That's exactly what I need, but I am having a bit of trouble wrapping myself around the illustration ? The written description makes a bit more sense. O.K. now I get it...so did we basically just construct a clear two piece 1" snuff box with a gas permeable bottom ? One could only hope the membrane just happens to be white. Man that's just as simple as it gets... almost poetic ? The one thing I might do would be to radius the outside edge of the inner ring on the membrane frame to prevent tearing the membrane, and perhaps thread the two sections, but for the simplicity of the design it could be built as a sealed throw away unit. If the membrane is gas permeable does the indicator solution ever evaporate or taint ? Grtz, Prof M