Training my sense of CO2 ppm | Hacker News Reader

Training my sense of CO2 ppm

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121 pointstruxs89 comments

I think the elephant in the room is calibration of the CO2 detector.

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.

I own an Awair unit, and as far as I can tell it attempts to calibrate itself based on outside atmosphere having 400 ppm... but that value is constantly going up and is now like 415, so I figure my unit must be at least a few percent off.

I may be misunderstanding, but if you claim outside is 415 and that's that, it's wildly inaccurate.

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.

Just how "wildly" inaccurate are we talking about? What should the outside CO2 readings actually look like under those conditions?

As an example, close to a busy road, you may as well double the number in parent. Calibration by outside air is strange.

Close to a busy road, sure.

The Aranet 4 claims it can measure up to 5000 ppm with +-3% accuracy. But you'd have to take their word for it, I guess. For $250 I'm hoping it's not a crap sensor.

The usual convention is that the accuracy refers to full-scale measurements. I.e., your device has an error of +-3%*5000ppm = +-150ppm. At ~400ppm you are about 37% off.

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.

I interpret the error as a sum, i.e. at 500 ppm, an error of up to 45 ppm (30+0.03*500).

If I understand correctly, they’re calibrated at the factory, which is probably in China, which might have vastly differing CO2 levels depending on whether the winds blowing up or downstream of any heavy industry or metropolitan centre?

Not an expert, but I looked into this a bit a while back to see if one could reasonably tell emissions via differences in atmospheric CO2 concentration, iirc the conclusion was that CO2 concentrations don’t vary much outside, air mixes pretty well, pretty quickly.

Even seasonal fluctuation is just a few percent.

I don't think this is what is meant by "calibration".

You can calibrate the Aranet4 at any time by putting it outside, and it will set that as 420ppm. Obviously the ambient CO2 levels are raising slightly each year, so this is not entirely perfect.

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.

Well, if you take it outside, and it says a bit over 400ppm, it’s at least roughly right at that point on its sensor’s curve.

If you don't care about errors of a few percent you should be able to make your own calibration chamber.

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 actually went down this path about 25 years ago, and I learned the hard way that CO2 hides in organic materials in a way that mostly invalidates the calibration.

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.

This is not easy at all. Your source of CO2 if it is a bottle will have a pressure. Your bicarb should be dry to make sure you weight it properly. If you want to measure ratios of 100 above 100ppm the home made way is decent, but for atmospheric/domestic variations that's not going to cut it. Plus as the other commenter said, CO2 creeps into plastics and leach slowly after so you have to let your system equilibrate which then opens you at the effects of porosity and diffusion.

The auto-calibration systems on cheaper consumer sensors also cause issues if they rarely see air that has low CO2 levels. Because these sensors can't measure absolute CO2 levels, only relative CO2 levels, they provide an absolute figure by looking for the lowest CO2 concentration they've seen over a period of time, usually around a 72 hour rolling window.

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.

> 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...

That's a shame, they've worked well for me. Unfortunately I can't recommend any others without doing some research, I found quite a few dual-NDIR sensor modules when I was looking and chose this one primarily based on availability and a reasonable datasheet.

A sibling post[1] mentioned a sensor that is listed as being dual-NDIR so should give reliable readings, and has a USB interface, so that sounds like one possibility.

[1] https://news.ycombinator.com/item?id=32122301

Manual for this unit is at https://cdn.shopify.com/s/files/1/0019/5952/files/Manual-RAD...

I assume 'dual-beam' is the same as dual-channel? Does the calibration technique mentioned in the manual indicate dual channel operation?

Auto calibration can be disabled, popular sensors that can do this are MH-Z19 and MH-Z14 (also cheap ones).

This is true, but I don't think any consumer-grade sensor systems offer this as an option? I guess it might be useful if you're building your own sensor, and can't afford/justify a dual-NDIR sensor.

My understanding is that with single-NDIR sensors (like the MH-Z14 and 19), the auto-calibration is intended to overcome gradual particle buildup and beam degradation in the sensor chamber. While disabling it will prevent the scenario I described, you'll instead end up with gradual sensor shift as the sensor ages. I guess this could be minimised by manually calibrating the sensor outdoors on a regular basis.

Dual-NDIR sensors split a single beam into two chambers, so any degradation of the sensor beam affects both measurements, and the particle buildup in both chambers should also be approximately equal over time, so they should remain accurate over an extended period without any requirement for calibration. I built mine about 4 years ago and I do occasionally check to make sure they read ~400ppm when placed outdoors, last check was around 420ppm which suggests they're behaving reasonably well as they age.

I have a few CO2 meters from very different sources (professional exetech, consumer product, electronics component) and they have always reported levels within 10% of each other.

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.

>pricing for those labs was prohibitively expensive.

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.

I use the same CO2 meter as the author has showcased. It's amongst the most accurate ones on the market at the moment (home grade). It uses a The Non-dispersive infrared sensor (NDIR) which is much more accurate than most sensors on devices sold online for $100-200. In fact, it actually measures instead of "estimates" the level of CO2 (as most cheap ones do).

Agreed!! I have also tried many different CO2 meters (all consumer-grade) and Aranet has been consistently the most accurate among the CO2 meters I have used.

How did you determine the accuracy of your CO2 meters?

I measured the CO2 levels in one room using several different CO2 monitors - under similar conditions (steady-state with one person in the room, windows partially open) on different days; Aranet readings were the most stable and consistent, while matching the "expected ppm".

I also tested the monitors by exhaling directly on them, and confirming the change in readings.

I have a fantastic way that I’ve trained my CO2 sense: an Italian greyhound. Anything above 900ppm and he barks and gets quite frantic above 1100ppm. I’m now able to anticipate the barking and normally realise at about 850ppm that we need to go for a walk and open the windows.

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.

On a related note: A friend of mine was exposed to high co2 levels while on a freight ship. He says it is the absolute worst feeling he has ever felt, with panic and convulsions and vomiting. He survived because a colleague of his managed to get him out.

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.

That sounds like it might have been carbon monoxide poisoning.

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.

Nope, that was co2. It is similar to CO, but in animals at least it gives a more aversive reaction.

CO2 concentrations are an ongoing topic of debate. There are studies that show no measurable cognitive changes up to CO2 concentrations in the thousands of PPM, while other studies seem to show cognitive changes at much lower amounts.

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

I wondered this too. Like yeah you’re going to be cognitively drained after being in a crowded closed meeting room for a long time. Is it because you were talking and working or because of the CO2.

If the sense lifts immediately (within seconds to a minute or so) after opening a door or window, I'd tend to suspect the atmosphere.

Good old placebo seems just as likely.

I've been using the uHoo air monitor for the last few months. I have one in the bedroom, living room, and office. It monitors many air factors, but CO2 was my primary focus.

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.

That I'd very interesting. Is ther data on how much C02 is consumed by lettece?

You need to approximately grow as many kg of plants each day as you eat food each day.

That's a lot of plants. Possible, but your house will feel more like a rainforest.

I run 2 dehumidifiers on top of air conditioning and stay under 60% (almost always under 54%).

Also the hydroponics water the roots of the plants in a closed system so very little of the moisture is evaporated.

Also I'm not sure about your calculation since poop. I would imagine it would be kg consumed - 0.45*kg pooped - house leakiness loss

From my calculations I was only expecting about an 80ppm difference, but in practice overnight when the humans and animals are in bedrooms, the living room with plants shoots down to 400 while before it would get about 550 in my leaky old house.

You need controlled ventilation with heat recovery to have both good air quality and temperature/humidity.

>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

Comments like this are why I come to HN to discuss. I honestly want to try this now lol

I've bought [1] and coupled it with Zabbix using [2].

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...

[2] https://codeberg.org/pf-monitoring/airco2ntrold

Not sure if they still make them with speakers that sound like life support monitors in the hospital, but if the beeping is too annoying you can just open the back and break off the speaker with pliers.

Well, so far I haven't heard any beeps.

Amazon link: https://www.amazon.com/CO2Meter-RAD-0301-Mini-Monitor-White/...

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”.

I'm not convinced this is truly the same product. A highly-rated review and some questions seem to say it does auto-zeroing and assumes the lowest concentration seen over some period of time is "outside", which as I understand it wouldn't be necessary if it were dual beam NDIR?

Does it allow you to read data via USB or is that just the power supply?

It does allow reading data via USB.

I've had an Awair monitor for a few years and enjoy keeping a fairly close eye on the numbers, but from what I've experienced so far I'm convinced sure I'd be able to train myself to discern CO2 levels. If anything it has mostly just encouraged me to open the windows, even just for a bit, whenever possible, and to always use the kitchen exhaust fan with our gas stove (and motivated me to buy an induction range in the future).

Same here. I was truly surprised at how fast the CO2 goes up with our gas stove on. The only thing worse is when we use our unvented gas logs in the fireplace.

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!

PM2.5 is also measurable and is arguably the greater long-term concern to human health with respect to indoor combustion.

(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.)

Yeah, I want to put in an ERV, too, to have the sense of fresh air all year round.

Especially if you do things like stirfry or anything which puts char on your food, particulate from the cooking food will be way more than from the cooking gas so induction won’t make a large difference.

Ventilation and good hepa filtration will make a much bigger difference.

> I’m looking forward to the day when I can walk into a room and say, huh, feels like 800 in here...

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.

Is there any kind of CO2 scrubbing that's at all practical for home? I'm curious what the effects would be of going below ambient/outdoor levels, especially since those are rising (if slowly).

From some (personal) research I did several years ago, I think good ventilation is by far the biggest concern.

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.

My issue with that is that in the summer, ventilation is ~impossible. It's 110 degrees out there, reasonable ventilation would mean either burning out my A/C or living with temps that are far too high.

Interesting problem. Mind sharing roughly where that is?

Ah, yeah, Phoenix summers. I believe parts of Texas and some other places are somewhat similar.

In the winter the situation is _much_ better, but probably 4 months out of the year, the outdoors are unlivable.

Due to climate change, that's could be pretty much anywhere it's summer. E.g. check out the UK:

https://www.metoffice.gov.uk/about-us/press-office/news/weat...

Simplest solution is a high efficiency air to air heat exchanger to let you circulate a lot of outside air. They are less efficient when dealing with high humidity, but you can have this as part of your HVAC system and never really think about it.

https://en.wikipedia.org/wiki/Heat_recovery_ventilation

Interesting, I don't think I knew that was an option. I'll look into it, thanks.

I rent right now, which means I doubt I can, but hoping to buy soonish.

If you do this (install an HRV), expect it to cost many thousands, and make sure all your duct-work is running through very well insulated spaces, or you will obviate the heat recovery part, and you will just have a very expensive ventilator.

Yeah, I'd assume it'd have to wait until I'm going to replace an hvac system anyway. But in my price-range of homes, that's fairly likely to be ~required anyway.

Sounds like something that would be worth it, I spend almost all of my time indoors and we have pets pumping out co2 as well.

> But in my price-range of homes, that's fairly likely to be ~required anyway.

TTBOMK, there is nowhere in the US that an HRV or ERV is required by code [1], regardless of the price range. There are a lot of very expensive homes built with no thought to indoor air quality.

In many places in the US, due to higher air-tightness requirements, mechanical ventilation is required, but that can be as minimal as having bath fans and a kitchen extractor fan.

1. Ontario CA requires them on new builds https://www.huskyair.com/blog/newsflash-ontario-is-making-hr.... Maybe Germany also?

I had this problem living in Dallas: it'll hit 108°F just about every day this week.

I also couldn't spend thousands on an energy recovery ventilator (ERV) installation because I've been living in rental properties.

My solution was to buy an ERV for ~$500 and rig it up to an open window: https://gen.co/air

I've since moved into a condo with a bedroom with no windows and french glass doors so I installed a cat door and mounted the ERV vents to that.

I think there's a lot of money here for someone to build an ERV designed to be window mounted.

> Is there any kind of CO2 scrubbing that's at all practical for home?

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.

By far the simplest solution is to just crack a window. Even a modest amount of fresh air exchange is enough to offset most of the CO2 generated by the people inside.

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.

You wouldn’t scrub the CO2. You would use an Energy Recovery Ventilator or Heat Recovery Ventilator to supply fresh air without impacting air conditioning too much.

Or if the temperature is right, just open a window.

Indoor plants do practically nothing.

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.

I'm currently playing with 2 NDIR co2 sensors connected to a pi (scd30 and MH-Z14), graphing the data using grafana.

Curious what sensor the aranet4 uses.

Edit: just found this teardown - https://kaspars.net/blog/aranet4-teardown

I feel terrible above like 900/1000ppm but I’m sure it’s also a good proxy for whatever other garbage and voc’s are in the air. When my monitor reaches 800 I’ll throw my window exhaust fan on for a few hours to circulate the air in my home.

How many people, in what square metreage, over how many hours, does it take you to go from ambient levels to 800?

I can take your figures and calculate the same for my conditions by rejiggering the input assumptions instead of buying a monitor.

Me any my dog in a small 2 bedroom house.

people on supplemental oxygen sometimes get CO2 problems; the oxymeter shows good numbers but they're acting drunk or feeling poorly otherwise.

Apparently its sneaky and cumulative; a few moments wont do much but as time goes on you get fizzier.

> ...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.

I think thats pretty much the case.

Supplemental oxygen usually causes hypocapnia (low blood CO2). Adding a little CO2 prevents hypocapnia.

I have a qingping air monitor lite, which was around $70.

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.

I have a cheap (about $20-25 a couple years ago) sensor from Amazon in my plane. It calibrates itself when I turn it on and I check it works on the ground before every flight - open the window after starting the engine and see co2 going up ;) Granted, I don’t really care about any small errors or small changes - I want the sensors to warn me if co2 concentration goes up significantly (e.g. because of leaks from exhaust system).

I've never heard of that. Do you have a CO sensor as well? Those are common, mostly in a very cheap one-time-use dot form.

I should say "disposable" instead of "one time use". https://www.aircraftspruce.com/catalog/pspages/asaCO.php

Are you sure this isn’t a carbon monoxide (CO) sensor? While exhaust might increase CO2 as well, pilots are usually far more concerned about CO poisoning and usually use CO detectors.

I don't know much about LoRa yet, but i would be interested to tinker with it. Can the LoRa radio of the Aranet4 PRO be used by alternative base stations or is it somehow locked down to their own Aranet PRO base station?

We need a study on what will kill you sooner, the CO2 levels or the need to anxiously monitor whether CO2 levels are too high in every single enclosed space you enter.

Nothing wrong with a little anxiety if it serves as motivation towards a healthier life — whether that be diet, exercise, or a healthy environment.

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.

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