Training my sense of CO2 ppm

If I understand correctly, they’re calibrated at the factory, which is probably in China, which might have vastly differing CO2 levels depending on whether the winds blowing up or downstream of any heavy industry or metropolitan centre?

If you don't care about errors of a few percent you should be able to make your own calibration chamber.

You just need an airtight container of a known volume, a source of CO2 and a maybe a fan. Put your CO2 detector inside the box, with fresh air, your CO2 source and the fan, and see if the result matches. You may need to do some calculations.

For your source of CO2, you have a few options: combustion of a known quantity of fuel, soda bottle, dry ice, acid + sodium bicarbonate,... If you want to remove CO2, you can use calcium oxyde (quicklime).

> I’m looking forward to the day when I can walk into a room and say, huh, feels like 800 in here...

I've been this way for well over a year now. Almost two years ago I picked up an air monitor from Awair (yes, this was a pandemic related purchase because I was working so much from home) and I frequently checked what the CO2 was. Nowadays, as it starts hitting ~750ppm, without looking at the monitor I can tell it feels "stuffy" and I open a window.

The auto-calibration systems on cheaper consumer sensors also cause issues if they rarely see air that has low CO2 levels. Because these sensors can't measure absolute CO2 levels, only relative CO2 levels, they provide an absolute figure by looking for the lowest CO2 concentration they've seen over a period of time, usually around a 72 hour rolling window.

This works acceptably if the sensor is frequently exposed to outdoor air, but in a residential environment that's not always guaranteed, particularly in winter when it's not uncommon to keep windows closed to retain heat. In these situations the sensor will consider the lowest level to be around ~400ppm, even if it's actually much higher. This, of course, scales all other readings, so a sensor might read between 400-800ppm, leading you to believe everything is fine, when the actual indoor range is 800-1600ppm.

Because the auto-calibration happens over a period of time, it can be quite difficult to determine that your sensor is misreading, and the only way to fix it is to expose it to fresh air to reset the baseline.

The best solution I found to this is a dual-NDIR sensor which measures two different light frequencies, one which is absorbed by CO2 and one that isn't. This allows the sensor to know the absolute CO2 concentration, rather than the relative CO2 concentration, and avoids the need for auto-calibration. (I believe for absolute accuracy it still needs calibration for altitude, but for consumer use this makes such a small difference to be irrelevant).

Unfortunately, when I last looked, I couldn't find any consumer-grade sensors which used dual-NDIR sensors, only more expensive and less aesthetic commercial sensors. In the end I built my own using a CDM7160 sensor connected via I2C to a ESP8266, which reports over MQTT.

I have a few CO2 meters from very different sources (professional exetech, consumer product, electronics component) and they have always reported levels within 10% of each other.

For my purposes I don’t really care about accuracy under 50ppm, higher precision for trends is useful but as long as the measures value is accurate to within 50ppm I’m just fine for effects on a human. If I was doing research to publish an actual calibrated meter for 10x the price would be warranted but having three separate measurements agree gives me the trust I need.