| > Every IAQ CO2 meter > For example, suppose I buy two CO2 meters from Amazon It sounds like you're looking at retail consumer products, which I think is the problem. The Sensirion SCD30 _sensor_ [0] for example allows forced recalibration [1] to an arbitrary concentration. Also, since you call it an "IAQ" CO2 meter, it might be that these aren't CO2 meters at all. They could be VOC meters that make up a projected CO2 reading based on the presence of other chemicals in the air. You need to make sure the sensor is NDIR-based and actually measures CO2. If you want accuracy, don't buy retail sensors that don't come with spec sheets. I'd find the sensor you want first then find or build a meter based on it. For example the SCD30 can be connected to an ESP32 and connected to WiFi using ESPHome [2], which can be set up with some YAML files. > self-calibration Just by the way, self-calibration is what you want. Real CO2 sensors drift over time and you definitely want them recalibrated at least once a year. Auto-calibration takes away that pain and when implemented in a suitable way, really works. Many calibration routines are not suitable for residential environments as they're based on the assumption that they will see pure fresh air at least once a day. This is a safe assumption in a business environment because most AC systems have an overnight fresh air purge. In a residential environment though, someone might not open the window once a day. Again referencing Sensirion, their auto-calibration cycle looks for two, hour-long valleys in the CO2 readings over the previous two weeks that are within a few percent of each other and uses those as a fresh air reference. In my experience, this works well at home. > Another downside to this difficulty in testing is that it's hard to truly evaluate a particular meter's performance. Which seriously limits side-by-side comparisons of different models, and makes it harder to hold manufacturers / retailers accountable for poor performance. You "just" need a reference to compare to. If you have a look at academic literature, they typically use a lab-grade setup to compare. You're right though, it's difficult for a regular home user to evaluate a sensor like this. [0]: https://www.sensirion.com/en/environmental-sensors/carbon-di... [1]: https://www.sensirion.com/fileadmin/user_upload/customers/se... [2]: https://esphome.io/ |