I can tell you that the calibration depends on the atmospheric pressure. I doubt very much if the readings are accurate to a full percentage point, i.e. that you can be sure that a reading of 32% is between 31.5 and 32.5%. When I mix nitrox I calibrate all 3 sensors to air. I check the mix into the compressor, the mix out of the compressor and then check the tanks. I get close, but not that close.In this whole mess does anybody check their air and adjust the DC according?
Oh wait, that's the calibration point.
I am quite sure in cities o2 in the air could be lower....