Somewhere in the description of the cheaper kit, it says that CO2 sensor is not included.
The auto-calibration on MH-Z -sensors seem to require the sensor to be on constantly for 24/48h. By powering off the sensor (via software?) every 23h should disable the feature (just like some anti-calibration command). I wonder if the calibration changes the slope, or the offset. If it just changes the offset (‘a’ parameter in the calibration line: [co2]=a+bx), then it should not even matter in chamber measurements.
Ideally, the sensor should be able to measure concentrations under 400ppm. This would mean that we could measure photosynthesis. SenseAir K30 can do it (but it also autocalibrates).
It would be interesting to do some further testing on the sensors, namely response time with/without a fan, response to CO2 concentration / temperature / air humidity / how does the calibration actually works. At University of Helsinki we have CO2 calibration gases, that could be used. Furthermore, we have some serious CO2 analyzers, that could be used as controls (although I have no contract, I have contacts). It would probably take a couple of days for some semi-serious tests. I could to it at some point. I now have K-30, MH-Z16. It would probably be a good idea to have MH-Z19B and T6713 as well.
Testing/calibration CO2 sensors is one thing, and the next thing to test would be the chamber + CO2 sensor combination. We have a test setup for that in Hyytiälä. It would take about a week to do some serious testing, so this won’t be happening instantly.
P.S. Related to open research (open science): Under the Hyytiälä web page, check the Open data page, SMEAR online data and https://openscience.fi. SMEAR II is the measurement station in Hyytiälä. The new version of the site is just put online, and some information still seems to be missing.