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Author here. I wrote this article a few weeks ago after discovering that a well known air quality monitor in the market that retails for more than USD 1000 act

Author here. I wrote this article a few weeks ago after discovering that a well known air quality monitor in the market that retails for more than USD 1000 actually uses a PM module that costs less than USD 20. Often people assume that the more expensive, the better but as you can see in the article this is often not the case -especially when it comes to the accuracy of the monitor. If you are looking for an open source / open hardware air quality monitor kit that uses high quality sensor modules and is very easy to assemble, have a look at the project we maintain. Instructions to built an indoor monitor [1], instructions to built an outdoor monitor [2] and overview of the kits [3]. All is open source (firmware, schematics, 3d files for enclosure, etc). [1] https://www.airgradient.com/open-airgradient/instructions/diy-pro-v42/ https://www.airgradient.com/open-airgradient/instructions/di... [2] https://www.airgradient.com/open-airgradient/instructions/diy-open-air-presoldered-v11/ https://www.airgradient.com/open-airgradient/instructions/di... [3] https://www.airgradient.com/open-airgradient/kits/ https://www.airgradient.com/open-airgradient/kits/

reassembled 3y ago
Have you compared the Sensirion SEN55 against the Plantower sensors? I ask because I’ve recently picked up both and have played around with them each individually but haven’t compared them for accuracy yet. The nice thing about the SEN55 is that it contains all of the sensors in one unit (PM1.0 to PM10.0, temperature, Relative Humidity, NOX and VOC). They do cost a little bit more than the Plantower units, about $30-40USD. For anyone looking at the Sensirion sensors, be aware that the SEN5x products span a range, and only the SEN55 has all of the above sensors in one unit. I’ve found the Sensirion software to be pretty nice to work with. I was able to port their embedded i2c example repo to ESP32 with little trouble.
- ahaucnx 3y ago
  Yes, we are looking at the SEN55. The reason we currently still use the Plantowers is that there is a lot more research out there on the Plantower and how it behaves in different environments.
  - sarabveer 3y ago
    I would definitely like some research into the SEN5x line of sensors. I have three air quality monitors in the house. Two of them are AirGradient kits (Basic and Pro), with the other one I made myself using a SEN55 and SCD41. All of them are using ESPHome for the firmware. With the current very bad air quality in the US Northeast, I noticed that the SEN55 was reading much higher than the two Plantowers in the AirGradients. At one point, the SEN55 was reading around 200µg/m³, while the Plantowers were reading around 100µg/m³. I am planning on buying a second SEN55 to check this.
- hyperbovine 3y ago
  +1 for the SEN5X. Incredible little sensor. I picked up a devkit a year ago when it first came out, intending to build an OSS AQM around it, but alas, time and young kids got in the way. I'm really hoping it gains traction in the community.
casenmgreen 3y ago
I would buy a couple of these in an instant, it's exactly what I want, except it's so BIG. I want something as small as reasonably possible.
- ahaucnx 3y ago
  We originally designed a smaller monitor but it makes it very difficult to get an accurate temperature reading because the enclosure heats up and you cannot have a good distance from the active components to the temperature sensor.
  - ilyt 3y ago
    Wouldn't just correction factor be enough ? If device takes constant power it should also heat the sensor by constant amount
    - ahaucnx 3y ago
      Yes to some extent but the correction factors will be influenced by absolute temperature levels, airflow in the room etc.
      - ilyt 3y ago
        Would probably be easier in all-in-one integrated sensor and not "a bunch of separate parts" as then the temp sensor could just be near incoming air so any heating from the board would be offseted by that
  - casenmgreen 3y ago
    Thank you very much for explaining. I'm happy with that, and I will place an order.
mnw21cam 3y ago
Please could you consider changing the CSS on the article. At the moment, the web page has "display: block" on almost everything, which does exactly what it says on the tin and produces a blank white page, which does make it a little difficult to read the content.
- whamlastxmas 3y ago
  Browser reading mode is good for fixing this
- DHPersonal 3y ago
  `Display: block` doesn't hide content; that's what `display: none` does.
- 83 3y ago
  https://www.w3schools.com/cssref/pr_class_display.php https://www.w3schools.com/cssref/pr_class_display.php
  - mnw21cam 3y ago
    I stand corrected. But anyhow, something in the css is making it show just a blank white page.
RobotToaster 3y ago
You appear to be using CC BY-NC (although it's ambiguous if that applies to the schematics/3d files or only the instructions?) which isn't an open source licence, as it violates point 6 of the open source definition. [0] Sorry to be pedantic, but it spoils an otherwise interesting looking project. [0] https://opensource.org/osd/ https://opensource.org/osd/
- ahaucnx 3y ago
  We use CC BY-SA 4.0, see for example here [1] which is open-source 'compatible'. We used -for a short time NC- but not anymore. Where did you see the NC license? Maybe we forgot to update it somewhere? We also wrote a blog post about our decision to use CC BY-SA [2]. [1] https://www.airgradient.com/open-airgradient/instructions/diy-pro-presoldered-v37/#license https://www.airgradient.com/open-airgradient/instructions/di... [2] https://www.airgradient.com/open-airgradient/blog/thoughts-on-licensing/ https://www.airgradient.com/open-airgradient/blog/thoughts-o...
  - RobotToaster 3y ago
    It's at the bottom of the first link you posted https://www.airgradient.com/open-airgradient/instructions/diy-pro-v42/#license https://www.airgradient.com/open-airgradient/instructions/di... It's great that you chose to switch to an open source compatible licence.
    - ahaucnx 3y ago
      Thanks for spotting this. I will change it later today.
      - ahaucnx 3y ago
        Fixed
joshvm 3y ago
> I wrote this article a few weeks ago after discovering that a well known air quality monitor in the market that retails for more than USD 1000 actually uses a PM module that costs less than USD 20. This may be true for off the shelf air sensors, but in general the base cost of the sensor is not necessarily correlated with achievable accuracy. There are even manufacturers like Maxim who will sell you sensors for cheap, but you pay a lot to license firmware to get the best performance (and in this case actually a smoke/particle detector if I remember right - the MAX30105). You might pay thousands for a third party calibration on top of the OEM device. For example an infrared calibration target is a puck of coarsely machined metal, a heater cartridge and some black paint, but you're paying $$$ for NIST traceability, guaranteed measurements of the emissivity, etc. https://hackaday.com/2017/04/11/ask-hackaday-how-does-this-air-particle-sensor-work/ https://hackaday.com/2017/04/11/ask-hackaday-how-does-this-a... https://www.sciencedirect.com/science/article/pii/S0021850221005644 https://www.sciencedirect.com/science/article/pii/S002185022...
kiney 3y ago
I am actually not surprised as ai noticed similiar non-correlation of price and quality with different products. For headphones someone made a small analysis regarding at leat frequency response and price: https://pubs.aip.org/asa/jasa/article/141/6/EL526/917945/No-correlation-between-headphone-frequency https://pubs.aip.org/asa/jasa/article/141/6/EL526/917945/No-...
wintermutestwin 3y ago
Just a note to say that I bought the Air Gradient pre-soldered indoor monitor kit. It was simple to put together and I appreciate the lack of a required cloud server connection. I'm glad to the the outdoor monitor kit is now available - just in time for fire season!
BestGuess 3y ago
You might know or know what kind of academics or something I could ask, but how do I go about photographing and videoing and proving cigarette smoke? Best I can figure blue lasers or very strong blue LED lights but "this smoke is cigarette smoke" is givin me trouble got asthma keep getting "smoke free" apartments that put me next to chronic smokers and all the law will do if I ain't got proof is let me break the lease. That's happened several times so far but if I can get proof "this is cigarette smoke it only comes from this apartment" I can get rid of the person instead of being forced to move for the 5th time
dognotdog 3y ago
While I admire the efforts of the SC-AQMD, and generally agree with the article, using R2 from a higher-end, but not quite perfect instrument, can be quite misleading, and is not a great indicator of actual sensor performance. Also, there are a lot of potential improvements in sensor tech, but instead almost everyone is relying on the same cheap sensor modules instead of innovating, which have have pretty bad deficiencies, especially in detecting particulates in the ultra-fine range, and don’t age very well. But, they are the cheapest.
- ahaucnx 3y ago
  The R2 is between the tested monitor and the AQMD reference instruments (BAMs and Grimms). They also test if the two reference instruments agree with each other before calculating the correlation to the tested monitor. So I believe their testing method is quite accurate.
  - dognotdog 3y ago
    This is probably a longer discussion, but PM0.3 (anything less than 0.3 microns) is quite difficult to measure, especially optically, as light scattering drops off very steeply once your particle sizes are around the wavelength of the light being used. Anything short of an SMPS class instrument that “grows” small particles through condensation so that they appear large enough to be counted, will not see much below 0.3 microns. Also, the total MASS of small particles might be low, but if you look at count, or lung-deposited surface area, we get a different picture. Especially around 0.3um, particulate matter is prone to get deep into the lungs, instead of being caught in the upper airways, and the small size means larger relative surface area, and thus higher reactivity. Even smaller nano particles might not go as deep, but are more likely to enter cells or the bloodstream due to their minuscule size.
hammock 3y ago
The same is true of digital bathroom scales. In fact almost all consumer grade digital scales have “fake accuracy” built into the firmware that ensures the same exact weight shows up when you step on the scale twice in a row, even if you pick up e.g a quarter-pound weight in your hand the second time
- xmdx 3y ago
  Surprisingly my random scales off of Amazon do not do this. Accurate both times.
- ilyt 3y ago
  Source on that ? Never saw that and just checked my cheapo, it doesn't do it
  - neogodless 3y ago
    My Bowflex scale does this. I tend to switch modes / user profiles if I care about accuracy.
    - ilyt 3y ago
      See mine is too cheap to have any logic or memory in it
      - neogodless 3y ago
        shrug I bought it for $39 about 11 years ago.
        ilyt 3y ago
        I paid $8 at least decade+ ago. I'm actually surprised it haven't fallen apart by now, it was "designed" by local company and of course made in china.
- RicoElectrico 3y ago
smcleod 3y ago
Out of interest what are your thoughts on the Aranet4? (other than it's quite high price of course).
- ahaucnx 3y ago
  As far as I know the Aranet uses NDIR CO2 sensor modules from SenseAir. We do use the same brand in our DIY kits and they are very accurate and probably one of the best low-cost CO2 sensors.
  - smcleod 3y ago
    Cool, good to know. Thank you for taking the time to respond!
psyklic 3y ago
Interesting analysis! Minor typo throughout: It's R^2 instead of R_2. Also surprised the dataset doesn't include Awair's products!
- ahaucnx 3y ago
  Thanks! Fixed.
supermatt 3y ago
What is the difference between the indoor/outdoor kit? It seems the outdoor one contains more sensors but is less expensive? (or is this a nod to the article in some way ;)) My partner has a pottery studio with a kiln, it can get pretty smoggy in there when a firing is taking place (its unoccupied during, but it takes a while to settle afterwards), so I want to get a sensor. Are there any add-on sensors you can think of that would be useful in addition to assess off-gassing of VOCs (or something else?) from the kiln firing?
- ahaucnx 3y ago
  The outdoor kit focuses on PM measurements with dual PM modules for enhanced accuracy. The indoor kit has in addition CO2 and an optional TVOC sensor. I think for a pottery studio to measure PM and VOCs is a good start (and CO2 to see how well the ventilation works).
  - gh02t 3y ago
    Have you looked into the longevity of the Plantower PM sensors outdoors? I have long wanted to have one outside but I've always been reluctant because there is a lot of pollen around where I am during the warmer months and I assumed that would clog them up fairly quickly.
    - ahaucnx 3y ago
      Outdoors we see quite different lifetimes with the Plantower. Sometimes insects are getting into it and reduce the lifetime. No experience with pollen.
  - supermatt 3y ago
    Thanks! Order placed :)

reassembled 3y ago

Have you compared the Sensirion SEN55 against the Plantower sensors? I ask because I’ve recently picked up both and have played around with them each individually but haven’t compared them for accuracy yet. The nice thing about the SEN55 is that it contains all of the sensors in one unit (PM1.0 to PM10.0, temperature, Relative Humidity, NOX and VOC). They do cost a little bit more than the Plantower units, about $30-40USD. For anyone looking at the Sensirion sensors, be aware that the SEN5x products span a range, and only the SEN55 has all of the above sensors in one unit. I’ve found the Sensirion software to be pretty nice to work with. I was able to port their embedded i2c example repo to ESP32 with little trouble.

ahaucnx 3y ago
Yes, we are looking at the SEN55. The reason we currently still use the Plantowers is that there is a lot more research out there on the Plantower and how it behaves in different environments.
- sarabveer 3y ago
  I would definitely like some research into the SEN5x line of sensors. I have three air quality monitors in the house. Two of them are AirGradient kits (Basic and Pro), with the other one I made myself using a SEN55 and SCD41. All of them are using ESPHome for the firmware. With the current very bad air quality in the US Northeast, I noticed that the SEN55 was reading much higher than the two Plantowers in the AirGradients. At one point, the SEN55 was reading around 200µg/m³, while the Plantowers were reading around 100µg/m³. I am planning on buying a second SEN55 to check this.
hyperbovine 3y ago
+1 for the SEN5X. Incredible little sensor. I picked up a devkit a year ago when it first came out, intending to build an OSS AQM around it, but alas, time and young kids got in the way. I'm really hoping it gains traction in the community.

casenmgreen 3y ago

I would buy a couple of these in an instant, it's exactly what I want, except it's so BIG. I want something as small as reasonably possible.

ahaucnx 3y ago
We originally designed a smaller monitor but it makes it very difficult to get an accurate temperature reading because the enclosure heats up and you cannot have a good distance from the active components to the temperature sensor.
- ilyt 3y ago
  Wouldn't just correction factor be enough ? If device takes constant power it should also heat the sensor by constant amount
  - ahaucnx 3y ago
    Yes to some extent but the correction factors will be influenced by absolute temperature levels, airflow in the room etc.
    - ilyt 3y ago
      Would probably be easier in all-in-one integrated sensor and not "a bunch of separate parts" as then the temp sensor could just be near incoming air so any heating from the board would be offseted by that
- casenmgreen 3y ago
  Thank you very much for explaining. I'm happy with that, and I will place an order.

mnw21cam 3y ago

Please could you consider changing the CSS on the article. At the moment, the web page has "display: block" on almost everything, which does exactly what it says on the tin and produces a blank white page, which does make it a little difficult to read the content.

whamlastxmas 3y ago
Browser reading mode is good for fixing this
DHPersonal 3y ago
`Display: block` doesn't hide content; that's what `display: none` does.
83 3y ago
https://www.w3schools.com/cssref/pr_class_display.php https://www.w3schools.com/cssref/pr_class_display.php
- mnw21cam 3y ago
  I stand corrected. But anyhow, something in the css is making it show just a blank white page.

RobotToaster 3y ago

You appear to be using CC BY-NC (although it's ambiguous if that applies to the schematics/3d files or only the instructions?) which isn't an open source licence, as it violates point 6 of the open source definition. [0] Sorry to be pedantic, but it spoils an otherwise interesting looking project. [0] https://opensource.org/osd/ https://opensource.org/osd/

ahaucnx 3y ago
We use CC BY-SA 4.0, see for example here [1] which is open-source 'compatible'. We used -for a short time NC- but not anymore. Where did you see the NC license? Maybe we forgot to update it somewhere? We also wrote a blog post about our decision to use CC BY-SA [2]. [1] https://www.airgradient.com/open-airgradient/instructions/diy-pro-presoldered-v37/#license https://www.airgradient.com/open-airgradient/instructions/di... [2] https://www.airgradient.com/open-airgradient/blog/thoughts-on-licensing/ https://www.airgradient.com/open-airgradient/blog/thoughts-o...
- RobotToaster 3y ago
  It's at the bottom of the first link you posted https://www.airgradient.com/open-airgradient/instructions/diy-pro-v42/#license https://www.airgradient.com/open-airgradient/instructions/di... It's great that you chose to switch to an open source compatible licence.
  - ahaucnx 3y ago
    Thanks for spotting this. I will change it later today.
    - ahaucnx 3y ago
      Fixed

joshvm 3y ago

> I wrote this article a few weeks ago after discovering that a well known air quality monitor in the market that retails for more than USD 1000 actually uses a PM module that costs less than USD 20. This may be true for off the shelf air sensors, but in general the base cost of the sensor is not necessarily correlated with achievable accuracy. There are even manufacturers like Maxim who will sell you sensors for cheap, but you pay a lot to license firmware to get the best performance (and in this case actually a smoke/particle detector if I remember right - the MAX30105). You might pay thousands for a third party calibration on top of the OEM device. For example an infrared calibration target is a puck of coarsely machined metal, a heater cartridge and some black paint, but you're paying $$$ for NIST traceability, guaranteed measurements of the emissivity, etc. https://hackaday.com/2017/04/11/ask-hackaday-how-does-this-air-particle-sensor-work/ https://hackaday.com/2017/04/11/ask-hackaday-how-does-this-a... https://www.sciencedirect.com/science/article/pii/S0021850221005644 https://www.sciencedirect.com/science/article/pii/S002185022...

kiney 3y ago

I am actually not surprised as ai noticed similiar non-correlation of price and quality with different products. For headphones someone made a small analysis regarding at leat frequency response and price: https://pubs.aip.org/asa/jasa/article/141/6/EL526/917945/No-correlation-between-headphone-frequency https://pubs.aip.org/asa/jasa/article/141/6/EL526/917945/No-...

wintermutestwin 3y ago

Just a note to say that I bought the Air Gradient pre-soldered indoor monitor kit. It was simple to put together and I appreciate the lack of a required cloud server connection. I'm glad to the the outdoor monitor kit is now available - just in time for fire season!

BestGuess 3y ago

You might know or know what kind of academics or something I could ask, but how do I go about photographing and videoing and proving cigarette smoke? Best I can figure blue lasers or very strong blue LED lights but "this smoke is cigarette smoke" is givin me trouble got asthma keep getting "smoke free" apartments that put me next to chronic smokers and all the law will do if I ain't got proof is let me break the lease. That's happened several times so far but if I can get proof "this is cigarette smoke it only comes from this apartment" I can get rid of the person instead of being forced to move for the 5th time

dognotdog 3y ago

While I admire the efforts of the SC-AQMD, and generally agree with the article, using R2 from a higher-end, but not quite perfect instrument, can be quite misleading, and is not a great indicator of actual sensor performance. Also, there are a lot of potential improvements in sensor tech, but instead almost everyone is relying on the same cheap sensor modules instead of innovating, which have have pretty bad deficiencies, especially in detecting particulates in the ultra-fine range, and don’t age very well. But, they are the cheapest.

ahaucnx 3y ago
The R2 is between the tested monitor and the AQMD reference instruments (BAMs and Grimms). They also test if the two reference instruments agree with each other before calculating the correlation to the tested monitor. So I believe their testing method is quite accurate.
- dognotdog 3y ago
  This is probably a longer discussion, but PM0.3 (anything less than 0.3 microns) is quite difficult to measure, especially optically, as light scattering drops off very steeply once your particle sizes are around the wavelength of the light being used. Anything short of an SMPS class instrument that “grows” small particles through condensation so that they appear large enough to be counted, will not see much below 0.3 microns. Also, the total MASS of small particles might be low, but if you look at count, or lung-deposited surface area, we get a different picture. Especially around 0.3um, particulate matter is prone to get deep into the lungs, instead of being caught in the upper airways, and the small size means larger relative surface area, and thus higher reactivity. Even smaller nano particles might not go as deep, but are more likely to enter cells or the bloodstream due to their minuscule size.

hammock 3y ago

The same is true of digital bathroom scales. In fact almost all consumer grade digital scales have “fake accuracy” built into the firmware that ensures the same exact weight shows up when you step on the scale twice in a row, even if you pick up e.g a quarter-pound weight in your hand the second time

xmdx 3y ago
Surprisingly my random scales off of Amazon do not do this. Accurate both times.
ilyt 3y ago
Source on that ? Never saw that and just checked my cheapo, it doesn't do it
- neogodless 3y ago
  My Bowflex scale does this. I tend to switch modes / user profiles if I care about accuracy.
  - ilyt 3y ago
    See mine is too cheap to have any logic or memory in it
    - neogodless 3y ago
      shrug I bought it for $39 about 11 years ago.
      - ilyt 3y ago
        I paid $8 at least decade+ ago. I'm actually surprised it haven't fallen apart by now, it was "designed" by local company and of course made in china.
RicoElectrico 3y ago

smcleod 3y ago

Out of interest what are your thoughts on the Aranet4? (other than it's quite high price of course).

ahaucnx 3y ago
As far as I know the Aranet uses NDIR CO2 sensor modules from SenseAir. We do use the same brand in our DIY kits and they are very accurate and probably one of the best low-cost CO2 sensors.
- smcleod 3y ago
  Cool, good to know. Thank you for taking the time to respond!

psyklic 3y ago

Interesting analysis! Minor typo throughout: It's R^2 instead of R_2. Also surprised the dataset doesn't include Awair's products!

ahaucnx 3y ago
Thanks! Fixed.

supermatt 3y ago

What is the difference between the indoor/outdoor kit? It seems the outdoor one contains more sensors but is less expensive? (or is this a nod to the article in some way ;)) My partner has a pottery studio with a kiln, it can get pretty smoggy in there when a firing is taking place (its unoccupied during, but it takes a while to settle afterwards), so I want to get a sensor. Are there any add-on sensors you can think of that would be useful in addition to assess off-gassing of VOCs (or something else?) from the kiln firing?

ahaucnx 3y ago
The outdoor kit focuses on PM measurements with dual PM modules for enhanced accuracy. The indoor kit has in addition CO2 and an optional TVOC sensor. I think for a pottery studio to measure PM and VOCs is a good start (and CO2 to see how well the ventilation works).
- gh02t 3y ago
  Have you looked into the longevity of the Plantower PM sensors outdoors? I have long wanted to have one outside but I've always been reluctant because there is a lot of pollen around where I am during the warmer months and I assumed that would clog them up fairly quickly.
  - ahaucnx 3y ago
    Outdoors we see quite different lifetimes with the Plantower. Sometimes insects are getting into it and reduce the lifetime. No experience with pollen.
- supermatt 3y ago
  Thanks! Order placed :)