I just tried one of my Ruuvis in the sauna for the first time, and though they have always seemed to be quite accurate in “normal” room temperature/outdoors scenarios, this time the device showed quite significantly - about 10°C - lower temperature than the thermostat of the kiuas.
I could chalk this up to the fact that the tag was a little bit lower (maybe 30cm or so) than the other sensor and farther away from the kiuas, but the tag was straight next to an old-fashioned bimetallic strip thermometer that did also show more or less the same result as the thermostat.
It looks like Bosch datasheet only specifies the accuracy of BME280 from -40°C to 65°C even though the maximum operating temperature is higher, so I don’t know if this is normal, but given that quite a few people are probably using these devices in a sauna, what are your experiences?
10 C sounds like a lot higher difference than expected. Placement of the tag makes a huge difference in the temperature readings, we have gone over 100 C a few times in Slush when we placed the tags near roofs, and under 60 C when we took the tags to sitting level on windows. It might be easiest to find the height where the tag shows 80 C and attach the tag to that level.
I have had a tag in my sauna for the past 2+ years, and every now and then I take an extra tag with me for various comparisons and experiments, so I do have some experience. Some of the experiences/observations I have:
A human is a terrible thermometer; I could barely notice a difference between two positions in the sauna, yet the difference exceeded 10ºC
The temperature variations inside a sauna are huge, especially close to doors and windows; a tag in my sauna on the wall 30cm from the door will show about 3ºC colder than a tag right next to it (further from the door), despite being closer to the stove (the door is between the stove and the tags)
The height affects the temperature a lot (as @otso pointed out), though I observed this with an infrared thermometer rather than a tag (you can more or less notice this yourself, the temperature at your feet is much lower than the temperature at your head, which is a lot considering the 1st point)
Walls are surprisingly cold, by hanging a tag from the ceiling 20cm from a wall I get about 7ºC higher reading than on the wall at the same height (for this reason my Saunatag is currently hanging from the ceiling at roughly head height; I get more precise readings on temperature changes as the cold walls are not “dampening” the temperature changes, though the tag is on the opposite wall to avoid it getting too hot)
