How to explain significant difference between Smoggie and Purple Flex Humidity?

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August 17, 2026 at 5:57 pm #30456

David White
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Hello. I have co-located both a Smoggie and a Purple Air Flex outdoors for comparison.

It is my understanding that the Purple Air pm2.5 readings are somehow “corrected” during its process of US EPA AQI. Further, I understand that relative humidity is used to some extent in the “correction” process.

It is not clear that the Smoggie does this or does so in the same manner. So in my effort to “compare apples with apples” I am trying to apply the same “correction” equation to the Smoggie data.

In doing so, I see that the Smoggie reports relative humidity shown as “Humidity:77.02RH”. At the exact same time, the Purple is reporting “RH: 54.00%” and this would seem to be a significant difference.

Any ideas on how I need to bring these two disparate readings into a more congruent match? Thanks much.!

August 18, 2026 at 5:22 pm #30460

David White
Participant

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.

August 18, 2026 at 5:54 pm #30461

David White
Participant

Ah! I now see here that this is a known issue with Purple and that the reported RH from that sensor is known to NOT reflect actual environmental conditions.

When I look at the values as mentioned:

* Conversions for temperature and humidity developed by Lance Wallace are used in the “Estimated Temperature” and “Estimated Relative Humidity” data layers.

those values are much closer to those seen on my hygrometer and Smoggies.

Strange that the more expensive and complex Purple Flex is so limited in this way compared to the Smoggie!
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