High periodic spikes from Senseair S8 CO2 sensor (and lower from one MHZ-19)

I was using an MHZ-19B to measure indoor CO2 for two years or something. As I needed a second sensor I planed to get a new one. After hearing that the Senseair S8 should perform better and less fakes one around I got one of those and to compare it a new MHZ-19(C).

Yesterday I build a simple ESP32 based test project with all three sensors connected at the same time.

This is the result:

That doesn’t make sense. Those spikes for the S8 and the little ones for the new MHZ-19 doesn’t make sense at all. I was just sleeping in the same room and the spike at 19:00, no one was at home at all.

The S8 and the new MHZ-19 always spike at the same time but the S8 much more and the MHZ-19 peaks at the end of the spike of the S8. While the old MHZ-19 has a smooth incline as expected.

What is going one? Two broken sensors but somehow synchronized? Something in the real world triggering this? There is nothing: No fan, no light. Maybe temperature, but this is floor heating it should be quite stable.

Some issue in the ESP32 with the UART communication or some issue in the sensor code of ESPHome? Why would that only effect two of the sensors?

I am at loss? Has anybody experienced something similar or any other idea?

(I am also not happy about how noisy the S8 is in general (you can see it between the spikes). That is even with a median filter with a window size of 7. Is that normal for an S8?)

Are you sure it’s not a fan?

The only time I see something similar is when my ducted heating turns on and blows the accumulated CO2 out of the bedroom past my S8 sensor in the lounge.

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How are you powering them?

Just a wild guess but I’ve seen some sensors which are sensitive to voltage.

Pretty sure. There is none visible in any of my rooms. And it also happenend with the window open with the sensors only two meters away.

I’ve tried a USB power supply but I think (not sure anymore) that I also tested with a USB power bank. Will do more experiments tomorrow.

The most important thing I want to try: Just one sensor on one ESP32 with a different power supply on each. If they still get in sync with those spikes, it has to be something real in my environment.

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