Keeping aerosol concentration low via a CO2 monitor

Hi everybody,

I’m fairly new to arduino (I have actually never done anything with Arduino so far), so I apologize in advance :slight_smile:

Discussing if it’s possible to keep class rooms and lecture halls safe in terms of keeping the probability for a corona-infection low, we came up with the idea to measure the CO2 concentration as this is proportional to the aerosol concentration in exhaled air (and aerosols are currently discussed to be responsible for further infections).

So I started with my first Arduino project ever (not even a single LED so far…) and came up with something that is working (see the picture). I uploaded everything to github, including a brief documentation and the program code.

Any comments, feedback or thoughts would be greatly appreciated.

Thanks!

I came across your fascinating project to infer air quality / aerosol concentration
from CO2. I am planning to use your code to make indoor air-quality gizmos
for compact/tiny houses - which often have air quality issues due to inadequate
ventilation.
Have you done any verification on the actual correlation of aerosols to CO2 as measured
by your device ?
Is the "assumption" that the rate at which aerosols drop to the floor about the same
as the diffusion rate of CO2 out of the enclosed space ?

While poking around on the sparkfun site to check out the SCD30,
I also came across their SPS30 particulate matter sensor and thought
it might make a good complement to your project

Of course this presupposes you are not burning fuel of any sort in the area. :grinning:

You need to look at the figures for what the CO2 concentration would be - you could do some rough calculations assuming number of people, breathing rates, room ventilation rates etc.
There might be some information around on what others have done too .

For example :

I would do this first to see if it can be sensibly measured .

I remember that in a school I was working in they have CO2 monitors all over the place. I remember that the monitors often showed concentrations in excess of 2000ppm. How this fits in with what you are doing I do not know.

My guess would be that CO2 is not a good proxy for aerosol levels because CO2 is a gas and mixes with the air and aerosols are not, by definition, a gas and settle out of the air fairly quickly.

PerryBebbington:
CO2 is a gas and mixes with the air and aerosols are not, by definition, a gas and settle out of the air fairly quickly.

And the concern is that aerosols do not necessarily settle quickly - depending on droplet size.

It is possible but unusual to monitor CO2 (carbon dioxide, the gas in soda drinks). Were these sensors actually for CO (carbon monoxide, the deadly gas used in some suicides) ?

vaj4088:
It is possible but unusual to monitor CO2 (carbon dioxide, the gas in soda drinks). Were these sensors actually for CO (carbon monoxide, the deadly gas used in some suicides) ?

No, it appears that the intent really is to measure CO2, with a view to using it to measure how well ventilated a space is and therefore how safe it is with relation to COVID. Whether that would actually work, I’m not at all certain.

PerryBebbington:
I remember that in a school I was working in they have CO2 monitors all over the place. I remember that the monitors often showed concentrations in excess of 2000ppm.

I was not questioning the project. My question (CO versus CO2) was about this quote.

I was not questioning the project. My question (CO versus CO2) was about this quote.

I am interested in the rise of CO2 in the atmosphere because of the damage it is doing to the climate, for this reason I noticed them. I also have a reasonably understanding on basic chemistry, I know the difference between CO and CO2. I am sure they are CO2 monitors.

Thanks !

Hi everybody, sorry for my very late reply, but I thought I would get a notifier if somebody replies to my post but that was not the case.

To answer one of the concerns mentioned here: there are actually by now a few examples where CO2 concentration was monitored to control infection rates (assuming that CO2 and aerosols are somewhat correlated) and the higher the CO2 level, the higher the infection rate (there is a very strong example of a tuberculosis outbreak only a few years ago, see my documentation mentioned above).

So the general conclusion is that the CO2 concentration is a good proxy for the aerosol concentration as long, of course, as you don't have other strong sources of CO2 in the room (open fires (e.g. candles), lot's of sparkling water).

And it is indeed the CO2 concentration, to address another concern, what the sensor I am using is measuring - in contrast to some cheaper sensors which measure the VOC concentration (volatile organic compounds) and then somehow calculate an eCO2 concentration - CO2 equivalent concentration. Comparing the eCO2 concentration from the cheaper sensors with the actual CO2 concentration one does see quite some differences.

What happens to your conclusions when someone turns on a fan..?

bluejets:
What happens to your conclusions when someone turns on a fan…?

Well, a fan might be a problem for the sensor: if you place it directly in-front of it, it gets wrong values (it is not meant to be operated in flowing air).

Apart from that, a fan just takes care that your potentially infectious aerosols are nicely spread across the room. It does not alter the CO2 concentration, whose pressure is very quickly “spread” across the room without a fan due to diffusion. That is the nice thing: if the CO2 concentration is high, you know that a lot of people have exhaled a lot of air, thus there are also a lot of aerosols and you should ventilate.

Too many if's or but's....someone decides the air con should be switched to outside air....flop goes your measurements without any indication.

bluejets:
someone decides the air con should be switched to outside air....flop goes your measurements without any indication.

sorry no native speaker here, what does con should be switched to outside air mean...? And is flop goes your measurement some kind of slang/urban language for saying the measurements are wrong ?

bluejets:
Too many if’s or but’s…someone decides the air con should be switched to outside air…flop goes your measurements without any indication.

I am a native American English speaker and I had some difficulty with this. My understanding of it is this:

There are too many problems with doing this type of measurement. For example, if someone turns on an air conditioner that brings in air from the outside, the measurement of CO2 will decrease and fool you into believing that the air is cleaner than it is, and you will not know that the air conditioner was turned on.

I am not saying whether or not I agree with this, I am just explaining what I think was intended. I think that the explanation would be best if it came from bluejets.

Ahh, thanks a lot for the explanation!

We rarely use air conditioning in my country and definitely not in schools and since this project was motivated by the situation in schools that will not be an issue here.

Let me, however, try to answer the concerns raised by @bluejets: there are actually some (super) spreader events linked to air conditioning without fresh air, where the air (and the potentially infectious aerosols) are just "nicely" (sorry for being sarcastic) moved around the whole room [1,2].

Air conditioning system with fresh air, however, helps to dilute both, the CO2 concentration and the potential infectious aerosols, thereby reducing the spread of the virus [3].

The CO2-monitor, as suggested by me, would cover both cases: in the case without fresh air, no CO2 is removed from the air, aerosol concentration is also growing over time; in the case with fresh air, the CO2 concentration is kept at a low level, same is then true for the aerosol concentration (due to the constant influx of fresh air).

[1] COVID-19 Outbreak Associated with Air Conditioning in Restaurant, Guangzhou, China, 2020 - Volume 26, Number 7—July 2020 - Emerging Infectious Diseases journal - CDC
[2] Abattoir's cooling system helped spread virus: German expert
[3] https://www.ecdc.europa.eu/sites/default/files/documents/Heating-ventilation-air-conditioning-systems-in-the-context-of-COVID-19-first-update.pdf