For home use a mid-range unit is sufficiently accurate, precise, and reliable. I use the Extech CO200 personally. Don’t but a cheap unit because those have calibration drift after a few months. And recalibration is not trivial.
https://www.adafruit.com/product/3566
What I was not able quickly to figure out is:
- What this eCO2 actually is?
- What is the real world accuracy I can expect from this kind of sensor, with and without calibration?
- is it enough for calibration just to go outside and assume some number between 400 and 500 (assuming outside means not just next to industrial chimney)? If not, what would be better calibration process?
It's an estimated reading of CO2, so it's not a direct reading, the sensor has to do some math to estimate the CO2
It's not very precise and/or dependent on temperature, etc
You can actually see when the device acquires a reading, because the tube flashes.
https://ec.europa.eu/eurostat/statistics-explained/index.php...
Re your other questions; yeah I gather there is some experimentation involved, even with out of the box commercial products.
If others have suggestions on chips or products to buy for the household, I would greatly appreciate it! The MH-Z19 looks like an option for me. Commercial products seem too unreliable for the price.
During the smoke event, they combined with my indoor/outdoor pm2.5 sensors have been extremely useful.
The protocol is compatible though.
Also note they come in up-to-3000ppm and up-to-5000ppm variants. Be sure to get the latter.
In case it helps anyone: a very fast way to get it working and get graphs is to connect it to a Raspberry Pi running HomeAssistant. Takes a few clicks and 3 lines of config.
One thing is that air diffuses out fairly effectively even with the door closed, presumably through gaps in the door frame. Quick calculation (welcome any corrections, numbers pulled from quick online searches):
Resting breathing rate is ~6 liters/minute.
Sleep ~8 hours = ~500 minutes = 3000 liters exhaled.
CO2 exhaled is 5% concentration, so 150 liters of CO2
Room = 30 m^3 = 30000 liters
Expected additional CO2 concentration in morning in perfectly sealed room = 150 / 30000 = 0.5% = 5000ppm
Actual additional CO2 = 700ppm (1100 reading - baseline 400ppm)
I've also found that when all windows are closed, an air purifier seems to help (or a fan, lowest level is fine). Obviously it doesn't remove the CO2, but it seems to prevent buildup in the one corner where you are (bed, desk). I'm sceptical it's a problem but the data shows it is.
1000 ppm is an absolute limit, the vast majority of the population will experience measurable effects above this threshold. And it’s surprising how often that’s exceeded in normal everyday environments.
First, if you start from 600pm instead of 300, you reach 1000 ppm faster. Adding to the insult, it is much more difficult to ventilate the excess CO2 away, if the air you use to ventilate contains in itself already 600ppm instead of 300ppm.
More ventilation cannot push the levels below the levels of outdoor air, but it can get them close.
Their argument that assuming today's already inadequate ventilation systems don't change, more outdoor CO2 = more indoor CO2, pushing it above some arbitrary threshold (which is far above predicted outdoor CO2 levels).