Not sure what people are interested in CO2 sensors for, but I use them as a proxy for ventilation in indoor spaces, which is then a proxy for covid risk.
It won't tell you if the air is being filtered (need MERV13 or better), but a lot of places for efficiency and/or due to old HVAC equipment don't bring in fresh air, which increases the risk in a more confined and/or crowded space.
Also, some places like airplanes, which have HEPA filters and have 10-20 air changes per hour (ACH), don't run those systems at full power or bring in outside air at fast enough rate until airborne, so PPM can't build up to 2000 ppm vs the ~420 ambient outdoor CO2 level and approx 800 "good" indoor air rate.
I use an Aranet4 device, which uses 2 AA batteries that last like a year.
As you said it won’t tell you if the air is being filtered. I think you’re better off measuring the small particle content of air if your concern is viruses, or both.
Devices like the Aranet4 are mainly useful for telling you how well ventilated a space is relative to how many people are breathing into it, which is a very good proxy for viral load and viral risk. It's pretty much only in spaces with high quality air filtration (aka high CADR for cleaning an enclosed environment) or significant far-UVC protective measures that the proxy relationship breaks down, and the breakdown will always be an overestimation of risk from the CO2 monitor, never an underestimation. For those reasons it's a very useful tool to have to put an upper bound on the risk of the space you're in and help you make informed decisions about whether you can e.g. hold your breath and take a drink under far-UVC cover or not.
PM2.5 sensors or other things like what you're thinking of do have their place, but they also have a bunch of other issues - they'll be detecting VOCs, brake and tire dust that's fine enough, any aerosol whether it's coming out of a mammal or not. I think it's inaccurate to say they're just a generically better solution than CO2 monitors for viral risk.
Exactly. CO2 is an imperfect proxy, but it's a pretty decent one. And, even for spaces that have high quality air filtration (e.g. a strong commercial central air system with good HEPA/MERV filters), the CO2 measurement can also tell you if the system is configured with a good recirculated to outdoor air ratio (residential systems usually have no such thing, unfortunately).
But yeah, as you said, the best aspect of a CO2 monitor is that it gives you a useful upper bound for the general respiratory virus risk in a space.
I have an SCD30 which is a dual-channel NDIR sensor and am pretty happy with it. Once a year I’ll take it outside to recalibrate it and it’s never drifted more than a few ppm. I make sure to turn off the auto-calibration and manually calibrate it because auto-calibration can lead to weird behavior if you don’t know what it’s doing (it re-baselines the lowest observed ppm in a time window as 400ppm).
FWIW, Sensirion's AEs told me they have empirical data showing that CO2 drops to outdoor levels at least once a week even in closed environments. Based on that they recommend using the auto calibration.
Anecdotally, I see my CO2 levels hover around 600ppm inside even over night.
I think that really depends on the environment it’s deployed in. I used to live in a poorly ventilated former warehouse apartment with a mini split for HVAC and at night my levels were around 700. I live in a much better ventilated apartment now and it’s made a noticeable difference in QoL.
700 is great, I see spikes up to 5000 in one of our rooms.
levels in the range 400-1000 are considered good quality
700 is pretty good, what are you on about?
If your sensor recalibratws to a baseline of 400 each day and you have a baseline of 700 that is a drift of 300 ppm per day or ca 100 000 ppm per year.
You see why this is a problem?
There are many cases where the level does not drop to ambient levels, e.g. offices in basements, low-energy houses etc.
We even saw schools where the classrooms were not able to drop to ambient levels over the whole unoccupied weekend because the ppm levels on Friday afternoon were beyond 3000ppm.
The automatic baseline calibration period of some of the NDIR CO2 sensors can actually be adjusted through the software. I believe the Aranet allows that and also we do have that feature on the AirGradient dashboard. So you can switch it to a much longer period or switch it off completely.
It always comes down to how well you can ventilate. If you can open windows / doors to different sides of a house / building there is usually some natural air flow, and the stronger it is, the faster it will bring CO2 levels down. In the apartment here it can be less than 5 minutes to have CO2 levels to ambient levels, but it also takes longer if the air is "standing" even outside. For that you will need active ventilation then, clearly, same for having windows to the same side of the building only etc.
There is also another thing: In multi-tenant buildings you may have others airing out their apartments which means you will not get ambient levels, but elevated ones due to added CO2 from the other apartment(s). For example, I see >500 ppm CO2 then. Also, even the ambient levels are not always the same; I have seen values between 416 and 432 ppm depending on the day, wind, and other factors (living in a larger city).
I strongly recommend the Aranet4 Home for personal/home use. Very accurate, very long-lasting, tells you other things like humidity etc, and only 1-2 hundred dollars when we bought it (may be more expensive now, been a couple years of inflation).
It's crazy how optimized the Aranet 4 Home is. It runs on AA batteries for a whole year, no need to plug it in.
That's unique in the space, and a case where e-ink makes a real difference. And very convenient in practice.
I bought one a year ago for 200 bucks. I'm still happy with the purchase, and I haven't had to change the batteries yet.
I chose the AirGradient[0] One. It's slightly more expensive than the aranet4, but has more sensors. It's open source, reports CO2, PM10, PM2.5, temperature, humidity, and more I'm probably forgetting offhand. Loaded it with the esphome firmware, so Home assistant integration is seamless.
And since it's open source, when a sensor inevitably wears out in a couple years (like the PMS5003), I can just replace the sensor instead of of buying a whole new unit.
Highly recommend it.
[0] https://www.airgradient.com/
Edit: I completely missed that AirGradient blog article
That sounds fantastic, I really love open source stuff.
Nitpicky but isn’t R correlation and R^2 coefficient of determination?
I’ve never heard of any of these. What’s going on in low cost oxygen sensors? What should I google? I can only find $100+ galvanic sensors that need to be replaced after a few dozen months
Do you really? You'll notice the effects of too much CO2 indoors long before a lack of O2.
As mentioned in TFA, these low-cost CO2 sensors aren't really exact, some don't even do well outside. Just look at the comparison with the reference CO2 sensor.
As I understand it processes that need O2 measurements require it to be exact and reliable, e.g. for industrial processes, medical purposes, or even scuba diving where it's life or death.
This. There are plenty of co2 but what I really need is to measure o2 level (plus co presence).
I've used both the SenseAir S8 and Sensirion SCD41 sensors, and the SenseAir is clearly my favorite.
The SCD41 seems to require a lot more manual calibration while the S8 is very stable - maybe once every 3 months or so, I'll see it reporting ~380 ppm in fresh air (which is actually within spec).
The venerable MH-Z19 is probably my second favorite sensor, ahead of the SCD40-series.
The MH-Z19 is similar from the performance of the S8 but we do see more noise in the readings and the MH-Z19 also seem to have less life time. We see some of them dying after around 5 years whereas the S8 has a ten year warranty and so far have been rock solid for us.
I see all these three (SD40/41, MH-Z19 and S8) on Aliexpress, but in yesterday's discussion on the sibling post it was said that the SD40 is likely a fake since Switzerland based Sensirion do not distribute there. Do you have a blog about sourcing these sensors and those needed for other air measurements? The cheaper options on AliExpress look attractive for someone willing to try a small hobby project but if there's a risk of getting malfunctioning fakes that option may need to be scratched. Thank you
That is good confirmation. I bought a MH-Z19 as a pandemic project so I might be approaching that timeframe.
What do you use these for? for indoor home use I cannot see the point. We have plenty of biochemical sensors in our body telling you when it's time to ventilate the room...
I mean, I can understand CO, maybe Radon, but CO2 really seems pointless and only driven by the modern green trends.
Higher levels of CO2 impair cognitive function [1].
In fact there's no floor - even small increases in CO2 have small but measurable effects.
[1] https://www.hsph.harvard.edu/healthybuildings/2021/09/09/imp...
Sure, point is we as humans are already equiped with efficient ways to detect it. Only home use I can imagine is to drive some ventilation automation.
Welp, I just ordered $1500 worth of the Cisco ones, so I hope they are the right type.
Is there a consumer model that you would recommend that one can use at home?
Aiegradient is great and they make indoor and outdoor monitors. Fully open source. They are the authors of this article and to a lot of research and advocacy which is great.
The cheapest one that is not eCO2 (estimated) will do the job of telling you when you’ve not ventilated enough just fine. No need to spend hundreds to thousands on lab-grade tech.
+- 50ppm, or even 100 or 200 is nothing. You will mainly care if the ppm is around 500 or around 1000-2000 (depending on preference).
The devices that have alarms are quite nice. Leave them on a desk and they’ll tell you when to open the window. The ergonomics of how you’ll use it are maybe more important than accuracy when it’s <200ppm off.
For NDIR sensors, the Aranet4 Home is an often recommended one. I have a couple, and they are great little units. A number of scientists recommended them during the heart of covid as a proxy for covid risk in indoor spaces (obviously, an imperfect proxy, but since there are no cheap, portable devices to measure viral concentrations in the air, you work with what you've got).
When I looked a few years ago the Await element was best consumer grade one on the market at a good price. I’ve been happy with it so far.
I use the Airthings (the $300 one). It detects more than just CO2, has an e-ink display, and has a decent app.
I like my qingping - works with both - homekit and home assistant.
a lot of these seem to be making the front page lately, i assume these are pointless if you regularly keep the windows open and air out the house?
In a small enough unventilated room you can fill it with enough co2 to make it unpleasant to be in with an hour. E.g. a study in the usual suburban house.
Yep. Even if the building as a whole is generally well ventilated, particular spaces in a building can still build up CO2 pretty quickly. It's actually quite dramatic to see the rise in chart form at times.
These sensors are really nice to help validate ventilation anywhere.
Yeah, location matters a ton. I have 2 Awair Elements and we ran them in different places until we knew what "normal" looked like, then move them on.
Because of what I saw in my house we switched from a gas stove/oven to electric induction, installed an air exchanger and set our central air/heat to recirculate more often (only important for us during spring and fall when we aren't heating or cooling much).
I don't like noise when sleeping, so I liked the bedroom door shut. CO2 gets way too high, so I switched to using an earplug and leaving the door open more.
It is so nice to have the data, make informed choices and see measurable results.
Yep, I also run central air in fan only mode much more often now. Since I'm in coastal California, I don't need to run heating/cooling for much of the year, so previously that meant central air was rarely on.
Now, I'll run it fairly often for the recirculation. And, since my house is old and leaky, aside from evening out levels, it no doubt results in more indoor:outdoor air exchange too. That alone made a massive difference at night, since I will usually have the bedroom door closed.
And, while I haven't switched the range over to induction yet (it's on the list to eventually do), I did get a portable induction burner that I use fairly often instead of the gas range. Induction is pretty amazing, so I look forward to eventually going all the way.
Let's be honest - the AirGradiant people/person are submitting these.
Which is fine - they seem like a good company with a strong open source commitment, but they do go a little overboard on HN submissions.
Achim, founder of AirGradient here. Just want to clarify that this submit was not done by me or our team.
Maybe not this one but look at your other submissions :)
I don't see how it is a repost. That submission was to an article that talks about a new model of photo-acoustic sensor from Infineon.
This submission is to an article that compares photo-acoustic sensors to NDIR sensors. Its only connection to the earlier submission is that the author of the comparison participated in the discussion of about the new Infineon sensor, updated the comparison article, and linked to it in one of their comments.
It's one of the fun things of internet forums in how you can see where someone gets nerd sniped but is able to bring back something from their trip down the rabbit hole.