A few weeks ago, I wrote quite a popular blog post titled “Air Quality Monitors: When Paying More Does Not Get You More Accuracy” with the key takeaway that the accuracy of PM2.5 does not get better the more expensive the monitor is. One of the things that this post did not dive in was WHY the accuracy is very similar across all these monitors.
So I thought why not take a closer look at what type of PM sensor modules these monitors use, and so I went through the South Coast AQMD list of their tested air quality monitors as they state in their field report the PM module used. I looked up the most popular brands across a large price range and this is what I found:
From the monitors I looked at, all except one, use low-cost PM modules costing between USD 10 - 20 in bulk quantities. The notable exception is the QuantQM that uses a more expensive AlphaSense OPC-N3 optical particle counter (in addition to the Plantower PMS5003).
I also added our own open-hardware air quality monitor Open Air as comparison into the table, and I am happy to see that it is very affordably priced for the sensors it offers.
Why do we see such a concentration of a few sensor modules?
It is interesting to see that even much more expensive monitors use very low-cost PM sensor modules, and it’s difficult to give a clear reason, but I think mainly two factors come into play:
The performance is good
Low-cost PM modules have quite a good performance for measuring PM1 and PM2.5 and with adjusted compensation algorithms that take into account impacts of humidity or specific particle concentrations, these low-cost sensors can be tuned to be very close to reference station accuracy. So in this respect, these sensor modules are “good enough” for a variety of purposes. What’s sometimes lacking -but this is for another post- is that most manufacturers don’t publicise their compensation algorithms so it’s not clear what corrections are made and how these correction factors have been derived. Where all of these low-cost sensors underperform is with larger particle sizes like PM10 but since the main focus is on PM1 and PM2.5 (as these are more harmful for the body), often PM10 is not such an important criteria.
Not much choice
As you can see from the table, there are 3-4 manufacturers in the USD 10-20 range, Plantower being the most popular one. Then on the other side of the spectrum you have reference grade sensors (like BAMs, GRIMMs, Palas) in the price range above USD 20.000. In between these two extremes is very little choice. There are a few in the price range USD 50-100 that deliver pretty much the same accuracy as the cheaper ones and don’t offer advantages. Above USD 100, the AlphaSense OPC-N3 is a good addition, especially as its performance for PM10 is much better. Then there are sensors that use special filter material that needs to be changed and maintained which is not ideal if the system should be low-maintenance.
Sensor module accuracy is not equal to monitor accuracy
It’s important to point out that not only the PM sensor module determines the accuracy of the whole monitor but there are other factors that can have a significant impact on the quality of the measurements. Here are a few:
- Airflow management: The enclosure design with the air-flow management can have a large impact on the measurement quality. Sometimes enclosures have internal loops that impact the measurement or are prone to condensation, thus leading to high humidity (that’s also impacting the accuracy). Some monitors have internal heating that brings the air into a specific temperature and humidity range that can increase the accuracy of the measurement.
- Measurement frequency: In general the more often the monitor samples, the better the accuracy. This is often an issue with monitors running on solar power as they could run on a low frequency thus not collecting enough data points for an accurate measurement.
- Application of compensation algorithms: The accuracy of the low cost PM modules can be increased significantly by applying compensation formulas that are derived for the specific location where the monitors are deployed.
- Testing & Co-Location of the monitors: There is a variance in the accuracy of the low-cost sensor modules, so it is important that the manufacturer tests each monitor before shipment and applies a strict performance passing criteria internally.
So what’s the conclusion?
In case you are considering getting an air quality monitor, I would recommend the following:
- Try and find out which sensor modules are used. It’s good to have a well known module like Sensirion, Plantower or AlphaSense that have extensively been tested.
- Look for test reports of the specific monitor you want to purchase.
- Check the hidden costs of the air quality monitor (e.g. forced subscriptions, expensive spare parts).
- Only pay for features you actually need. For example if your main concern is PM, you don’t need to purchase a monitor that also measures NO2. Also, monitors with cellular connectivity often have relatively high ongoing costs.
- If you want to support an open-hardware air quality monitors, check out the Open Air Outdoor Monitor or AirGradient DIY Pro Indoor Monitor that we offer.
Please share your thoughts with us and let us know what kind of topics you would like us to write about.
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