Oxygen-quenched luminescent paints, commonly used as pressure sensitive paints, can be used to characterize concentration distributions of oxygen and non-oxygen gas mixtures. When quenching from pressure effects are small relative to concentration effects, a porous plate in the exit plane of a mixing region, painted on the back side, can be used to measure the molar fraction of oxygen at any point. A painted coupon was used to calibrate a sample of PF2B paint in the normal method, using a range of pressures. A porous plate, painted with the same plate, was affixed at the exit plane of both a chamber with high gas mixing and a chamber with minimal mixing. oxygen and nitrogen gasses were mixed in the chambers, which were left open to atmosphere. By measuring the intensity of light emitted by the paint and assuming that pressure in the wake of the plate was very nearly ambient, the Stern-Volmer equation was applied to determine the molar fraction of oxygen at all points. These measurements were compared by the measured mass flow rates of each gas into the chambers, which did not match completely. Potential reasons for the disconnect are discussed.