In addition to measuring VOCs, specifically alcohols, instead of CO2 using NDIR, the cheap ones can have additional algorithms that skew the readings. I purcha

In addition to measuring VOCs, specifically alcohols, instead of CO2 using NDIR, the cheap ones can have additional algorithms that skew the readings. I purchased an INKBIRD Air Pollution Monitor. It has an internal battery, doesn't connect to other devices or networks, is inexpensive, and is irrationally inaccurate sometimes. A reviewer found the same behaviour and confirmed what I knew through tests with dry ice (solid CO2) and isopropanol alcohol. It measures CO2, not VOCs, but some constantly running calibration or data fitting algorithms lead to wild overshoot and undershooting compared to higher-quality monitors. Finding an inexpensive and accurate CO2 monitor may be possible, and aliexpress.com is the home of inexpensive. But without testing and possibly a teardown, you will not likely get an accurate and affordable monitor.