms the basis for all our computations. He shows the largest Klinkenberg effect as it has a long mean-free-path due to its small molecular size. SF6 flow shows the largest inertia effect due to its large molecular mass. N-2 is intermediate in both cases. We introduce a model based on a dynamic skin factor for spherical flow near the probe tip. With this model we obtain a simple relation between the injected now and inlet pressure in terms of gas-permeability and inertia effect. Comparison of this simple relation to numerical model results gives a geometric factor for the inertia effect in addition to the conventional geometric factor for permeability. We show that the pressure versus time plot can be used for a quick and accurate determination of both the gas permeability and the inertia factor. Reliable inertia factors, however, can only be obtained for lower-permeability samples.