I have been watching the O3 with the new power supply and it’s quite the same. It is just that – during the daytime – there is a lot of activity that covers up the tendency to “stick” around 120 ppb. Since this sensor is designed to measure PPM, it is probably unrealistic to ask it for readings in PPB range. The resolution is 10 ppb and I think the zero point could easily drift by 100 PPB.

I would be nice if there was a way to set the zero calibration in software. In my case, zero gas is giving a reading of around 100 PPB (and has always been so).

If replacing the sensor with another one from China would be certain to fix the problem, I would be willing to give that a try.

Ken