A method for measuring the transmission rate of an analyte through a film. The method includes the steps of (i) separating a chamber into a first cell and a second cell with a known area of a film, (ii) flushing the first cell with an inert gas to remove any target analyte from the first cell, (iii) introducing a gas containing a known concentration of an analyte into the second cell, (iv) sealing the first cell to gas flow through the first cell, and (v) sensing any analyte in the first cell with a sensor that consumes the analyte at a rate greater than the rate at which the analyte is passing through the film, until a steady state rate of analyte consumption is measured by the sensor.