I checked the response time for this latest membrane drop checker. It took about 30 minutes to make about 2/3 of the change in color from one ppm of CO2 to another, both for increasing ppm and decreasing ppm. Disappointiing.

It looks almost like something like Heisenburg's Uncertainty Principle - you can either get fast reaction or ease of reading the color, but not both at once. Maybe I will call it "Hoppy's Uncertainty Principle" and find the constant that connects response time to readability. (T/R=K, time constant divided by readability equals a constant.) If I get to it tomorrow I will try this with a sponge in the cavity to reduce the fluid volume, but I know that will make it even harder to see the color.