True, when I've used PT100 and PT1000 sensors I've added a separate A/D converter. The added precision from these sensors would not be needed in a sauna...
With DS18B20 you could even put a couple of them together on the same wire (they use the 1-wire protocol) if you wanted temp outside as well as inside the sauna.
That is the route if you want to compile the firmware on your own, if you don't want to Tasmota works well for this as well, and you already use it so know how.
EDIT: One thing, for a sauna I'd want a humidity sensor as well...
Tasmota is a firmware that runs on ESP8266-based devices. It definitely has a matching driver and app, one which would even auto-detect the number of sensors you put in a device. I've tested it with 8 sensors in one device, all showed up as separate child devices without any extra configuration. No compiling of your own firmware, no editing and choosing drivers. But yes, Hubduino has other uses, like for motors and for those that want special features and settings not available in Tasmota.
Code is mostly ready, just some configuration depending on use, but you do have to compile it.
When in use, yes, but not before or after. I'd monitor the after values to make sure that ventilation is right after use.
Thanks guys but I “need” a battery powered device that can be mounted inside the sauna.
Will keep looking and keep an eye in this thread for further suggestions.
I have a Xiaomi Aqara temp/Humidity sensor in my Sauna.
It's main use is so that Echo Speaks alerts me that the Sauna is up to temperature and ready for me to go get cooked..... Once I'm in there there's the traditional temperature and humidity dials that tells me at a glance how it's doing.
Also - if you don't have any Xiaomi's in your setup - you need to be aware of the issues around them routing via repeaters.... As Xiaomi are not truly Zigbee compliant not all repeaters will handle them well. Xiaomi's work fine directly with HE (even if they are not on the approved device list as they not compliant). There's a big thread here in the community on Xiaomi's.
Jupp, live nearby a lot of store that sell all kind of electronics and smart home stuff.
Do you had to play with the offset temp? My is off by about 10-15 degees. My Aqara is showing ~61 when two other thermometers (real once, not the on the wall in sauna stuff) shows around 75 at the same position.
I saw they were a bit different when I first put it in, but didn't consider it as being significant. 6 degrees is a good bit - I must re-check mine to see what the difference is when I fire it up this week.
I've read other people post about the differences between temp readings of devices when in the same room, even between multiple units of the same device model.
It seems it gets off when there is a higher temp. Back to ~ 21C it seems more in line with the other temperature devices. Will check some more but maybe that’s why the specs states up to 50C, after that it might not be accurate.
It’s not really a big deal since I just want to know when the sauna has reached a good temp. 5 degrees isn’t an issue for me.
I'm a bit biased though, I wrote the app and driver for HE and made the latest version of the modified Tasmota firmware for use with HE. 
