The Vacuum Method is a key application of the differential pressure technique used to measure the diffusional permeability of gas filtration/separation membranes. Here, the permeation cell is separated into two distinct chambers by the membrane. Both chambers are initially evacuated, after which one side is filled with a testing gas at 0.1 MPa while the other side remains under vacuum. This creates a pressure difference of 0.1 MPa across the membrane. The gas mixture permeates through the membrane into the low-pressure side, inducing a pressure change there. The permeability then can be calculated based on this pressure variation, providing important data for the evaluation of membranes for the oil & gas industry.