I wondered how much the ecobee Smart Sensors vary from each other, so I moved them all temporarily to a location without any air currents for about an hour (temperatures are Celsius). This screen shows they don’t vary much (I did occasionally see ± 0.1C variation):
Edit: From the Sensor Detail graph:
I think the advertised accuracy is ±1.0°F. Pretty good to get a bunch right on!
And all three 2-packs from different sources. One 2-pack was clearly “old stock”.
Looks like you were very lucky. I have 5 myself, and found that they vary quite a bit. They are all within spec, but no where near as close as your ones are.
See https://community.beestat.io/t/sensor-calibration-offset/137 for my results…
That’s very interesting.
I placed mine on a large table, with none of the sensors near the edge, & closed HVAC vents in the room.
From my prior experience with the same sensors on shelves in my family room, I learned that temperatures can vary significantly with sensors fairly close to each other on shelves in an area with human movement & HVAC airflow. And that was nothing compared to the variation when the sensors were further apart in the same room.
You might try your calibration again, in a room with no south-facing windows & no air circulation. Maybe the floor of a garage (assuming the engines in any vehicles there have not been run for at least 2-3 hours). Or maybe a closed bathroom?
ps: After a couple decades of using Celsius in everything (because aviation weather uses Celsius world-wide), I just (two days ago) switched back to (ugh) Fahrenheit, because consumer temperature products these days (including the ecobee) report temperatures in tenths, regardless of Celsius or Fahrenheit. That means I get more resolution (not necessarily accuracy) using Fahrenheit. It’s easier to see small movements in temperature in Fahrenheit (in this case).
I was trying to calibrate them with reference to each other, as well as the ecobee itself. I had a shelf stuck to the wall at ecobee height. All 6 devices were no where near a HVAC vent, and within 6" of each other in a straight line against the wall. Can’t see the temperature changing that much within 6 inches on an interior wall 
If you look at my graph, they all varied in the same pattern with a delta between them that stayed fairly consistent.
I did do the math, and they are all within spec of the +/- tolerance specified.
I should try again. Put my 5 in a small box. they would be isolate from any airflow, and the temperature in the box should be very constant.
Do you have the newer Smart Sensors or the older sensors?
All newer Smart Sensors.
