Question: T and RH compensation for PM measurements?

subir is asking a question about air-quality
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by subir | May 28, 2020 18:29 | #23732

Hi. subir here from Pune, India.

We have been developing some low cost openhardware for PM measurements and have good amount of data over the past 6-7 months over 5 locations in our city. When compared to government published PM data, our values approximately match government values during colder days than during hotter days. This observation is seen repeatedly and consistently and is especially significant (20-30 ug/m3 difference) now during the hot summer months.

Our system uses Sensirion's SPS30 sensors for PM measurements and Bosch's BME280 for ambient temperature and RH.

I would like to know if we need to cool the air we sample to steady the air sample temperatures to a constant value for consistent results? Anyone who does this on their low-cost systems? Should we also worry about removing moisture?


Hi subir, Great work. I have a few doubts that help to get more insights, 1. Does the SPS30 sensor measure Indoor or Outdoor data alongside RH and T? 2. Is the SPS30 sensor placed inside the casing or outside? If there is a window/ passage for airflow, what is the size? 3. The distance of Govt. installed Air quality measurement station to yours? Since you installed across 5 locations in Pune and the government installed station is only 1, near Karave road.

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Hello silentsairam. 1) SPS30 and BME280 are all on 1 single PCB enclosed in PVC piping with an inbuilt 12V fan. The fan's air rushes underneath the PCB (protecting all components from dust, while the SPS30 and BME280 are exposed to the air flow through holes in the PCB. Everything is made to be used outdoors. 2) Good point. The distance of government sensor is 2-3 Km from the nearest Breathe2 sensor. I know its too much, so theoretically there's no reason to compare. But we'v only got that kind of data so compare we must. In a sense, despite spatial and temporal unique variations between each sensors (including government) we see similar patterns after (running/boxcar) averaging over 1 hour for days and days.

Hope this clears my original question, will air sample conditioning help? Has anyone tried it.


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