So I built a room air monitor that stays unobtrusive as long as everything is in the green zone, but becomes deliberately noticeable once thresholds are exceeded. For my personal love of statistics I also visualise the measurements in a clear dashboard.
Most of the affordable CO2 meters are relative, not absolute. They set their 400 PPM level based on the lowest value they ever see. That's usually OK, but it's not good enough for places with permanent people occupancy, such as nursing homes. Absolute detectors with NIST calibration are available but around US$500.[1]
[1] https://www.forensicsdetectors.com/products/carbon-dioxide-d...
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.
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.
I have a similar monitor for equipment metrics, and the cardboard design is similar to the stand I made out of metal. I powder coated the metal light beige and it looks professional.
Fun - I have the opposite problem, humidity goes down to almost nothing and we have to use humidifiers to keep it around 40% to avoid horrible nose/throat/skin dryness.
Forced air hvac is probably why.
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.
Wouldn't it be easier to just turn on the air conditioner?
Home Assistant has been running here for several years, and there are quite a few mammals in a relatively small space (humans and dogs). Air quality plays a significant role in well-being. I spent some time tinkering around to find good sensor solutions (I still use esp32 with bme280/dallas and mhz19 for other rooms), and after some back and forth, I purchased an Awair Element. At first glance, it seems quite expensive, but the sensors alone would cost me 1/3 of the price.
We love it. The little LED that indicates air quality, which I didn't even notice at first, is extremely helpful. The sensors are so accurate that I can see when someone has cooked something, when cleaning products have been used, or when we have a dog visiting. A simple API+web server (which I never needed), as the Home Assistant integration works great.
Great device.
zucked•2mo ago
The sky is the limit as to what you can do with Home Assistant automations.
It's surprising how quickly a room with a closed door and one person can go from ~ambient CO2 levels to 1000ppm+.
broabprobe•2mo ago
rifty•2mo ago
Yeah, having seen myself how quickly it happens i've recently been thinking of finding automatic window openers that would respond to CO2 levels reported from either my aranet or on its own.
buckle8017•2mo ago
rifty•2mo ago
buckle8017•2mo ago