[werk_ag]

Thank JT118 for this valuable info

the diurnal range will be in the region of 50ppm

So it seems that the diurnal range that we are getting is far above this value.

I am not sure if there is some other phenomena forcing this huge range, or perhaps it is the way the sensor is being read.

According to the manufacturer manual:

"You can read gas concentration via UART directly, no need to calculate."

This is what the software is doing, issue a command that asks for a reading, an then read the answer.
There is checksum to verify the validity of answer, if it fails the answer is rejected. There isn't any calculation involved.

I wasn't aware of the normal diurnal range, but since the beginning that the measured diurnal range seemed too large.
I have already asked to the others, info about their model of the MH-Z19, but I get no answer. This could be important, because there are two models of this CO2 sensor, mine is the first model, called only by MH-Z19, but there are also a newer model called MH-Z19B.