I made my own silicone membrane between 2 pieces of wax paper.
It's 1 mm in thickness about 3x as thick as the breather bag. It's also twice as reactive allowing about 2ppm per minute.
I also finished building the relay connections and will hook it up later today. I'll run for a month and observe fish reaction and BBa.
** oh, I just figured there's a mathematical limit to this sensor. You can only read 0 - 44.01 ppmw (this is what we want) and 0 -10,000 ppmv. It's good for growing plants. I probably had higher CO2 levels in my tank. I thought it looked suspicious that it stopped at 44.01ppm every time. I knew there's a reason why my fishes were so skittish.