Air exchange rates and air quality - understanding

We usually solve this with a timer, then the system runs twice a day at level 2. I'm not a fan of too much technology and sensors, they all just break down...

But at the beginning you describe that your approach does not work satisfactorily, so what now?

Our Q350 runs continuously in automatic mode at level 2 with 175m³/h. The volumes of the individual levels can be set independently during installation (or by the installer). When we shower, the system automatically (there is a function in the menu) switches up to level 3 until the humidity value is appropriate again. I estimate that takes about 30 minutes. We are actually satisfied with our air quality. I have not noticed anything negative yet. However, I have never set up a measuring device.

You describe at the beginning that your approach does not work satisfactorily, so what is it now?

But I am absolutely satisfied. I just wonder whether it is really supposed to be that the measurable air quality decreases with two people in a room and does not remain constantly at the achievable minimum of 450 CO2. But I can see, normal people probably don't have such thoughts.

CO2 is a dog. Actually heavier than air particles, CO2 gets swirled around the entire room when entering or opening a door.
Plaster can absorb and release CO2.
CO2 sensors drift over time. Many calibrate themselves. Some have to be manually calibrated over time (0 point calibration).
The CO2 measuring cells have fluctuating tolerances depending on temperature. Typically, tolerances at 21° are specified as 10% of the measured value or ~ ±75ppm. Now, of course, it depends on the measuring range. In the worst case, you have 2 sensors with maximum tolerance up and down, which corresponds to a difference of 150ppm.

All this causes measurement deviations or fluctuations.

You see, a CO2 sensor is a better indicator for air quality and, metrically speaking, rather an estimation tool.

Roughly applies:
0...750ppm = very good air quality
751...1250ppm = good to moderate AQ
1251...2000ppm = moderate to poor AQ.

With two people in a closed bedroom, you easily reach between 5000-10,000ppm (depending on room size and sleep duration).

Besides the CO2 sensor, it would be interesting to install a VOC sensor as well. It measures multiple gases, such as adhesives from materials, cleaning agents, cigarette smoke, food odors, and many other gases that can cause headaches and migraines.

Yes, exactly, that is the approach that is most likely to lead to the desired results.

CO2 + VOC strategically arranged into one system which can make use of the values.