nmea protocol has a few PGNs relevant for “temperatures”
PGN 130310 environmental
PGN 130311 environmental
(cant remember which of above has “air temp” , “humidity” its assumed this is outside humidity,
these are the PGNs typically used by a weather station/wind instrument)
and
PGN130312 extended temperature
PGN 130316 extended temperature
across above, they support a number of different temperature types (“cabin”, “engine room”, “refrigerator”
“freezer”, various “tanks”, “air duct”, “under bolts” ... and so on)
as timo says, it then becomes dependant on the display used (MFD, dedicated data display etc)
as to what PGNs it will support, and how it displays them (& any level of customisation ... eg renaming the way its displayed)
In my case Im interested in “engine” room data ..I dont have a weather station/wind sensor
so i will be using “humidity” to actually display the engine room humidity, and likewise “air temp”
for engine room temp - as my MFD (raymarine) doesnt support PGN instance for “engine room temp”
(later on .. I might build a small dedicated display and use timos arduino libraries to display a bunch PGNs that are not supported by my MFD)
from what i can see - nmea protocol only supports one humidity instance. but it certainly
supports multiple temp instances.