Yeah, I've read that some monitors take the lowest reading of the week and recalibrate on the assumption that that was 400 ppm, which is *awful*. 😬 The one I got instead has two emitter LEDs and one sensor; the first LED blinks every few seconds to sample the air, and the second blinks on once a week or so in order to calibrate the first. (The assumption is that decalibration is primarily via weakening of the first LED with use, and that the second one does not meaningfully weaken over time.) There's also a calibration port but I don't know where to get 1000 ppm calibration gas. :-)