PurpleAir Touch and EPA correction

Sensors
1

I have a PurpleAir Touch located in our main bedroom. I’ve noticed that when the outdoor air is clean, the windows are closed and a HEPA filter is running in the room the raw PM2.5 concentration reported by the device sits around 0 µg/m³. But the EPA corrected concentration hovers around 3.7 µg/m³. As a result, the AQI reported is almost never below 20.

I’m using the correction algorithm described in https://www.mdpi.com/1424-8220/22/24/9669.

Is this correction just not appropriate for use with the Touch?

Line chart with two series: PurpleAir Touch PM2.5 (EPA corrected) and PurpleAir touch PM2.5 (not corrected). The chart depicts a one week time span of uninterrupted readings for both series. The corrected series appears to average just under 4 µg/m³ while the non-corrected series appears to average 0 µg/m³.
Line chart with two series: PurpleAir Touch PM2.5 (EPA corrected) and PurpleAir touch PM2.5 (not corrected). The chart depicts a one week time span of uninterrupted readings for both series. The corrected series appears to average just under 4 µg/m³ while the non-corrected series appears to average 0 µg/m³.2432×736 76.4 KB

2

The EPA correction was developed for use in outdoor spaces, where humidity can cause false readings on nephelometer-type sensors like the ones used in PurpleAir devices. That constant offset you’re seeing on your graph is due to that humidity adjustment.

Indoors, you don’t have to worry about fog confusing the sensor.

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3

No you don’t need to worry about fog because assuming you use AC, there’s no humidity, LOL!!