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Comment by bizzleDawg | Hacker News Reader

I've worked on environmental sensing iot projects and have seen the cost of co2 sensors. Almost all the cheap ones are VOC sensors which infer a co2 level by assuming the VOC output is from humans (rather than paint for example).

Given the cost of the device I would be amazed if it wasn't a VOC based sensor. The cheapest CO2 sensors I've seen are well over $250, even in bulk. CO2 is quite innert, so it requires very precise and accurate components

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Looking at their materials[1], it's IR based and needs calibration. I'm sure it's output is interesting, but likely not super accurate given their started assumptions for the calibration process. Better sensors may be required before drawing any conclusions!

[1]: https://www.netatmo.com/en-gb/helpcenter/air-care/4/installa...

i’ve noticed this casually as well. i bought an airthings wave plus [0] a few months ago and the co2 and total voc readings seem pretty (but not perfectly) correlated.

i live in a drafty pre-war building and my co2 peaks at around 1500-1600 ppm at night with doors and windows closed. it troughs at around 500 during the day when i have the doors and windows open (~400 is ambient from what i understand). tvocs typically peak around 600 ppb and trough at around 20 (under 100 is considered safe iirc), with the same cycle as co2.

[0] https://airthings.com/us/wave-plus/

413ppm is now ambient. Goes up higher every year (with a small amount of seasonal variation).

By 2100, we may be at 800ppm if we continue and/or accelerate fossil burning (4 degrees C trajectory). https://www.co2.earth/2100-projections

Already a middling stuffy room, and would increase indoor levels 400ppm over today.

I wonder at what point will people have active systems to lower their CO2 below ambient in their homes.

You mean plants?

You need an enormous, full sun greenhouse just for one person’s CO2:

“An active person exhales about 1 kg of CO2 per day. So, you would need 1000 square meters of grass to absorb one person's CO2. That's a square about 32 meters on a side, or in archaic units, a quarter acre. A one-square-kilometer park could support 1000 people, just with the grass.“

(Or: Need about 16 trees per person.)

http://highfrontier.com/forum/showthread.php?tid=20

ha, i actually bought a bunch of plants from the nasa list a year-ish ago to help reduce co2 & voc levels: https://www.healthyandnaturalworld.com/best-air-filtering-ho...

Did you notice a difference? We have a small office with two people, multiple PCs, printer, etc. It gets warm and stuffy regardless of the season and climate control, and I have trouble focusing especially in the afternoon.

I'm thinking of picking up a plant or two and pointing a fan at the door to send out some of the stale air.

To get a better filtration effect you need to force air into the root zone through an active carbon layer in the soil media. That is the method used in the original nasa paper and subsequent public labs designs when trying to remove VOC's such as those emitted by printers. Active carbon adsorbs the VOC's so that the root microbes can metabolize it. Since it is a microbial filter, it becomes more effective at filtering the contaminants that it's most exposed to. Active carbon can also remove CO2, but must be recharged with lower CO2 air periodically.

The public labs design uses a aquarium bubbler to push the air, but the flows are pretty low - with retention time in my 10x12 office at 1 month. Since the VOC's are usually heavier than air, if you place the pump inlet on the floor it performs better though.

i felt a little better but honestly i think it was more placebo than anything. i didn’t have the IAQ monitor before getting the plants so i can’t know for sure what the air quality effects were.

i got 6 plants (but 1 died) and they make my living space feel more lush and comfortable, which i count as a win.

i did also eventually get a air purifier (blueair 211+) and that’s probably the main workhorse at night with the doors and windows closed (which is when i need it the most). i noticed a clear drop in allergy/asthma symptoms (sneezing, coughing, headaches sometimes) after getting the air purifier.

so a fan at the door will probably have the best bang for the buck for you, but plants are nice too. =)

Can you recommend the lowest cost CO2 sensor that one can buy that would give reliable readings?

The "TFA Dostmann AirCO2ntrol" devices are popular in Germany. Searching for "co2 meter" on Amazon.com one of the top results is an identical looking device.

It measures with an infrared spectroscopy-based sensor, which should be quite accurate. There are a few good software packages for recording data, e.g. https://github.com/vfilimonov/co2meter.

Repost from my comment above, but for a couple hundred bucks you can get an accurate CO2 sensor that reads out via USB-serial to a computer. I use them at work and we've shown they are quite good over the long term. Plus to recalibrate, you can just take them outside. https://www.co2meter.com/products/k-30-co2-sensor-module

MH-Z14

I have developed custom Ethylene sensors in the past for customers, and Dynament (UK) is a manufacturer that I can definitely recommend if anyone wants to do this and go down the DIY electronics route without having to dig through a bunch of Chinese products to find the "right one". https://www.dynament.com/products/gas-sensors/standard-non-e...

Cool! I worked at an ethylene-sensing IoT startup (fruit ripeness natch). What was the core sensor you used? (We were using off-the-shelf, but somewhat pricey, electrochemical sensors)

"MSH-P/HC/NC/5/V/P/0 - 3% Volume Ethylene = 0.4V - 2.4V"

We built that into a custom SS body with a Delrin mounting insert, and ran the serial connections to a custom board that interfaced with up to 8x sensor locations (in conjunction with some other sensor elements). The elements were ~US$200/ea ~4.5 years ago.

These are fancy interpolated IR absorption sensors internally, so any gases of similar concentrations could throw off the readings; depends on the environment. Would be BEST to calibrate to local atmosphere conditions of the installation, but that's not really something you can sell. :P

I remember looking into this years ago, and the prices were in the $100-$200 range for low volumes. Was this your experience?

Yes

I got an NDIR CO2 sensor on AliExpress for $20. Do you think one of those would just have low accuracy or be uncalibrated?

I suspect both! I should compare a $250 one with the $20 one

That would be a worthy HN post.

Is that something HN is interested in? I do work and research with remote environmental monitoring stations and am always comparing sensors. Currently doing a precipitation gauge intercomparison study.

I can only speak for myself and what I've seen discussed here over the years -- yes, it would be interesting. Why is better stuff is more expensive, how to make the cheap stuff work closer to the better stuff, and interesting findings from actually using them. Lessons learned are always interesting...

Please do! I just bought a couple of cheap co2 sensors on Amazon (2k positive reviews) only to find out that they may not be accurate.

It would be amazing if you or anyone really could make a comparison between different models - and show whether the cheap ones on amazon and the likes are worth it.

Very interesting.

Just test/calibrate them outside (400-ish ppm). Even if absolute measurements are bogus, they should do just fine to figure out whether it’s time to open the window..

Would be interested to find out because if its just uncalibrated plotting out the relative differences it still would be useful (e.g. seeing a X% increase on whatever scale the sensor using at night versus day could provide enough of an indicator that something is going on and more investigations are warranted.)

Also if someone is searching for PM2.5 particle monitoring:

https://smartairfilters.com/en/blog/how-accurate-are-common-...

Thanks. I was about to get a cheap one on Amazon :D

Sort of related, I'm living in a newly constructed house and I'm interested to know the indoor air quality. Any recommendation for a comprehensive in door air quality test solution (either device(s) or hire someone maybe)?

MH-Z14 or Z19 are cheaper, around 25€. With optical measurement. I have a few of them, they seem accurate enough. I calibrate them every 6 months by placing them outdoors.

Are there any brands or products you would reccomend for measuring CO2 in my house? Ideally with some way to hook it up to my computer.

It looks like things have moved on quickly since the other year when I was more involved in this! This company does some good products, some of which are fairly easy to interface: https://www.gassensing.co.uk/products/

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