Hello,
I bought this Xiaomi Thermometer and flashed it so I can use it with zigbee on HA. I also bought this Sonoff Thermometer.
Now, when I check the Dashboard, I have differences in temperature, and in humidity.
It’s only 0.5 degree, but how do I know which one is the correct one?
Does anyone have any experience with one of these? Which one shows the correct numbers?
Likely neither one.
These are not calibrated measurement instruments, but cheap temp sensors. At 10’C one might be closer to correct and at 25’C another one.
For the humidity accuracy is even lower.
A cheap calibrated thermometer easily cost over €70 euro, and that is without ‘smarts’. You can’t expect a € 5 to €15 smart thermometer to have the same accuracy.
Buy a calibrated alcohol thermometer (because mercury thermometers, though more accurate are illegal in most locations). Put it next to one of your sensors and manually plot a graph of the two measurements. If you are lucky the two graphs will be parallel, and a simple offset will work. More likely, they will cross and the algorithm to correct that is beyond my skill level. (Maybe a lookup table??)
Actually within 1° surprises me.
The xiaomi one uses the SHT3 on most hardware revisions.
The datasheet says it’s calibrated, but after all the soldering and assembly steps in factory you probably have to calibrate it yourself again.
Actually, when i bought a set of 5 pcs xiaomi thermometers i put them all together on a same spot for a week just to test accuracy. Surprisingly all 5 were showing +/- one decimal identical temperature. I don’t remember for humidity, since that wasn’t my primary concern, though.
I’d say that xiaomi’s are definitely cheaper 
and as a bonus they have LCD, so apart from having data on HA you can see temperature and humidity locally on the spot, too, if needed.
I flashed all of them to pvvx firmware, though.
If they used genuine Sensirion sensor, I would expect pretty accurate readings. Difficult to immagine someone needs anything better to measure ambient temperature in his house.
