That sounds like a great-looking stand. Maybe you could send me a picture by email, you can find my address on my website. I’d love some inspiration. I also thought about 3D-printing a stand; I just need someone with experience in 3D design to create (or help me create) one, and someone who can print it.

I can’t really compare it to the Aranet4 because I don’t own one. But from what I can see, it offers similar functionality. In terms of hardware, though, the Aranet4 is more expensive than my setup. Of course, I also had to do some programming, which should probably be factored into the overall “cost” as well.

The accuracy of the Aranet4 is likely a bit better, at least based on the specifications I found (Aranet4: https://www.galaxus.ch/Files/6/6/1/0/2/6/6/2/Aranet4_datashe... , SCD40: https://m5stack.oss-cn-shenzhen.aliyuncs.com/resource/docs/d... ).

With my setup, I can also check the readings using my phone — currently only when I’m at home, since I host the stats website locally — but that’s enough for my needs.

I live in Switzerland — we normally don’t have air conditioners. At least not yet...

For my use case — simply getting some statistics about the air quality in our shared flat, improving concentration, and having a reminder to ventilate — the accuracy of the SCD40 is sufficient. It also has a self-calibration feature, which works as long as it is exposed to atmospheric CO₂ levels at least once a week. See the detailed specifications here: https://m5stack.oss-cn-shenzhen.aliyuncs.com/resource/docs/d...

I built a meter around a SCD30 six years ago. The self-calibration routine was not very good, for two reasons.

1) It had an unrealistically low default level, something like 380 ppm. The atmospheric concentration keeps going up! The linked pdf says "The automatic selfcalibration algorithm assumes that the sensor is exposed to the atmospheric CO2 concentration of 400 ppm at least once per week." Atmospheric CO2 definitely is not 400 ppm anymore.

2) As far as I could tell it made no effort to choose a local minima. In a regularly ventilated space, if it decided to "calibrate" when a door was closed, it could abruptly declare 600 ppm to be 380. I just hard coded an offset value and disabled ASC.

> as long as it is exposed to atmospheric CO₂ levels at least once a week

That's much less likely than most people would think.

A modern building without active ventilation and windows closed is absolutely not going to see atmospheric CO2 levels.

I measured this once and found it took almost a full week of no human occupancy for such a building to be equal to outside.

Made some changes to my blog, now the link above does not work anymore. Please use the following link: https://www.nicolin-dora.ch/blog/en-epaper-room-air-monitor-...

Thanks for your feedback. I just updated the blog with an English version. Please feel free to add any comments. English version: https://www.nicolin-dora.ch/blog/en-epaper-room-air-monitor-...

My suggestion is to make a new post in HN with the English version. HN is in English, and post in other languages are usually ignored or flagged. [Hi from Argentina!]

PS: I get a 404 in the German version.

Thanks, I created a new post: https://news.ycombinator.com/item?id=45962266 [Hi from Switzerland]

In der kalten Jahreszeit bleiben die Fenster oft geschlossen. Die Luft wird «dick»: Die Luftfeuchtigkeit steigt häufig über 60 %, was dem Wohlbefinden schaden und zu Schimmel führen kann. Gleichzeitig erhöht sich der CO₂-Gehalt in der Luft, was sich negativ auf die Konzentration auswirkt. Darum habe ich einen Raumluft-Monitor entwickelt, der so lange unauffällig bleibt, wie alles im grünen Bereich ist. Wird ein definierter Schwellenwert überschritten, wird er bewusst auffällig. Für meinen Bedarf an Statistiken visualisiere ich die Messwerte zusätzlich in einem übersichtlichen Dashboard.