(German below)

We have two Sensirion SCD41 sensors in the space. One is on top of the on top of the shelves between the seats and the couch in main room, and other is in the soldering workshop. Locations may change, this page should be updated accordingly.

They're connected to an ESP8266 with an 8mm PL9823 RGB LED (which uses WS2812B protocol, except it's RGB, not GRB). They run esphome and data is available over Club Assistant.

The LED changes between green and yellow between 400-1000 PPM, yellow and red between 1000-2000 PPM and stays red above 2000 PPM. This can give a quick overview of how fresh the air in the room is, however, you should apply your own safety model to the values.

In my personal (ave - Public Page) experience, due to the size of the room and leaky windows, the main room rarely goes above 2000PPM, but it is also really hard to lower the level when there's a large amount of people in there. The soldering workshop, when the door and windows are closed, has its CO2 values go up very fast, you should pay attention to air it regularly when it is crowded.

A quick rundown of how to interpret the values:

A detailed look into research on CO2's effects on human cognitive health can be read on this paper.

As a note, while air filters make the air feel fresher and reduce the Covid risk, they do not lower the CO2 levels. You should open the windows to air the room as appropriate.

For sensor choice justification see https://queer.party/@ao/111779978098588711, tl;dr: NDIR is the only cheap and good way to get CO2 data currently, and many other cheaper options require a lot more maintenance due to their calibration code being worse.

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German goes here