I have recently switched to a Stages gen 3 left power crank. Within the stages app when I complete the zero offset calibration process the temperature reads a few degrees C lower than a thermometer reading (all inside).
How big an impact does this have on power readings? Should I be concerned or is the difference negligible?
Id say it’s negligible. I calibrate mine before every ride and don’t even bother to check the temperature. Since you’re calibrating indoors the temp delta is probably minor.
Now if you were to go from your warm indoor temperature to an outdoor cold ride I’d imagine then it would matter and you would want to let the crank acclimate a bit before running the calibration.
How confident are you in the accuracy of your thermometer?
I ask because there is a chance that your Stages temp measurement could be the “right” one. Just like checking power data, you really need either an officially calibrated data source, or a few devices to see which correlate or differ.
Stating that, I suspect your thermometer is probably irrelevant. Presumably the Stages is calibrated at the factory (note: a true Calibration and not the Zero Offset that you perform… because they are very different) and that calibration uses the Stages temp device. As long as that device is consistent and operating like it did at the factory, you are probably good to go. The potential fact that it may differ from your other temp device may well not matter at all.
A few degrees isn’t going to make a difference.
In regards to temperature. Don’t assume that the air temperature and the temperature of an object will be the same if the room temp varies throughout the day. There’s typically going to be some lag between the PM internals and the air temp.
Also, radiated heat does weird and wonderful things sometimes. It could be something as simple as the large dark coloured flat surface on your crank facing a cold wall in the room.
Or your thermometer is rubbish 
