Probe permeameters have been widely used to characterize the spatial variations in permeability in core plugs, outcrops, rock slabs, and slabbed cores. Constant pressure, rate, and volume boundary condition methods are in nse. The portable, rapid, accurate, precise, inexpensive, and durable nature of our computer-interfaced electronic flow rate-controlled probe permeameter makes it an ideal tool for making the large number of laboratory and field permeability measurements necessary for quantitative reservoir description and characterization. This equipment utilizes a precision mass-flow regulator to maintain constant flow rate, allowing steady-stare permeability to be calculated from a modified form of Darcy＇s law, while avoiding the need for inertial corrections. This approach requires no calibration to core-plug Hassler Cell standards and allows flexibility in adjusting seal-tip size or material. Hassler Cell permeability measurements were matched within 1 to 12% for homogeneous core plugs from 1 md to 45 darcys with a precision of typically 1 to 3%. The volume of investigation by probe-permeametry methods has been addressed through analytical modelling and laboratory experimentation. According to our modelling efforts, streamlines and probed volume are independent of permeability and coincide in compressible and incompressible steady-state flows. The effective radius of investigation was found to be about three to five times the internal tip radius for injection. For negligible tip thicknesses, 80% of the injected fluid was found to exit the face of the prepared surface within a distance of one additional tip radius, and 90% exited within five radii. In-situ polymerization experiments involving the injection of tracer compounds with styrene indicated hemispheroidal flow during steady-state probe-permeametry operation for homogeneous porous media. For a seal tip with internal diameter of 4 mm and outer diameter of 7 mm, the volume displaced by tagged styrene was 19 mm deep and 20 mm in diameter when stable instrument readings were obtained. This confirms the Goggin flow model, the use of the geometric factor, and the modified form of Darcy＇s law. These findings also impact surface-preparation procedures and the maximum sampling density for independent data collection.