Are home CO₂ meters as reliable, accurate and precise as commercial ones?

Last modified 8 months

This question about the reliability, precision and accuracy of home CO2 metersI felt it was time to write a few lines about it because there is a lot to be said for it. little informed opinion on the internet and it can be difficult to know the truth among so much opinion.

Although the answer is not that simple, I will start by saying that yes, that home meters are just as reliable, accurate and precise as commercial meters. (of the range we are talking about), although there are always some nuances. In the following, I will explain why.


I have been wanting to publish this article for a long time, but have not quite dared to do so.

I will, of course, start by saying that this is just another opinion, my informed and sincere opinion.. I hope no one comes down on me ....

I don't have to be rightI have simply dedicated myself to writing my views on this issue.

I will not be held responsible if my views provoke new political conflict, world disorder or the zombie apocalypse.

I'm aware that I'm getting myself into a swampy terrainwhere there are important economic interests in a booming industry. I will accept any kind of criticism in the comments, as long as it is polite, although I will prefer the constructive ones.

First things first: What is the difference between reliability, precision and accuracy of a meter?

Reliability, precision and accuracy are three important concepts in the context of measurement and the assessment of the quality of data obtained through measuring instruments. Although they are often used interchangeably in everyday conversation, they have different meanings in the field of metrology and science. Here I explain the differences between them:

  1. Reliability:
  • Reliability refers to the consistency and stability of the results of a measurement over time and across repetitions of the same experiment.
  • A measurement is considered reliable when, when performing the same measurement under the same conditions, very similar or consistent results are obtained.
  • Reliability relates to the reproducibility of measurements and is important to ensure that an instrument or measurement method is consistent and reliable in its performance.
  1. Accuracy:
  • Accuracy refers to how close the measured values are to each other, i.e. how repeatable the measurements are.
  • An accurate measurement implies that the results are very close to each other, even if they are not close to the true or exact value.
  • Accuracy is measured by the standard deviation or dispersion of the results of repeated measurements.
  1. Accuracy:
  • Accuracy refers to how close a measured value is to the true or known value.
  • A measurement is accurate if it is close to the true value or within an acceptable margin of error.
  • Accuracy is assessed by comparing the measurements with a known reference standard.

In summary, reliability relates to the consistency of measurements over time and between repetitions, precision refers to the consistency between individual measurements, and accuracy refers to how close the measurements are to the true value. It is possible to have measurements that are precise but not accurate, and it is also possible to have measurements that are accurate but not precise. The goal is to achieve measurements that are both precise and accurate and, at the same time, reliable.

The reliability, precision and accuracy of a home meter

The first difference we find between a user who has a commercial meter and one who has a self-made meter, is that knows what materials it is made of.

There are many inexpensive (and not so inexpensive) commercial meters that they look very good, but they have very bad sensors. which are not valid for our purposes when we measure CO2 in the environment. Sensors must be NDIR, non-electrochemical and other types.

When you build the meter yourself, you know that the sensor you are using is NDIR and not some other type.

It should be noted that, unless you compare it with a €10,000 meter, the NDIR CO2 sensor that will be used in the "professional" meter is exactly the same as the home-made one. (or worse, because the Senseair S8, which we are using, is one of the best, within the "what you can afford".and there are many commercial meters that use sensors. terrible). In addition, this CO2 sensor is already calibrated by the manufacturer at the factory, so it should read acceptably well from the very first moment and improve over the days thanks to its automatic calibration system.

I'll bet my ass that the vast majority of commercial CO₂ meter manufacturers are not calibrated by their manufacturers one by one, but simply rely on the sensor manufacturer's calibration, as we do. The difference is that there are many other CO2 sensors that come very poorly calibrated from the factory.

Sensors, over time they become unbalanced (also the 200 or 300 € meters) and that commercial meter for which you have paid 100, 200 or 300 € is decalibrated in the same way as your home meter, so its measurement accuracy will be the same as yours and will depend exclusively on the care you put into calibrating it. (the same as yours, homemade, as the 300 € one).

Here we can talk about something important, and that is that "education". (information and knowledge) that most people who purchase a commercial sensor have is little to none (not to mention the rickety manual you will normally receive with the commercial meter), whereas a user who builds his meter with love, following a tutorial like the one in this blog, has the interest and the desire to learn, and does not have a rickety manual, but a whole blog with a lot of information about it and a community of users who participate, help and contribute their knowledge and experiences.

How many commercial meter manuals correctly explain to the user how to calibrate the device?

Note that the only commercial meter I have ever bought in my life, of which there is a review in this blog, came badly calibrated and measured high above 150-200 ppm and it is still measuring high because It doesn't even have a procedure to calibrate it, how can you rely on commercial meters!

In short, the homemade CO₂ meter will measure very well. if it is based on a good project (exactly the same as the 300 € commercial) but the exact CO2 concentration will depend on what the actual concentration is at the time of calibration (same as in the €300) and how, both need to be calibrated from time to time.You will never know the exact CO2 concentration unless you have it calibrated in a laboratory (either one or the other).

What can we expect in terms of measurement accuracy in either case (and provided the calibration is done well) - 100 ppm seems sufficient to me! When we talk about health and CO₂ (or about Covid and CO₂) it's not those small differences that make the difference.

What makes the difference is that some users live with very high concentrations of CO₂ without even suspecting it. and it is having a CO₂ meter that opens their eyes. Some users have said in the Telegram group (drop by and see). "I have built the meter, but it doesn't work properly, it reads over 1000 ppm and keeps going up". to realise that these figures were correct. Not 1000 ppm, but it is easy to have 2000, 3000 ppm and more, if we neglect ventilation (I myself in the room where I work, if I don't ventilate, at the end of the day I am always above 3000 ppm).

What does a CO₂ meter look like?

A CO₂ meter is a device used to measure the concentration of carbon dioxide in the air and is used to measure the concentration of carbon dioxide in the air. consists of a detection sensor, electronic components and firmware.or software, which makes these three parts work.

Really CO2 measurement is performed by a single component, the CO₂ sensor, and the rest of the components and firmware only serves to for this sensor to work and tasks related to the use of these readings.

When I say "that sensor works", I mean things like:

  • Provide you with the feeding Correct
  • If there is batterykeep it charged and in optimal condition.
  • Etc.

When I say "tasks related to the use of those readings" I mean things like:

  • Allowing us to see the measurements in some way (a screen, for example). Can you imagine a meter that had no way to see the measurements?
  • Allow the user to communicate with it (turn it on, turn it off, calibrate it, set alarms, if any, etc.).
  • Various things such as self-checks, measuring and displaying the temperature or humidity (in models with these functions), etc.

Actually, the accuracy of the meter will depend primarily on the following things:

  • Of the sensor used (there are better and worse ones)
  • The quality of the electronics (stable and clean power supply to the sensor).
  • From the user: Know what he/she is doing, calibrate it properly, use it properly, maintain it, etc.

In the image below you can see the inside a commercial CO₂ meterKecheer brand (although it is also sold under other brand names such as Baugger, KKmoon, KKTECT, S SMAUTOP and Brisunshine). As you can see, is not very different from ours. In this case it uses a CO₂ sensor from the manufacturer Honeywell, but this same meter is also used to measure CO₂ emissions. also sold with Senseair S8 sensor that we usually use in our project (and probably with other sensors).

What does a commercial CO₂ meter provide?

If you think about it, a commercial meter provides the following advantages:

All ready for use

Normally with a commercial meter all you have to do is buy it, open the box and press the power button..

This, which is fine in principle, has a major disadvantage: the user has no idea how the device works and, therefore, it is very likely to be misused and poorly maintained.

Many users are unaware that their meter must be calibrated by themselves from time to time to provide correct measurements (and there are even commercially available meters that do not have no calibrationas the DM306 meter I reviewed here).

Clear instructions (or not?)

Typically, a commercial quality meter will come with clear instructionseven if it is in English, on most occasions.

Clear instructions, often meaning instructions brief and scarce without going into anything that might seem complicated to the user.

Brands are very concerned that their product is presented as easy to use. Often, this means that in the instructions they are going to disregarding warnings, precautions and procedures relatively complex that can give the user the appearance of difficulty.

This applies to quality commercial CO2 meters, but what about non-quality ones?

The market is flooded with meters of Chinese origin of very low qualityand this is reflected in the manual. In many cases they are very bad translations into English, which leave a lot to be desired. When the translation is into Spanish, the result is often even worse, there are many manuals that are better not to read.

An aesthetically pleasing and well-finished product

Here we have an important point in favour of commercial meters.

A meter it enters through the eyes and most people wary of a homemade-looking meterThe idea is that their measurements cannot be of high quality, confusing the part for the whole.

To the fans, no doubt about it, the most difficult thing for us is to give a good finish to our projects.. No matter how hard we try, we cannot compete (with honourable exceptions) with commercial terminations.

Even the most basic and cheapest commercial CO2 meters look good (regardless of whether their construction materials look cheaper or cheaper). No dangling wires or holes that are not perfectly round will be visible.and usually have a good signage.

The manufacturer's warranty

The manufacturer's warranty, I said? Forgive me for laughing!

There is no doubt that there are manufacturers who are concerned with providing a exceptional servicewith a service quality after-sales service (selling your product at a price commensurate with this service).

But most of the time, we will not have this quality service.

When we talk about the lower-middle range of commercial CO₂ meters, which is what most users buy, what we find are very low-quality Chinese products that, like so many other low-priced Chinese products, are intended to give the minimum and fail at a minimum. They are designed to be cheap.

Approval of CO2 meter

In order to sell a CO2 meter in Spain (or anywhere in the European Union) it is necessary that it complies with European regulations and displays the CE marking. Therefore, in theory, if you buy a CE meter in Europe it should have the CE marking. (in practice I have seen many commercial models of CO2 meters without CE marking).

Does CE marking mean that the meter will be more accurate, precise or reliable than a home-made one?

The answer is NO.

To understand this, let's take a closer look at this aspect in order to see what a CO2 meter must comply with in order to obtain CE marking.

Although it is a measuring device, it is not required to comply with the 2014/32/EU directive regulating the placing on the market of measuring instruments.

As it has no moving parts, it is not a machine either, so the Machinery Directive 2006/42/EC does not apply.

Once we have excluded the above rules, we are left with the fact that this type of devices is in the electrical and electronic equipment category.

Electrical and electronic equipment must apply the following standards and directives:

▪ Directive 2014/30/EU on Electromagnetic Compatibility (EMF).

▪ Directive 2014/35/EU Low Voltage Electrical Equipment (Electrical Safety Directive).

In addition, it may be necessary to comply with the Harmonised Standard UNE EN 61010 of safety requirements for electrical equipment for measurement, control and laboratory use.

As you can see, everything a CO2 meter needs to comply with legislation and be sold (and eventually manufactured or imported) in Europe, none of these things do anything to make the commercial meter any more accurate, precise or reliable than a home-made one..

It should also be borne in mind that the components we use (at least the ones corresponding to the tutorial on this blog to build a homemade CO2 meter) are already approved and CE marked.This indicates that these components have been tested, tested and documented for electromagnetic compatibility (no interference) and low voltage electrical equipment compatibility (safe component).

I clarify that constructing equipment with CE marked components does not automatically give the CE marking to the constructed equipmentTherefore, we could not sell them as commercial equipment without first obtaining the CE marking of the product (with its corresponding laboratory tests, technical documentation, user manual, etc.).

What can go wrong (or right)?

In fact, most home CO2 meters are not really are fairly simple devices:

  1. They receive the measurement from the CO2 sensor and
  2. They show it to the user in some form (as the sensor sends it to them).

In principle the meter does nothing else, it does not manipulate the measurement and does not have to calculate anything special, however, there are things that can go wrong...

There are two main types of problems that can arise with a home meter: software and hardware.

Software problems

Although most meters do not have to interpret the measurements provided by the CO2 sensor, nor do they have to modify them at all, problems can occur.

  • A bug in the firmware that modifies the measurement before displaying it to the user.
  • A bug in the programming of the communication with the CO₂ sensor that causes it to not "understand" what the sensor is sending and take one thing for another.

In both cases I believe that the solution is simple (and it is exactly the same as you would use with a commercial meter, which is not free of these problems either): Choose a homemade CO₂ meter project. that is well documented, has been built by enough people, with good results and has a good reputation.

Hardware problems

Here, the range of problems may be somewhat wider. Basically, we can find the following problems (of the project, not of the assembly made by each user, which could give other problems if he is careless in doing so):

  • Food failures. The most common and that if you follow a good tutorial you should not suffer (and not only teach you how to avoid them, but also how to foresee, prevent and identify them).

Did I say that the range of failures was wider? No, I didn't. I lied. It's just that power supply problems are not uncommon (not in commercial meters either) and can be a problem.

Normally in the instructions for a self-respecting project, they will talk at length about food. Just read and follow the instructions. If the project does not talk to you at length about this issue, it is better not to take it seriously and look for another project.

If you have to pay attention to one thing in particular, meter projects that include rechargeable batteries:

It is not easy to design the power stage of a meter that can be used either on mains or battery power (when it is one or the other it is not so critical). I guarantee that more than 80% of the project makers have problems in the power supply stage (also many commercial ones).

As in the case of software, I believe that the solution is simple (and it is exactly the same as you would use with a commercial meter, which is not free of these problems either): Choose a homemade CO₂ meter project. that is well documented, has been built by enough people, with good results and has a good reputation.

User testing

Some of the users with access to professional meters have made some very interesting measurements. Below, I include the data from some of them, with the permission of the users:

User L.V.

He used a professional CHAUVIN ARNOUX C.A 1510 meter. It is a CO2 meter, not a commercial one, but a a professional and certified CO2 meterwhich costs more than 400 €.

The user has performed two batches of measurements with the two meters simultaneously (the Chauvin Arnoux and the eMariete meter with Senseair S8 LP sensor). Here are the results, in their own words:

During the day

"On air for 1 hour: S8/CH 412/426. Empty room: 656/665. Room occupied by me with door closed, evolution in half an hour: 850/868, 948/939, 1032/1020, 1063/1080 (on some occasions the Senseair went higher). Empty room again: 726/732 and now with the door open, 798/793, the data from the photos of 10 minutes ago 802/811".

In the evening

"Comparing during the night, very similar measurements, i.e. if the Chauvin is "certified", the Senseair is too, and even better, because it reaches lower values outside (as I said above, I am in an unpopulated area with a small town of 8000 inhabitants below, at the bottom of the valley, 1 km away). Consulting both graphs, I report the hourly data in a room of about 25 cubic metres, sleeping one person with door and windows closed: S8/CH. At 3 1371/1345, at 4 1686/1667, at 5 1833/1882, at 6 2024/2060, at 7 2165/2191, at 8 2335/2352 at 8.50 (maximum) 2374/2437, window is opened at that hour, at 9.30 534/643 and the final minimum at 10: 431/452."

If you too have been able to test with a home meter and a commercial one, let me know in the Telegram group to include it!

1 thought on “¿Son los medidores de CO₂ caseros tan fiables, exactos y precisos como los comerciales?”

  1. Dear Mariete,
    I just want to drop a general "thank you so much!" for all the valuable content in your block.
    It helped me already big time and gave me so much information and inspiration.
    I'm currently testing 3 Wemos (1x MH-Z19 digital output, 1x MH-Z19 analog (PWM) output, 1x with Senseair) beside a Netatmo in our sleeping room. I'm running an air-ventilation system with heat-recovery in our apartment. And controlling it via ioBroker on basis of the CO2 concentration. Until now, I did it with Netatmo in each room, which already worked pretty well. But I had some (external) trouble with the Netatmo cloud API, so this woke me up, that I need to get rid of the cloud and need a local solution with reliable CO2 measurement. Over my research I've stumbled over your blog - and I'm happy I did. Great work you are doing here, thank you again!
    Coming to this article:
    From my testing until now, I totally agree with your findings. Especially the calibration is a big topic. Just today I've ordered 3 more Senseair S8.
    What I see in my measurements: the MH-Z19 are running pretty parallel with the Senseair S8 - max 50ppm difference. I can see in my graphs how the Senseair is self-calibrating. And what I see as well: the Netatmo, although just calibrated again, is parallel in the area around 400ppm. But as soon as the concentration is rising, the Netatmo is showing way higher values - up to 400ppm higher than the MH-Z19/Senseair S8.
    I'm looking forward for the ongoing testing. Just ordered 5 OLED displays for my sensors. And soon my 3D-printer will be here, that I can print myself nice cases. It's really fun.
    Take care, all the best and keep up the good work!


Leave a comment