I was with MikeK; when I took the pics I thought it might be lower than 20. But that's probably interpreting "the center is no longer darker than the outside" rather than the black dot "just disappear".

Interestingly (well, for photo geeks), these pictures overestimate the actual readings due to a combination of two factors: (a) they were taken very close to the top of the vial rather than from a distance (i.e., not waist high), (b) the small depth of field of cameras in general, but particularly at this small distance, causes the cloudiness to have a greater effect in the picture than in person. Indeed, although I can hardly see any variation in brightness in the 20 ppm picture, I could see definite variation in person.

Scott