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« Managed Beans, XPages and Testability | Main| MAMIL gone mad »

Air Quality Measurements

Singapore is facing a severe haze crisis. Besides the subjective "this is bad", I got curious how the official air quality gets reported. Singapore's National Environment Agency uses the US devised Pollution Standard Index, but reports that onlt only as 24h, but also 3 h average for different places in the republic. Besides that they report the 2.5µ particle (PM2.5) count that isn't part of the PSI ( PM stands for particulate matter).
So far it feels like the information is genuine. Nevertheless knowing how an abstract number is formed is the first step to independent verification. With a little digging NEA provides the detailed instructions how the value is computed. The substances measured are: Sulphur dioxide (µg/m³). Particles with 10µ or more size (µg/m³), Carbon Monoxide (mg/m³), Ozone (µg/m³) and Nitrogene dioxide (µg/m³) which NEA publishes separately including a haze map:
Regional Haze Map
Source: NEA
A quick check on Google reveals, that air quality measurement is quite a popular application both with Arduino as well as the Raspberry PI (there are more). It seems the Arduino has an edge when it comes to coupling the sensors, while the Raspberry PI shines in aggregation and web accessibility. Probably a combination of both would be a winner. Something like the Pinoccio meshed with a Raspberry Pi powered CouchDB for aggregation.
The real challenge however are the sensors (and their calibration later on). The PSI measurements are in weight per volume, while most of the data sheets for the sensors I found state sensitivity in particles per million (PPM) or particles per billion (PPB). Luckily LennTech provides an online calculator to transcribe the values for us:
Substancerange (weight/volume)Parts per millionParts per billion
Sulphur dioxide80-2620 µg/m³0.0284 - 0.928928.4 - 928.9
Carbon monoxide5-57.5 mg/m³4.05 -46.64050 - 46600
Ozone118-1180 µg/m³0.0558 - 0.55855.8 - 558
Nitrogen dioxide1130 - 3750 µg/m³0.558 - 1.85558 - 1850
The PM concentration can't be converted from weight/volume into ppm using a static formula, since particles can be anything with different weight. It needs to be measured.
So the hunt for suitable sensors starts. SO2 seems to be the biggest item on the list. I've looked so far at the following providers: I would be grateful for more hints of suppliers. Once the sensors have been settled, further planning will commence. stay tuned!


Gravatar Image1 - Another aspect of air quality is the Smoke measurement BS2742:1969 measured through the Ringelmann scale


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