I have not replaced the sensor all this time and, old as it is, seems to be working. My conclusion is there must be some source out there that is producing intermittent bursts of VOC’s. Also, I have recently seen that, by refreshing the browser page, the spikes go away.

You know that competing sensors (of which there are several) all use 10-minute averages for the default plots. This would flatten out spikes and would greatly improve the appearance of the PM graphs. This would be done at the server end of things I suppose.

This summer, the formaldehyde sensor is – for the first time ever – starting to produce some readings. I wonder if you have changed some software? or if my air is dirtier this year than ever before?

The VOC and formaldehyde readings tend to track each other (mirror image) indicating a common gas is being detected. Probably not actually formaldehyde. Both sensors respond to a a variety of gasses.