Please note the VOC sensor operates in reverse. Detected gas gives the low reading. Clean Air gives a high reading. When the unit was new, it looked like this curve upside down.

Ken

You are missing the point. The reason the graph looks like that is because the unit is producing readings of 30Meg to 50Meg for short periods of time – perhaps only 1 or 2 samples. Historically, the “sensible readings” have been in the range of 300K. The ratio of 30M to 300K is 100:1. According the specs the Winsen MP503, the range from quite polluted to quite clean is on the order of 10:1. So such big readings cannot be coming from a healthy sensor.

see the graph within this document https://www.winsen-sensor.com/d/files/ZP07-MP503.pdf

This sensor gives lower readings with more VOC. These >10Meg “spikes” would therefor represent sudden spikes of amazingly clean air amongst a background of almost continuous high pollution. This is nonsensical.

These very big readings of >>10M have only appeared in recent months (coinciding with colder weather) that is why I suspect it is the start of a failure. I see that the device has a small built-in heater, perhaps that is what is failing. I suppose time will tell.

Ken