Is the undocumented TT value on the responses of command 0x86 really related to the ambient temperature?
Dispite of no mention in nether MH-Z19 nor MH-Z19B official document, it is widely believed that the TT value on the responses of command 0x86 mentioned document1 might be related to ambient temperature. But, verification of this value with other thermal sensor shows random mismatch of the value.
IMHO, it might be just intermediate temperature value from infrared heater & thermal sensor pair for CO2 concentration measuring and it might not related to the ambient temperature.
The Principal of how to measure CO2 concentration.
The principal of measuring CO2 concentration is called NDIR(Nondispersive Infrared), this is based that CO2 absorb the thermal infrared ray. Unlike the near-infrared, which wave-length is near visible light and used to remote controller device, there are neither inexpensive diode emit heat infrared nor sensor detect it. Instead, a pair of infrared heater and thermal sensor is used in place of emission diode & detector because infrared heater emit heat infrared and this conduct heat from emitter to the object as the name indicate. The figure 1 of document2 might be informative to grasp intuitive understanding.
The random mismatch between TT value and other thermal sensor value.
The observed mismatch are random. document1 suggest that "TT is the temperature in degrees Celcius, plus 40". But rzarajczyk reported this conversion value shows higher value than real room temperature as 29-30℃ where actual temperature is 22℃. Despite, in my observation, it shows lower value than real room temperature as 21℃ even the temperature measured by dht22 shows 24℃ (In fact, the room was something like the latter at that time).
I guess that the TT value might may NOT be related to the ambient temperature, but might be the thermal sensor value near the infrared heater for measuring CO2 concentration.