We unintentionally ran a brief experiment at Advent this morning. We have a stand-alone CO2 monitor from Greg that we hadn't seen go above 415ppm or so in several ringing sessions. (400 is baseline/background/as low as the detector goes). After closing the windows this am due to cold, the levels did go up to 560 or something. So we reopened them.

So the good news is twofold: a) the CO2 monitor does seem to be working and is not just stuck displaying 406, and b) having open windows really does air out the tower. At least with these winter breezes.