I think there is nothing wrong with my computation. What I believe is the difference between my Smoggie and the Purple Flex is that the Flex reports fractional values for pm2.5 whereas the Smoggie seems to give only integer values for this item.

Someone over on Reddit said: For US EPA AQI, 0-50 = 0-9ug/m3. So this is LIKELY a product of PA only showing the whole concentration value (I.e., 2.1 vs. 2.2 ug/m3).

So I decided to start running pm2.5 values between 1.5 and 2.5 thru the EPA formula I have (got elsewhere here). I found that at pm2.5 = 1.6 I get AQI = 9. At 2.0 I get 11, at 2.2 I get 12 and so on. So I guess the Reddit answer is correct – the display of the pm2.5 value is being rounded while the computation for AQI is using the raw value.

This then raises the question of why does the Smoggie not provide fractional data for pm2.5? Both sensors use the same mechanism, as I understand it. I know that the Smoggie data is somehow “corrected” and perhaps the data is rounded up/down or something and the fractional value is lost by the time the json I get has been produced?

I am trying hard to compare/contrast the Smoggie with the Purple Flex and this sort of thing makes that a bit more difficult. Is there some way to access the un-corrected data that might include the fractional portion?

Best

I should add to this that, this morning – after initial clear skies – a huge fog rolled in here (we are proximate to the water in Puget Sound, WA USA). I notice that the outdoor Smoggie is showing RH at 81.48 and the Purple Flex is showing only 61. It happens that we had all of our windows open while this change was occurring. So both our indoor Smoggie shows 73.74 and our completely analog hygrometer shows 79. So it appears that it is the Purple Flex which is possibly off-kilter.