The relationship between the permeability of proppants and applied crushing stresses are conventionally determined using a linear flow cell compresses a 1" wide bed of proppant between platens. As well as inlet and outlet pressure for the fluid flowed through the proppant, the cell has three pressure measuring ports which enable the pressure drops to be measured separately across an upstream portion of the proppant bed and a downstream portion. Usually these readings are averaged and used to produce a single permeability of the proppant bed at any given closure stress. The paper describes the equipment used and presents results obtained by conducting "routine" fracture permeability tests on a variety of proppants. It is shown that the previously unreported downstream migration of proppant fines in the linear flow cell can have a considerable effect on the fracture conductivity and permeability.