When I looked into this a few years ago, I couldn't find any accuracy guaranties for CO2 meters marketed for households / greenhouses.
The closest thing I could find were laboratories that could test CO2 meters in chambers with known CO2 concentrations. But IIRC the pricing for those labs was prohibitively expensive.
- There were literally no guaranteed accuracy bounds for the meters I looked at. So they could be off by 3x reality for all I know.
- Their calibration systems relied on an assumed CO2 concentration for outside air. But even if the calibration system ensured that the meter would report the right number for that CO2 concentration, there was no information about how accurate the calibrated meter would be at other CO2 concentrations. Nor information about how the meter's numbers would be off when the CO2 levels observed during calibration differed from the level assumed by the calibration logic.
These limitations might not be a problem for some applications. But they could be an issue when people want to relate their meters' readings to the numbers used in various research publications.
This works acceptably if the sensor is frequently exposed to outdoor air, but in a residential environment that's not always guaranteed, particularly in winter when it's not uncommon to keep windows closed to retain heat. In these situations the sensor will consider the lowest level to be around ~400ppm, even if it's actually much higher. This, of course, scales all other readings, so a sensor might read between 400-800ppm, leading you to believe everything is fine, when the actual indoor range is 800-1600ppm.
Because the auto-calibration happens over a period of time, it can be quite difficult to determine that your sensor is misreading, and the only way to fix it is to expose it to fresh air to reset the baseline.
The best solution I found to this is a dual-NDIR sensor which measures two different light frequencies, one which is absorbed by CO2 and one that isn't. This allows the sensor to know the absolute CO2 concentration, rather than the relative CO2 concentration, and avoids the need for auto-calibration. (I believe for absolute accuracy it still needs calibration for altitude, but for consumer use this makes such a small difference to be irrelevant).
Unfortunately, when I last looked, I couldn't find any consumer-grade sensors which used dual-NDIR sensors, only more expensive and less aesthetic commercial sensors. In the end I built my own using a CDM7160 sensor connected via I2C to a ESP8266, which reports over MQTT.
Looks like that's been discontinued. [1] Any advice for folks trying to build one now?
[1] https://www.figarosensor.com/topic/2020/11/Discontinuance-of...
For my purposes I don’t really care about accuracy under 50ppm, higher precision for trends is useful but as long as the measures value is accurate to within 50ppm I’m just fine for effects on a human. If I was doing research to publish an actual calibrated meter for 10x the price would be warranted but having three separate measurements agree gives me the trust I need.
You just need an airtight container of a known volume, a source of CO2 and a maybe a fan. Put your CO2 detector inside the box, with fresh air, your CO2 source and the fan, and see if the result matches. You may need to do some calculations.
For your source of CO2, you have a few options: combustion of a known quantity of fuel, soda bottle, dry ice, acid + sodium bicarbonate,... If you want to remove CO2, you can use calcium oxyde (quicklime).
I had a semi-sophisticated device consisting of a lightweight plastic cylinder that could move up and down when filled with gas, and a way to know the volume with accuracy (single-digit milliliter error out of five liters). I had a tank of pure CO2 and an air intake, coupled with valves that let me fill the cylinder with any desired mixture of CO2 and air. I wrote an automatic program that created a calibration curve in various proportions (100ppm CO2, 200ppm CO2, ..., up to 5000ppm) and collected the sensor value.
The results of this procedure made no sense, because the sensor reading collected during the calibration, e.g. at 1000ppm, was totally different from the sensor reading in response to a 1000ppm concentration created outside the calibration loop. After several days of investigation, it turned out that the problem was that I was using tubes of some carbon-based plastic material. Somehow the CO2 mixes with the plastic and is slowly released afterwards, altering the mixture. Everything worked fine after I replaced the tubes with silicon-based silicone tubes.
Human exhaust breath contains about 5000ppm CO2, so this device is decent for measuring humans. It's less decent to measure atmospheric CO2.
Edit: looking at the datasheet, the device claims +-30ppm and 3% of reading, which I interpret as "whichever is greater". Thus, the device would be +-30ppm up to 1000ppm, and 3% of the reading above 1000ppm.
It's good to have reference instruments like chromatographs and infrared analyzers, where I could use reference gases to calibrate and verify.
OTOH very low-cost sensors could be ideally suited if well designed with accompanying electronics, but ultimate calibration would require the same expensive laboratory materials for reference.
Where you live, time of day, lay of land, all matters.
Winter (no trees or vegetation with photosynthesis), and there is naturally more local CO2.
Live deep in the country, in a forest, in the summer, when trees have loads of water and are at max output? Less CO2.
At night, more CO2, for all those trees, that greenery, is breathing and exhaling CO2, with no sun for photosynthesis.
It's variable, not static.
We have tried a number of different CO2 sensors over the years (mainly from Ali Express), and recently purchased 2 Aranet4’s. We were pleasantly surprised that they report identical readings in the same location.
Even seasonal fluctuation is just a few percent.
A vet friend theorised that because they have a completely disproportionate and crazy cardiopulmonary system (huge lungs and heart compared to overall volume) they are particularly sensitive to CO2 because they inhale more of it and it goes into their blood and brain much faster.
As soon as we open a window he calms - in winter I don’t take him out, I just ventilate the room.
Kinda makes me wonder why it is still allowed in animal slaughter. It is cheap and gets the job done, but there must be better gasses. There are loads that just makes humans pass out, and it must be the same for animals.
https://www.mayoclinic.org/diseases-conditions/carbon-monoxi...
Though symptoms are quite similar to those of CO2 poisoning:
https://www.emedicinehealth.com/wilderness_carbon_dioxide_to...
One of the symptoms of CO poisoning I've heard mentioned is of a sense of doom or dread.
I tend to think that people with CO2 sensors tend to over-focus on CO2, when really what they’re feeling is the general stuffiness of a room. CO2 isn’t the only thing that accumulates in the air in an enclosed space, but once you have a CO2 meter in hand you might think it’s the only thing that matters.
That said, CO2 levels can be used to somewhat estimate the amount of rebreathing going on in a space. That is, the amount of air that you breathe in that has been exhaled by someone else. It’s not entirely pleasant, which can explain why people feel like they don’t like high CO2 levels
My original goal was to have a fresh air intake activate automatically at certain concentration levels, if the outside ambient temperature/humidity were appropriate.
However, living in a hot humid place, the data I got made that solution seem futile. Ventilation for CO2 alone takes so much air that it becomes basically a full reset on dehumidifying and conditioning.
However, this failure lead me to the fortuitous discovery that by adding 36 hydroponic heads of lettuce, I am now going beyond outside CO2 levels, dipping down to 390-400 and only reaching ~700 overnight when it's the worst.
Comments like this are why I come to HN to discuss. I honestly want to try this now lol
That's a lot of plants. Possible, but your house will feel more like a rainforest.
Now I have a pretty realtime graph and a strong reasoning to air my room more often.
[1] https://www.tfa-dostmann.de/en/product/co2-monitor-airco2ntr...
Artwork slightly different, but appears to be same product: “Using NDIR CO2 Sensor Technologies; Technical Specifications: CO2 Measurement Range: 0-3,000 ppm; Response Time: < 2 Minutes; Accuracy: 0-2,000 ppm ± 100 ppm or ± 7% of reading; Sensor Life Expectancy: 15 years”.
We're switching houses next month and I'm seriously considering putting in an ERV. It's just a little tricky to explain to friends and family because it kind of makes you sound like a crazy person. But CO2 is measurable! And there is clear science about bad effects when it gets high!
(Not to mention the physics of fireplaces makes them counter-productive. They are effectively air pumps which suck in the warmest air inside your home and send it up the chimney. And that air has to be replaced from somewhere, most likely bitter cold outdoor air, through the many concealed cracks and crevices in your home.)
Ventilation and good hepa filtration will make a much bigger difference.
I've been this way for well over a year now. Almost two years ago I picked up an air monitor from Awair (yes, this was a pandemic related purchase because I was working so much from home) and I frequently checked what the CO2 was. Nowadays, as it starts hitting ~750ppm, without looking at the monitor I can tell it feels "stuffy" and I open a window.
I use a relatively small 10x11ft spare bedroom as my home office. If I close the door and window, it'll quickly climb above 1200ppm after 15-30 minutes (and set off an alarm on my sensor). It'll cross 1500ppm easily if left unchecked. HVAC helps but gets outpaced quickly if my apartment windows are all closed.
That said, keeping doors open, running HVAC normally, and cracking a small window open, even 1-2 inches and on the opposite side of my apartment, is enough to keep CO2 levels around 550ppm while sleeping and 700ppm (in occupied rooms) while awake.
A well-sealed, occupied room can build up significant CO2 levels pretty quickly. In my particular case, one person, not exercising, could bring CO2 above 800ppm (reported) in just a few hours.
So I think lack of ventilation causes way, way more excess CO2 than is caused by recent increases in atmospheric CO2. At least in my suburban neighborhood; maybe it's a lot worse in a dense city.
Not scrubbing in the way that a power plant does, but you can improve ventilation, but you probably need to do more than crack a window.
Options from cheapest to most expensive (in upfront cost):
1. Run your stove extractor fan (assuming it is externally vented), and open the window that is furthest from it in the house.
2. Open 2 windows and put a fan in 1 facing out, and if it's winter, wear a sweater.
3. Install a whole house ventilation fan (i.e. Panasonic Whisper Green fan) and keep the furthest away window slightly cracked.
All of the above will cost less upfront but will result in a significant increase in your heating/cooling energy use during hot/cold seasons. If you want to minimize the increase to your heating/cooling energy use, then you need to spend more upfront and:
4. Install a whole house <heat|energy>-recovery-ventilation system. This system brings in fresh air 24/7 while transferring much of the heat/"cold" from the conditioned space to the fresh air from outdoors.
What all of these solutions share is that they don't rely on wind/convection to ventilate the house, they are all mechanical.
The most effective is cross ventilation. I no longer live in places without cross ventilation, it's too hard to air out quickly unless the windows are huge.
It's also good to have windows facing the direction of typical wind direction in whatever geography you're living in.
Or if the temperature is right, just open a window.
Curious what sensor the aranet4 uses.
Edit: just found this teardown - https://kaspars.net/blog/aranet4-teardown
I can take your figures and calculate the same for my conditions by rejiggering the input assumptions instead of buying a monitor.
Apparently its sneaky and cumulative; a few moments wont do much but as time goes on you get fizzier.
While I'm fairly certain you meant "fuzzier" I can't help but thinking about bubbly blood full of dissolved CO2.
It also measures PM10 and PM2,5 which helped me show that my wood furnace is indeed leaking something sometimes when I noticed it smelling weird.
Besides, as a society, we have access to orders of magnitude more information than ever before and life expectancy doesn’t seem to be suffering for it.
