Old sensor gives strange values

I have recently started to measure the fine dust in the air with two rather old “PurpleAir Flex Air Quality Monitor”-sensors:

The local measuring stations with professional sensors give a value of 5-15. The purple air sensor give a value 40-70. I have thought of two possible problems:

  • There is no unit given for the measured values. Normally in my region it is µg/m³. Is this also true for these sensors?

  • Or maybe it’s because they are a bit too old and therefore a bit dirty inside.

Have you got an idea where the problem is?

Make sure you define the units/options you are looking at in the map. The default is AQI (Air Quality index).

5-15 ug/m3 of PM2.5 is about 50 - 75 AQI.

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Thank you for the hint. Do you mean this option?:“Tasmanian PM2.5 scale (ug/m^3)”
When I apply this the values are still a bit to high but now much lower.

Who is your local air quality authority? And you have PA-II Classic sensors, not Flex :slight_smile:

You might try setting the map as follows, if you are looking for PM2.5 in μg/m³ to compare with locally calibrated equipment:

  • Data layer: Raw PM2.5
  • Apply conversion: US EPA

It may be difficult to get close agreement if your air pollutants require specialized correction factors. US EPA’s adjustment was developed with wood smoke as a primary pollutant.

You might also try performing the sensor maintenance described here, especially if the A and B sensors are wildly different:

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