The technology of the MH-Z19B instrument is gas selective and precise. It exploits the phenomenon of absorption by CO2 of infrared ray around 15 microns. It certainly has a Fabry Perot interferometer.
But it is not intended to be used outdoors according to the specifications of Winsen the manufacturer. Moreover, to obtain the announced performance, it is still necessary to calibrate it regularly and scientifically.
The daily calibration (mode ABC) should be deactivated before the first 24h of use, otherwise the factory calibration will be lost. This mode of calibration is that of the “poor man” who does not have a source of reference gas. The sensor considers that the lowest value recorded during the last 24 hours corresponds to 400 ppm which is about the concentration of the external atmosphere in the absence of pollution (campaign). This assumes that during this period, the sensor for at least 20 minutes is placed outdoors in a slightly polluted atmosphere. Placed indoors, if the air is never renewed during this period of 24 hours, the sensor will take low values, > 1000 ppm in general and consider that this corresponds to 400 ppm. From there, the values ​​provided will be erroneous.
A more serious calibration approach is to make a true zero with the Z19B placed in a closed container where a maximum of CO2 has been removed. For example by using charcoal filter (foam for extractor hoods) or lime water or soda placed at the bottom of the jar. The concentration can thus be reduced to a few ppm in a few hours. Be careful not to have condensation or water on the sensor. You can also try using nitrogen, argon or bottled helium if you know the specifications of the producer. One can even imagine a vacuum chamber with a primary vacuum. The sensor will calibrate itself to 400 ppm but whatever, in the software, it will be necessary to withdraw this value to that coming from the UART or PWM to obtain the true value. On the other hand, I do not know how temperature compensation is carried out and possibly humidity that can be sources of significant uncertainties. Small chemical reminders to consult:
en.wikipedia.org/wiki/Limewater
http://fred.elie.free.fr/soude_et_gaz_carbonique.pdf