Setting the worldwide standard for permeation testing, the OX-TRAN Model 2/21 system uses a patented coulometric sensor(Coulox®) to detect oxygen transmission through both flat materials and packages. This high performance sensor provides parts per-billion sensitivity even in the presence of water vapor. The Coulox sensor is an intrinsic or absolute sensor that does not require calibration. N.I.S.T. films are available to ensure the entire system is performing to the highest MOCON standards in precision and accuracy. Flat film samples are clamped into the diffusion cell, which is then purged of residual oxygen using an oxygen-free carrier gas. The carrier gas is routed to the sensor until a stable zero has been established. Pure (99.9%) oxygen is then introduced into the outside chamber of the diffusion cell. Molecules of oxygen diffusing through the film to the inside chamber are conveyed to the sensor by the carrier gas.