Product review (3 stars): Automatic calibration is a bad feature
Unit seems to work well, nice large display and temperature and humidity are fairly accurate.  My issue is with the automatic calibration.  After 7.5 days of operation, it will reset the baseline, matching the lowest recorded reading to 400 ppm regardless of what the actual reading was.  So, if you're lowest reading for the week is 700 ppm, the unit will set that as 400 ppm and all future readings will be inaccurate by 300 ppm.  If you can't have the unit in fresh air at least once a week, it must be manually calibrated outside for 30 minutes (each week).  I don't know many home applications that will have it exposed to outside air every week.  Honestly, it's a real pain.  Otherwise, it does what it's supposed to.