A measurement apparatus is disclosed in which a barrier separates a chamber into a first region and a second region. The barrier is of a known thickness, and it comprises holes having a known radius. Gas flow means provide a flow of a carbon dioxide in the first region and a flow of a nitrogen in the second region. A gas detector measures the concentration of carbon dioxide in the second region and a processor calculates the theoretical diffusive flux from one region to another based on the known properties of the through-holes. The processor is arranged to calculate a calibration parameter in order to relate the measured parameter to the theoretical diffusive flux, based on the known properties of the through-holes.