In the graph below, some tests with two sub-100€ CO2 sensors: Airsense K30 (I think 2000ppm model), and MH-Z16. MH-Z16 is about half of the price of K30. Shortly: they seem to have very similar response, suggesting that they work as they should.
Both sensors were inside the same chamber that was kind-of airtight. The air space inside the chamber was mixed with fans. Using 9V instead of 3.3V seemed to decrease the response time of the sensors considerably (this is good news). MH-Z19B testing was inconclusive, because of some technical-practical issues. However, it seems that MH-Z19B (the golden box and also the cheapest of the sensors) has in practice very long response time (some minutes), and it does not get shorter by ventilation. K30 can explain only about 30% of the variance of MH-19b, which is low. This is because of the problems with the experimental setup, and the very different response time of the sensors.
Anyway here is the picture, showing that in the today’s test environment the sensors respond very similarly (which is great news).
Edit.: Here is an additional picture of the measurement, just to get the idea what was done.
